Superalloys 2024

Nickel-based superalloysNickel-based superalloy remainNickel superalloy the essential enabling materials technology for high-temperatureHigh temperature componentry in gas turbines. Alongside architectural designDesign, nickel alloysNickel alloys have permitted substantial improvement in specific fuel consumption and durability; indeed the Rolls-Royce Ultrafan aero engine is set to be ~25% more fuel-efficient than the first generation Trent Engine (Trent 700) and 10% better than the XWB—currently the world’s most efficient aero engine [1]. There exists a multitude of complex alloy compositions consciously optimised over 70+ years which have been designed or selected for specific applications. The use of certain constituent elements or processingProcessing methods for targeted property improvement must be considered in the context of the full product life cycle to assess the sustainability trades of new manufacture and through life impact on customer and business value. Original Equipment Manufacturers (OEMs) are now increasingly contracted to deliver ‘customer outcomes’ rather than just the equipment. The principles of effective Asset ManagementAsset management and Through Life Engineering ServicesThrough life engineering services (TES) are discussed and supported by case studies pertaining to lifecycle management, particularly repairRepair methodologies. ‘Digital Twins’ are a key enabler to optimising the Through Life Engineering ServicesThrough life engineering services value levers; understanding/controlling the physics of how superalloysSuperalloys degrade in use is pivotal to delivering better customer, business, and sustainability outcomes. The paper presents a framework for optimising product offerings whilst simultaneously working towards a sustainable future.

Recent advances in hardware technology as well as sophisticated methods for post-processingProcessing of Electron Backscatter DiffractionX-ray diffraction (EBSD) and Energy DispersiveX-ray characterization X-Ray Spectroscopy (EDS) data have opened up new possibilities for detailed quantitative microstructure characterizationMicrostructure characterization of polycrystalline Ni-based superalloysNi- based superalloys. However, combining EBSD and EDS scans to reconstruct the true morphology of primary γ′ particles remains challenging, as some important microstructural features exist at a scale below the EDS method’s lateral resolution limit, which leads to undesired artifacts at γ/γ′ interfaces. We present an automated computer vision architectureAutomated computer vision architecture capable of resolving the meso-scale features of polycrystalline γ/γ′ microstructuresMicrostructure with a level of detail that has not previously been demonstrated. Our methodology involves the following steps: 1. The combination of multiple elemental EDS maps. 2. Edge-preserving filtering of EDS maps using a non-local-means algorithm. 3. Unsupervised machine learningMachine learning phase segmentation based on k-means clustering and 4. An automated artifact correction for the combination of EDS and EBSD information based on morphological conditions. In this manner, digital micrographs are reconstructed in a way that allows for quantitative determination of meaningful numeric metrics by utilizing methods from the field of algorithmic geometry. Various microstructural entities such as discrete primary γ′ particles, mixed γ/γ′ grains, or γ grains can be characterized separately, including properties of related boundaries. Geometric characteristics can be quantified in terms of the local arrangement and cluster behavior of particle groups, as well as their spacings. The present work contributes to the development of digital workflows for precise and automatic microstructure characterizationMicrostructure characterization.

SuperalloysSuperalloys are inherently complex alloys to designDesign due to their multicomponent nature; designing alloys to take advantage of the newly discovered Local Phase TransformationLocal phase transformation (LPT) strengthening creates constraints on alloy composition beyond the conventional considerations for polycrystalline, precipitatePrecipitates-strengthened microstructuresMicrostructure. The basis for designDesign is precipitationPrecipitation of χ/η on superlattice stacking faultsStacking faults and microtwinsMicrotwin while remaining thermodynamically inaccessible to form in bulk. This approach to LPT strengthening has now been demonstrated by optimizing η-LPT in an empirically designed alloy, NA1, the performance of which is shown here by testing [001] oriented single crystalsSingle crystal at several conditions. Computationally designed alloy, NA6, in polycrystalline form, was shown to perform similarly to single crystalline NA1 at 760 °C 552 MPa and outperform single crystalSingle crystal CMSX-4CMSX-4 as well as all LPT-strengthened polycrystalline alloys at this temperature. The deformation substructure of NA6 was investigated via HR-STEM, showing both η-LPT at SESFSuperlattice Extrinsic Stacking Faults (SESF) and χ-LPT at microtwinsMicrotwin. Compositions of these LPT were elucidated using atomic resolution energy dispersive X-ray spectroscopy and compared to LPT compositions in relevant alloys and discussed considering recent studies on fault propagation velocities.

With its precipitationPrecipitation strengthening effect, the L12-ordered γ′ phase contributes substantially to the mechanical propertiesMechanical properties of superalloysSuperalloys; therefore, understanding the microscopic mechanisms by which it can be sheared is of key importance. A commonly used method to study these mechanisms involves high-resolution imaging in the transmission electron microscope in $$\langle {110}\rangle $$ ⟨ 110 ⟩ projection which enables straightforward discrimination between intrinsic and extrinsic stacking faultsStacking faults as well as microtwinsMicrotwin. However, the complex or superlattice nature of these stacking faultStacking faults structures, which provides key information on their formation mechanism, is not necessarily revealed in this projection. In the present work, an experimental approach is presented to resolve this ambiguity and reliably determine the complex or superlattice nature of a stacking faultStacking faults in the L12 structure by additionally imaging the fault in a nearby $$\langle {211}\rangle $$ ⟨ 211 ⟩ projection, which is achieved by tilting the specimen by 30°. The method is demonstrated using two different examples in single-crystalline Co-base superalloysSuperalloys. In the first example, the approach enabled the direct experimental verification of two key aspects of the well-known Kolbe mechanism for superlattice extrinsic stacking faultStacking faults formation, namely, the complex nature of the leading intrinsic stacking faultStacking faults segment and the occurrence of diffusionDiffusion-mediated re-ordering. In the second example, microscopic details of the shear-based transformation from the cubic L12-γ′ to the hexagonal D019-χ phase are revealed, accounting for additional complexities—again including a re-ordering process—arising from the superlattice ordering of both phases.

PrecipitationPrecipitation hardening is the dominant method of achieving high strength in most Ni-based superalloysNi-based superalloy. The formation of nanoscaleNanoscale precipitatesPrecipitates during thermal exposure is often studied to determine the optimal methods of attaining high strength. The commercial Ni-based superalloyNi-based superalloy, HAYNES® 244® alloy, is strengthened through a novel $$\gamma^{{\prime}{\prime}{\prime}}$$ γ ′ ′ ′ -Ni2(Cr, Mo, W) intermetallic phase that forms during a two-step aging cycle. The precipitationPrecipitation kinetics of this intermetallic $$\gamma^{{\prime}{\prime}{\prime}}$$ γ ′ ′ ′ phase are sluggish for single-step aging in comparison to the γʹ phase in precipitationPrecipitation-strengthened Ni-based alloysNickel-based alloys, but a two-step aging treatment has shown to reliably harden the alloy and improve high-temperatureHigh temperature properties compared to a single-step aging heat treatmentHeat treatment. To investigate the formation and coarseningCoarsening of this phase, heat-treated samples of the 244 alloy were analyzed with high-energy in situIn situ and ex situ X-ray techniques such as small angle X-ray scattering and wide angle X-ray scattering as well as Vickers micro-hardness, electron microscopyElectron microscopy, and atom probe tomographyAtom probe tomography. The relationship between hardness, aging parameters, and microstructureMicrostructure evolution is discussed. The enthalpy of formation and precipitatePrecipitates solvus temperature were determined with high-temperatureHigh temperature differential scanning calorimetry and dilatometry analysis.

This paper presents the development of AMS20, a novel third-generation single crystal superalloySingle crystal superalloy designed for the next generation of aeroengines. The alloy designAlloy design utilizes a computational approach that combines empirical models and CALPHADCALculation of PHAse Diagrams (CALPHAD) calculations to identify compositions meeting targeted specifications. This methodology facilitates the selection of the final AMS20 alloy composition, aiming to optimize high-temperatureHigh-temperature oxidation creep propertiesCreep properties while preserving excellent high-temperature oxidationHigh-temperature oxidation resistance. The alloy was cast and single crystalSingle crystal bars were grown using the conventional Bridgman method. Subsequently, appropriate solution and aging treatments were developed, and the tensile and creep propertiesCreep properties of AMS20 were evaluated. A comparative analysis was performed against reference second- and third-generation single crystal superalloysSingle crystal superalloys, namely, CMSX-4CMSX-4, CMSX-10K, CMSX-4 PlusCMSX-4 Plus, and AGAT. Results indicate that AMS20 exhibits a remarkable combination of high-temperatureHigh-temperature oxidation creepCreep and oxidationOxidation resistance. Specifically, AMS20 demonstrates a creepCreep life at 1200 °C comparable to CMSX-10K alloy, while its cyclic oxidationCyclic oxidation performance at 1150 °C is akin to second-generation CMSX-4CMSX-4 alloy and notably superior to third-generation reference alloys. However, it is important to note that initial aging results highlight the sensitivity of AMS20 alloy to TCP phasesTCP phases at 1050 °C and its relatively narrow high-temperatureHigh temperature processingProcessing window. Yet AMS20 remains a promising single crystal superalloySingle crystal superalloy for the future generation of aircraft engines.

Ni-base superalloysNi-base superalloys of the latest generation are prone to the formation of topologically close-packed (TCP) phases due to their high content of refractory elements. The quantitative correlation between the TCP phaseTCP phases precipitationPrecipitation, the evolution of the lattice parametersLattice parameter, and the γ/γ′ lattice misfitLattice misfit in a highly alloyed single crystalSingle crystal Ni-base superalloy Ni-base superalloySuperalloys is investigated here. Even in the standard heat-treated state TCP phasesTCP phases are found in the dendrite cores. With additional annealing at 1100 °C further TCP phaseTCP phases precipitationPrecipitation occurs until a constant fraction is reached after 100 h. High-resolution X-ray diffractionX-ray diffraction experiments conducted at room temperature revealed that the lattice parameterLattice parameter of the γ phase decreases with increasing exposure time and reaches, like the TCP phaseTCP phases fraction, a kind of plateau after long aging durations. In contrast, the lattice parameterLattice parameter of the γ′ phase hardly changes throughout aging, which results in a decrease of the initial γ/γ′ lattice misfitLattice misfit from about −1% to a plateau of about −0.45% after 1000 h. Comparison of thermodynamic and lattice parameterLattice parameter calculations with the experimentally determined values revealed that the depletionDepletion of the main TCP phaseTCP phases-forming elements Re, W, and Mo associated with the ongoing TCP phaseTCP phases formation is the main reason for this behavior. It is also shown that the effect of both stress relaxationStress relaxation and TCP phaseTCP phases precipitationPrecipitation results in a reduction of the tetragonal distortion and coherency stresses in the γ matrix channels.

Nanomechanical testing methods are very well suited to complement combinatorial materials researchCombinatorial materials research using diffusion couplesDiffusion couple. Diffusion couplesDiffusion couple allow the investigation of the microstructureMicrostructure and the mechanical propertiesMechanical properties of multiple alloy compositions in a single sample without the need to cast many different alloys. To expand the investigation of mechanical propertiesMechanical properties on diffusion couplesDiffusion couple to higher temperatures, indentation creepIndentation creep is a highly suitable technique. In this work, the diffusion coupleDiffusion couple approach was utilized in combination with a high-temperatureHigh temperature indentation creepIndentation creep testing technique, using a 25 µm flat-punch indenter, to conduct combinatorial investigations on a Co–Ni diffusion coupleDiffusion couple and on a multinary γ/γ′ CoNiCr-base superalloySuperalloys diffusion coupleDiffusion couple with increasing Cr content. For the Co–Ni system, the highest room temperature hardness and highest creep resistanceCreep resistance at 550 °C was observed between 50 at.% Co and 80 at.% Co. For the γ/γ′ CoNiCr-base superalloySuperalloys CoWAlloy2, an optimum Cr content of 14 at.% has been found. Up to this Cr content the hardness and creep resistanceCreep resistance is not altered and the precipitationPrecipitation of undesired phases does not occur. With further increasing Cr content, the γ′ volume fraction decreases significantly and undesired W- and Al-rich phases precipitatePrecipitates, leading to an increase in hardness but a deterioration of the creep propertiesCreep properties.

A multicomponent high-Cr CoNi-based superalloySuperalloys was developed for potential use in industrial gas turbine materials. In response to the enhanced overall performance of alloys, the microstructural stabilityMicrostructural stability and strength of alloys with different Re additionsRe addition at 950 °C were investigated. During long-term thermal exposure, the γ′ precipitatesPrecipitates remained nearly cuboidal and the addition of Re led to a mild decrease in the γ′ volume fraction. The Re additionRe addition can effectively reduce the coarseningCoarsening rate of γ′ precipitatesPrecipitates due to both the significant reduction in the effective diffusionDiffusion coefficient in the γ matrix and the decrease in interfacial energy between the γ and γ′ phases. Despite a reduction in γ′ volume fraction, the addition of Re improved the creep propertiesCreep properties of the investigated alloys at 950 °C/200 MPa-300 MPa. Firstly, the introduction of Re causes the inversion of W partitioning from γ matrix to γ′ precipitatesPrecipitates to improve the γ′ strength. And it also elevates the content of Re within the γ phase, thereby enhancing the solid solution strengthening effect. Moreover, this improvement is also attributed to a decrease in the effective diffusionDiffusion coefficient of the alloy. All these factors contribute to improving the creep propertiesCreep properties and yield strength of high-Cr CoNi-based superalloysSuperalloys.

ATI 273ATI 273™ alloy was developed to be a fabricable, creepCreep-resistant, and oxidationOxidation-resistant superalloySuperalloys. This paper summarizes material characterization of early vacuum-melted lab heats focusing primarily on its oxidationOxidation resistance, though high-temperatureHigh temperature oxidation strength properties will also be addressed. Short- and long-term continuous isothermal exposure data for 871 and 982 °C testing in dry air will be presented with an emphasis on understanding the impact of Ta addition on scale formation. Furthermore, cyclic exposure testing in flowing air was carried out to assess oxide scale adhesion and healing. ATI 273ATI 273 has an intentional tantalumTantalum addition of 2.5 wt.%, and this impactful element increases the alloy’s resistance to both oxidationOxidation and creepCreep. This paper will show the alloy’s improved properties are linked to Ta addition and Ti reduction in the chemistry leading to improved properties compared to ATI 263™ and Haynes® 282® alloys. ATI 273ATI 273 is being considered for high temperatureHigh temperature structural applications, especially those for land-based gas turbine and aerospace engines.

Additive ManufacturingAdditive manufacturing (AM) is revolutionizing the production of complex superalloySuperalloys parts, yet its advancement is hindered by the high costs and extensive time required for atomizing powders from which new alloys are produced and tested. This study presents our approach to expedite and economize alloy developmentAlloy development for AM. At the heart of this method is the CALPHADCALculation of PHAse Diagrams (CALPHAD)-based multi-criteria optimization algorithm PyMultOpt, utilized for selecting potential alloys, which are then produced through arc-melting. Our innovative testing framework involves two key processes: (i) assessing alloy processability via electron beam remelting of bulk material, and (ii) evaluating mechanical propertiesMechanical properties after refining coarse-grained material via deformation and recrystallizationRecrystallization. Focusing on Alloy 247 and derivatives selected through PyMultOpt, our approach successfully emulates AM-like microstructuresMicrostructure in arc-melted material. The compressive creepCreep tests on the recrystallized microstructureMicrostructure of Alloy 247 indicate a minimal creepCreep rate comparable to that of AM specimens. Moreover, profilometry-based indentation plastometry tests at 760 °C offer a rapid, high-temperatureHigh temperature evaluation method, allowing for preliminary alloy ranking before extensive creep testingCreep testing. This study demonstrates that remelting and recrystallizationRecrystallization of bulk material can reproduce AM-like microstructuresMicrostructure, enabling a faster and more cost-effective assessment and ranking of new alloys in terms of their mechanical propertiesMechanical properties and AM-processability, as opposed to the traditional, powder-dependent alloy developmentAlloy development methods.

Exp-G27 is a new low-cost Ni base wroughtWrought alloy developed by Carpenter Technology Corporation for applications such as jet engineJet engines turbine casing and internal combustion engine exhaust valves. Mainly strengthened by γ’, Exp-G27 has higher temperature stability compared to alloy 718. Within the targeted service temperature range between 704 and 871 °C, Exp-G27 demonstrates comparable mechanical performance as WaspaloyWaspaloy but with a significantly lower raw material cost. However, the creepCreep and stress ruptureStress rupture of Exp-G27 is slightly lower than WaspaloyWaspaloy. In this study, a cost-performance tradeoff study was carried out to help make modification to the original Exp-G27 to achieve superior creepCreep/stress ruptureStress rupture performance than WaspaloyWaspaloy while still being cost competitive.

There is significant industrial demand for low-cost polycrystalline Ni-base superalloysPolycrystalline Ni-base superalloys that can be utilised for rotating and static applications at intermediate temperatures in aircraft engines. In this work, we report on the microstructureMicrostructure, heat treatmentHeat treatment response, mechanical propertiesMechanical properties and oxidationOxidation behaviour of a newly developed alloy for these applications. The alloy has been designed to improve on the thermal stability and mechanical performance of Inconel718 (IN718), whilst offering lower processingProcessing costs than advanced cast & wroughtWrought alloys such as Alloy 720Li and Renè 65. The study highlights key areas of alloy developmentAlloy development, comparing the alloy properties where possible to current commercially available alternatives.

The temperature capability of current state-of-the-art high-pressure diskDisk superalloysSuperalloys is around 700 °C. To further improve their temperature capabilities, new diskDisk alloy designAlloy design approaches with a focus on blade alloy compositions, which was designed for applications at temperatures of above 900 °C or higher, may be effective. In this study, novel Ni–Co base diskDisk superalloysSuperalloys were designed based on a combination of Ni-base blade superalloySuperalloys TM-47 and Co-12.5 wt.% Ti, both of which possess a γ–γʹ two-phase structure. First screen results using single crystalSingle crystal casts revealed TM-47 to be a potentially promising candidate as a base alloy for Ni–Co base diskDisk superalloySuperalloys, while additions of Co-12.5 wt.% Ti to TM-47 was found to improve creepCreep strength of the alloy. A later investigation using P/M alloys revealed that limited additions of Co-12.5 wt.% Ti to the base alloy also improves powder manufacturability, phase stability, and high temperatureHigh temperature proof stress. Thus, TM-47 M2 (20 wt.% addition of Co-12.5 wt.% Ti to TM-47) was selected as a candidate alloy for subscale manufacturing trial. CreepCreep tests at 760 °C/630 MPa demonstrated that the selected TM-47 M2 provides superior creep propertiesCreep properties compared to other conventional diskDisk superalloysSuperalloys, especially for the 0.2% creepCreep time, which is a critical property for high-pressure turbine disksDisk.

Enhancing the aspect ratio of the γ′ precipitatesPrecipitates in the raft structureRaft structure of Ni-base single crystal superalloysNi-based single crystal superalloy for high-pressure turbine bladesTurbine blades is a fundamental approach to improve the creep resistanceCreep resistance at higher temperature and lower stress conditions. To obtain a larger aspect ratio, the alloy designAlloy design strategy to enlarge the lattice misfitLattice misfit between γ and γ′ phases towards the negatively larger side has been applied. However, the fundamental driving force for the raft structureRaft structure formation, “interfacial strain anisotropyAnisotropy”, has been ignored for the alloy designAlloy design. Here, interfacial strain anisotropyAnisotropy is a difference of the interfacial strain between horizontal and vertical γ/γ′ interface, and this is caused by the interaction of loading stress with the elastic misfitElastic misfit between γ and γ′ phases. In this study, model alloysModel alloy imply that the aspect ratio is modulated by kinetic factor caused by Re with a presence of interfacial strain anisotropyAnisotropy. The lattice misfitLattice misfit and the elastic misfitElastic misfit can be used as new alloy designAlloy design parameters. Regression analysis using database of creepCreep ruptured lives indicates that negatively larger lattice misfitLattice misfit and the positively larger elastic misfitElastic misfit can enhance the creepCreep rupture lives although the relationship between the lattice misfitLattice misfit and the elastic misfitElastic misfit was basically trade-off. However, additional regression analysis of the lattice and the elastic misfitElastic misfit indicates that the addition of Re, moreover, co-addition of Mo and W, Ti, or Ta could intentionally realize both negatively larger lattice misfitLattice misfit and positively larger elastic misfitElastic misfit.

In the present work, comprehensive investigations were conducted to examine the distribution behavior of the minor alloying elementMinor-alloying element Zr, its impact on microstructural evolutionMicrostructural evolution and effect on the high-temperatureHigh temperature stress-rupture propertiesStress-rupture properties in IN 100 superalloysSuperalloys. The results indicate that the presence of Zr influences the eutectic phase in as-cast alloys, resulting in a reduction in the γ channel width and an increase in both the volume fraction and size of the γ′ phase. Through the time of flight-secondary ion mass spectrometry and transmission electron microscope analysis, Zr is found to segregate along the eutectic front in the form of Ni5Zr in 900 °C stress-ruptured samples, rendering IN 100 alloys enhanced creep resistanceCreep resistance at elevated temperatures. Heat treatmentHeat treatment at 1200 °C could effectively eliminate the eutectic structure, thus mitigating the segregationSegregation of Zr. A maximum endurance life in IN 100 alloys at 900 °C/314 MPa and 950 °C/225 MPa is reached when Zr content is approximately 700 ppm. Synergistic effects including the eutectic content, volume fraction, and the degree of raftingRafting of the γ′ phase together with the dislocation networkDislocation networks structure were analyzed and discussed, contributing to the improved stress-rupture propertiesStress-rupture properties in IN 100 alloys.

DevelopingSingle crystal superalloy novel and affordable Ni-based single crystalSingle crystal (SC) superalloysSuperalloys is crucial in order to address the escalating expense of advanced industrial aero-engines. The present work introduces a novel low-cost SC superalloySuperalloys DD93, designed to meet the performance requirements of third-generation superalloysSuperalloys, which have a Re content of only 3 wt.%. The alloy designAlloy design proceeded focusing on reducing the Re additionRe addition whilst still maintaining a high-temperatureHigh temperature capability at 1120 ℃. Comprehensive performance tests confirmed that DD93 has great microstructural stabilityMicrostructural stability, and its stress ruptureStress rupture properties and tensile strength are comparable to those of commercial third-generation SC superalloysSuperalloys. In addition, DD93 was evaluated and shown to have favorable fatigueFatigue strength, environmental resistance, coatingCoating compatibility, and castability. More importantly, the reduced Re content of 3 wt.% leads to a considerable cost reduction of almost 40% compared to CMSX-10 and René N6. The competitiveness of DD93 in terms of cost and mechanical propertiesMechanical properties improves the affordability of turbine bladesTurbine blades and shows promising prospects for its application in advancedLow-cost single crystal superalloy aero-engine turbine bladesTurbine blades.

FrecklesFreckle are casting anomalies generally detrimental to the mechanical propertiesMechanical properties of superalloysSuperalloys. This work presents a new freckleFreckle formation propensity criterion calculated using only alloy composition to rank the freckleFreckle propensity of alloys when processed via the same melting route at common conditions This is achieved by combining literature established changes in density of solidifying liquid (Δρ/ρ) with an approximation for permeabilityPermeability within the mushy zoneMushy zone, using enthalpy of fusion, and solidificationSolidification range of the alloy. Also, a methodology to calculate Δρ/ρ using electron probe microanalysis (EPMA) on as-cast coupons of superalloysSuperalloys is outlined. Additional factors influencing the permeabilityPermeability within the mushy zoneMushy zone, such as the presence of primary carbides, can also be calculated based on alloy chemistry, and incorporated to refine the freckleFreckle formation propensity criterion.

Five computationally designed single crystal superalloysSingle crystal superalloy have been developed for aircraft engine airfoils submitted to high temperaturesHigh temperature. The broad steps of the alloy designAlloy design procedure are presented in this study. Tensile and creep propertiesCreep properties of these SX superalloysSuperalloys are evaluated in regard to the predictions of the creepCreep-life model and/or in regard to reference SX superalloysSuperalloys. Effective and predicted density show a good consistence. The γ′ phase fraction predicted by CALPHADCALPHAD calculations at the designDesign stage and the ones evaluated in this work are consistent as well. For AMSP2 alloy, atom probe tomographyAtom probe tomography measurements were conducted. We observe that excessive additions of rhenium to a platinumPlatinum containing SX superalloySuperalloys have a strong impact on creep propertiesCreep properties, reducing the creepCreep lifetime compared to the reference alloy TROPEA, and strongly affect the γ/γ′ coherency, leading to the absence of raftingRafting at intermediate temperatures (950–1050 °C).

Increasing the fractions of certain refractory elements resulted in the development of modern single-crystalSingle-crystal superalloysSuperalloys with not only improved high-temperatureHigh temperature performance but also increased solidificationSolidification segregationSegregation of elements. The importance of solution treatment to reduce segregationSegregation and homogenize the microstructureMicrostructure has been well verified; however, guidelines for achieving simple but effective homogenization practices have yet to be developed. In this study, the effects of different solution treatments on the creep propertiesCreep properties of a fourth-generation single-crystalSingle-crystal superalloySuperalloys were investigated, and two guidelines for designing homogenization practices for modern single-crystalSingle-crystal superalloysSuperalloys were proposed. First, the degree of homogenization achieved by solution treatment will be saturated once the dendritic segregationSegregation coefficient of specific elements reaches a certain limit (lower than 1.2 for Re and W, and higher than 0.9 for Al and Ta). Second, if the residual segregationSegregation is strong, different degrees of local deformations between the dendrite core (D) and interdendritic (ID) regions induce uneven fractures in the alloy. An effective, fully homogenized structure should exhibit similar local strengths in the D and ID regions. These guidelines will help assess the effectiveness of solution treatments and guide the development of new solution treatment strategies for modern single-crystalSingle-crystal superalloysSuperalloys.

A follow-on study was performed on NASA’s recently developed nickel-based P/M TSNA-1 diskDisk alloy to evaluate the influence of processingProcessing on the alloy’s mechanical propertiesMechanical properties. The composition of the alloy was tailored to improve the high-temperatureHigh temperature creepCreep strength through transformation strengthening of precipitatePrecipitates phases. Initial alloy developmentAlloy development was done utilizing HIP processingProcessing. The follow-on study evaluated the properties of the forged version of the alloy which provided for a more realistic processingProcessing history similar to that utilized in the engine industry. The creepCreep performance of the forged material significantly exceeded the original HIPHot Isostatic Pressing (HIP) material and outperformed alloys like LSHR and ME3 by an order of magnitude. The TSNA-1 alloy’s cyclic and dwell FCG behavior was also characterized. While the HIP condition of the alloy exhibited very poor dwell FCG resistance in comparison to the LSHR P/M diskDisk alloy, the forgingForging processed TSNA-1 drastically improved the crack growth behavior. In the forged condition, both cyclic and dwell FCG behavior of the alloy were equivalent to a current LSHR P/M alloy.

Modifications to INCONEL® alloy 725 (IN725) with higher Nb or Ta and high Ti/Al ratio previously revealed the formation of γ″ precipitatesPrecipitates as the sole precipitatePrecipitates strengthening phase which was accompanied with significant increases in elevated temperature strength and resistance to creepCreep deformation. In this study, the γ″ alloys (named M725-Nb and M725-Ta) were chosen to observe the microstructural evolutionMicrostructural evolution of γ″ precipitatesPrecipitates during creepCreep. Following aging, γ″ was found to be the main precipitatePrecipitates strengthening phase with no detectable γ′ in both M725 alloys. After creepCreep, the grip/gage sections of the crept M725-Nb/Ta exhibited preferential coarseningCoarsening and inhomogeneous distributionInhomogeneous distribution of γ″ platelets, and sandwich-like structures, comprising of cubic γ′ precipitatesPrecipitates with small γ″ discs on each facet. This γ″ variant selection behavior taking place in the grip section with no stress applied was different from the typical stress-induced variant selection reported in previous literature. However, creepCreep tested specimens with prior high-temperatureHigh temperature exposure to promote varying γ″ characteristics prior to deformation demonstrated a similar creepCreep behavior when compared to aged specimens tested under identical conditions. This suggested that the coarseningCoarsening and inhomogeneous distributionInhomogeneous distribution of γ″ precipitatesPrecipitates was not the determinant factor for creepCreep life in bulk M725-Nb/Ta alloys. The failure of bulk M725 was linked with the development of cracks along grain boundariesGrain boundary (GBs), where the emergence of Nb-rich δ and/or Ta-rich η phases at the GB markedly depleted the γ″ strengthening phase in the surrounding GB regions whichGamma-double prime led to localized weakening.

Spark Plasma Sintering was used to join powders of two nickel-based superalloysNickel-based superalloy containing over 40% volume fraction of γ’ phase and considered very difficult to weld. After 1 min of sintering, a very high relative density was reached, and both alloys were successfully solid-state joined without precipitationPrecipitation of detrimental phases in the diffusionDiffusion-affected zone. Solutioning of samples showed that a dual-alloy microstructureMicrostructure, composed of a coarse-grained alloy in the rim and a fine-grained alloy in the core, could be produced by this technique. In both as-received and fully-heat-treated states, assemblies exhibit tensile strengths at least higher than the weakest parent material. The heat-treatment sequence enables modulating strain partitioning during tension, leading to a more homogeneous deformation at very high stresses. Thus, failure occurs in the weakest parent material, far from the weld, with no alteration of the ductility, suggesting a sound joint has been produced during the short sintering time. The study on the viscoplastic response by means of tensile stress relaxationStress relaxation tests shows that a dual-alloy diskDisk technology has a high potential, resulting in improved creep propertiesCreep properties, chemical stability, and corrosion resistance in the rim section.

This study focuses on the challenging characterization of the microstructuresMicrostructure and the viscoplastic behavior (creepCreep, relaxation) within the transition region of dual microstructureMicrostructure disksDisk made of γ/γʹ Ni-based superalloysNi- based superalloy. This transition microstructureTransition microstructure, consisting in spatial variations of grain sizesGrain size and/or γʹ precipitationPrecipitation, poses difficulties in providing insights into effective modeling strategies in aero-engines. To characterize viscoplastic behavior at intermediate temperatures, stress relaxationStress relaxation tests were employed using AD730™ superalloyAD730TM superalloy. A single-crystalline version of AD730™ was used to isolate the effects of different precipitationPrecipitation states (ranging from 15 to 300 nm in size), at 700 °C and 800 °C. The results reveal a significant impact at 700 °C, while no substantial impact was observed at 800 °C. Additionally, precise specimen machining was conducted in different radial zones of a dual diskDual disks to explore the influence of various grain sizeGrain size gradients, on viscoplastic behavior at both 700 °C and 800 °C. The findings indicate a progressive viscoplastic behavior along the gradient of grain sizeGrain size for both testing temperatures. Finally, the viscoplastic behavior of transition specimens was analyzed within the framework of a grain-sensitive phenomenological model. The study provides valuable insights into understanding and modeling the complex behavior of dual microstructureMicrostructure disksDisk in aero-engine applications.

Crack growthHigh temperature characteristics under dwell-fatigueDwell-fatigue loading are investigated in this paper for a new PM nickel superalloyNickel superalloy developed by Rolls-Royce plc. In order to achieve a good balance of mechanical propertiesMechanical properties, it is critical that an appropriate heat treatmentHeat treatment be defined for the alloy. Here, the influences of cooling rate after solution heat treatmentSolution heat treatment on γ′ size and distribution, grain boundaryGrain boundary serration, and dwell-fatigue crack growthFatigue crack growth resistance are first investigated. A medium cooling rate has been found to produce good dwell-fatigue crack growthFatigue crack growth resistance and has been carried forward to a comprehensive matrix of dwell-fatigueDwell-fatigue crack growth testing in air with varied temperatures (700 and 760 °C), initial ΔK values, and testing procedures (constant amplitude loading and load shedding). Tests have also been interrupted to allow detailed examination of crack tips using scanning electron microscopyElectron microscopy. Notably, the dwell time is 120 s (positioned at maximum load), and the applied stress ratio is fixed at 0.1. Significant variations in behaviour are observed at both temperatures. It is concluded that the variation in crack growth rates results from the selection and interactions between two different time-dependent mechanisms: environmentally assisted oxide forming and crackingCracking, and environmental independent creepCreep deformation and creepCreep crack growth. When the environmentally related mechanism operates alone, the fastest crack growth rates are obtained. This study demonstrates that the environmentally assisted, sustained fast crack growth can be inhibited with a combination of an appropriate microstructureMicrostructure and a defined range of mechanical driving force.

Tensile tests on Alloy 718 Ni-based superalloyNi-based superalloy at 650 $$\,^{\circ }$$ ∘ C at different strain ratesStrain rate revealed a strain-rate dependency on the fracture mode. A change from intergranular to transgranular fracture was observed in air as the strain rateStrain rate increased, mainly when Portevin-Le-ChatelierPortevin-Le-Chatelier (PLC) mesoscopic deformation bands were present. To better understand the link between strain rateStrain rate and fracture mode, a description of the strain localization in the early deformation stage is needed. In this study, high-resolution digital image correlationDigital image correlations (HR-DIC) was carried out at the onset of strain localization, a low strain rateStrain rate (LSR, $$\dot{\epsilon }$$ ϵ ˙ = 10 $$^{-4}$$ - 4 s $$^{-1}$$ - 1 ) and at high strain rateStrain rate (HSR, $$\dot{\epsilon }$$ ϵ ˙ = 10 $$^{-2}$$ - 2 s $$^{-1}$$ - 1 ). This latter condition aimed at investigating the microplasticityMicroplasticity development within PLC bands. The in-plane and out-of-plane displacement components of each single plastic event were measured to accurately assess and distinguish morphological sliding at grain boundariesGrain boundary (i.e., grain boundary sliding) andGrain boundary sliding dislocation slip. The deformation within the PLC bands was examined at macro, meso, and microscales. Statistical analyses highlighted the distribution and partitioning of these strain localization events related to different microstructural features, including grains, and grain and twin boundaries. Grain boundary slidingGrain boundary sliding was found to be more prominent at LSR. Interestingly, events near and parallel to twin boundaries are particularly intense regardless of the strain rateStrain rate. At HSR, grain boundary slidingGrain boundary sliding is less pronounced, and a high density of intragranular slip bands developed within the PLC bands; based on observations before and after the occurrence of the PLC band.

Residual stressResidual stress is one of the major factors that reduces the dimension stability of superalloySuperalloys disc components during either machining or in service. A pre-spinningPre-spinning operation has been proven to be an effective technique to modify the unfavorable residual stressResidual stress in disc forgingsForging. By spinning the semi-finished disc forgingsForging up to a certain rotational speed, a small amount of plastic strain is generated and accordingly results in a redistribution of the internal stress. The spinning-generated plastic strain, though quite small, significantly affects the mechanical propertiesMechanical properties of the disc forgingsForging. To ensure the efficiency and reliability of the application, a clear understanding of the effects of the pre-spinningPre-spinning-generated plastic strain needs to be obtained. The two widely employed disc alloys GH4065A and FGH4096 were evaluated using a specifically designed, full-scale spinning experiment, the microstructureMicrostructure and the mechanical propertiesMechanical properties prior to and after the spinning were analyzed in detail. The results show that the pre-spinningPre-spinning generated a higher dislocation density, which is heterogeneously distributed. The dislocations tend to accumulate at the various types of internal interfaces and within primary γ′ precipitatesPrecipitates. The formation of heterogeneous dislocation structures significantly affects the mechanical propertiesMechanical properties, including, but not limited to, the tensile strength, creepCreep, and fatigue crack growthFatigue crack growth rate. By optimizing the spinning parameters, the mechanical propertiesMechanical properties can be well controlled to meet the designDesign requirements for disc forgingsForging.

This article investigates grain boundaryGrain boundary serration and its effects on high-temperatureHigh temperature creepCreep behavior of Udimet-720Li. Grain boundaryGrain boundary serration is induced by controlled cooling during heat treatmentsHeat treatment, with continuous and discontinuous precipitationDiscontinuous precipitation of γ′ phase identified as the competing mechanisms affecting serration formation. Continuous precipitationPrecipitation of coarse γ′ particles pins grain boundariesGrain boundary and leads to a slight serration termed the continuous precipitationPrecipitation type (type-I) boundary, while discontinuous reaction forming cellular γ/γ′ behind the mobile grain boundaryGrain boundary causes larger serration known as the discontinuous precipitationDiscontinuous precipitation type (Type-II) boundary. Samples with straight (STB), Type-I (SRB-1), Type-II (SRB-2) grain boundariesGrain boundary were produced, and creepCreep behaviors under 700 °C/700 MPa were investigated.The SRB-1 sample exhibits a notably lower minimum creepCreep rate at about 2.66 × 10–7 s−1 and a prolonged 17% rupture lifetime compared to the STB sample. These improvements are primarily attributed to the different size distribution of γ′ precipitatesPrecipitates, which contributes to a higher hindrance to dislocation movements. The presence of Type-I serration also hinders intergranular crack propagationCrack propagation, thereby extending creepCreep life. In contrast, the SRB-2 sample exhibited a higher minimum creepCreep rate, around 1.74*10–6 s−1, and a considerable 42% reduction in rupture lifetime in contrast to the STB sample. This is attributed to the enhanced grain boundaryGrain boundary fractions through the formation of Type-II serration in the SRB-2 sample, promoting grain boundaryGrain boundary diffusionDiffusion creepCreep. Additionally, the presence of incoherent cellular γ/γ′ interface associated with Type-II serration facilitates void nucleationNucleation, leading to increased creep damageCreep damage and a shortened creepCreep lifetime for the SRB-2 sample.

The effect of non-fully recrystallized (NFRX) microstructuresMicrostructure on the fatigueFatigue performance of γ-γ′ Ni-base superalloySuperalloys GH4065A was investigated by comparing the fatigueFatigue properties of specimens with a homogeneous fine-grained structure and a heterogeneous grain structure containing many NFRX regions. The fatigueFatigue life was characterized by performing strain-controlled fatigueFatigue tests at 500 ℃ with a maximum strain amplitude ∆εmax/2 ranging from 0.8% to 0.45%. In order to study the fatigueFatigue slip, crack initiationCrack initiation, and propagation behaviors associated with the NFRX structures, interrupted stress-controlled fatigueFatigue tests of dog-bone shape specimens were carried out at 400 ℃, employing two loading levels with the maximum stress amplitude ∆σmax above and below the yield strength, respectively. The NFRX grain structures were characterized and classified into two distinct types: unrecrystallized (URX) and partially recrystallized (PRX). Their effects on the slip band formation, crack initiationCrack initiation and propagation, and propagation were investigated and compared with that of the fully recrystallized (FRX) structure. The superior resistance of NFRX structures to the fatigueFatigue damage, particularly restricting slip-induced crack initiationCrack initiation and inclusion-induced crack propagationCrack propagation, makes the rest of the FRX regions accumulate most of the fatigueFatigue damage, resulting in unnoticeable variation in fatigueFatigue life.

Polycrystalline γ/γ′ CoNiCr-based superalloysSuperalloys have a promising combination of properties for aerospace and stationary gas turbine applications. In this study, four alloys, CoWAlloy1, 2, 3, and 6, were selected for further investigation. MicrostructuresMicrostructure of fully heat-treated materials were characterized by scanning and transmission electron microscopyTransmission electron microscopy, and long-term stabilityLong-term stability at 750 °C was examined for various durations up to 1000 h. Tensile tests were performed between 25 °C and 700 °C, revealing dynamic strain agingDynamic strain aging in CoWAlloy2, 3, and 6 at intermediate temperatures of 400–650 °C. Hydrogen charging induced a decrease in tensile ductility at 25 °C in the µ phase containing CoWAlloy3. CreepCreep experiments were performed on CoWAlloy1, an alloy with increased Ti and Ta content, at 750 °C and 800 MPa, with variations of the secondary γ′ precipitatePrecipitates size and grain sizeGrain size to identify optimal microstructural parameters. It was found that smaller grain sizesGrain size lead to a significantly higher creep resistanceCreep resistance at these testing conditions.

The impact of non-metallic inclusionsNon-metallic inclusions (NMIs) and the forgingForging process on the low-cycle fatigueFatigue (LCF) durability of a γ/γ′ superalloySuperalloys is investigated. X-ray synchrotron tomography was performed to characterize the NMI populations. Low cycle fatigueLow cycle fatigue was tested at 400 °C on specimens taken in various orientations on forged disksDisk using the ring rollingRing rolling process or the die forgingForging one. The results show anisotropic tensile and fatigueFatigue properties for the ring rolled disksDisk, especially in radial and axial directions. On the other hand, no reduction of the lifespan was observed for the die-forged ones. Fractographic observations and a macroscopic approach in modellingModelling of a particle during forgingForging were conducted. It showed that abnormal low lifetimes are connected to crack initiationCrack initiation at NMIs. However, it is not a sufficient condition. The length of NMI stringers and their orientation in the diskDisk are important parameters. The forgingForging process has a strong impact on the plastic flow and the initial state of the particles. Indeed, ring rollingRing rolling promotes decohesion at the interface between the inclusion and the matrix. The crack initiationCrack initiation time and the first stages of the propagation to the matrix highly contribute to low lifespan. A better understanding of the detrimental configurations of NMIsNon-metallic inclusions would allow optimized forgingForging processes.

To improve the low-cycle fatigueFatigue (LCF) strength of Alloy 247Alloy 247, refinement of the alloy microstructureMicrostructure through the forgingForging process has been trialed, and the mechanical propertiesMechanical properties and microstructureMicrostructure of the forged alloy were compared with that of the conventional cast alloy. By utilizing the MH (Mitsubishi Hitachi) process, an ingot of Alloy 247Alloy 247 with high volume fraction of γ′ phase was successfully forged without crackingCracking. In contrast with cast Alloy 247, forged Alloy 247Alloy 247 exhibited uniform microstructureMicrostructure with γ′ precipitatesPrecipitates, the grain sizeGrain size was significantly reduced from 10 mm in the cast alloy to 200 μm in the forged alloy, and the eutectic structure was eliminated completely. Results obtained from high-temperatureHigh temperature tensile testing showed remarkable improvement in elongation, which led to higher ultimate tensile strength (UTS) values. Owing to the fine microstructureMicrostructure, forged Alloy 247Alloy 247 showed 15 times longer LCF life than that of the cast alloy although creepCreep strength slightly decreased in low stress regions. Significant improvement in LCF life of Alloy 247Alloy 247, which has intrinsically high creepCreep strength, allows for tuning of the mechanical propertiesMechanical properties based on requirements and broadens the application of the alloy to hot section parts.

One key performance attribute of cases and seals in advanced gas turbine engines is resistance to thermal fatigueFatigue, which is dictated by strains associated with thermal cyclingThermal cycling of the component and the material’s intrinsic resistance to fatigueFatigue crack initiationCrack initiation and propagation. Frequently isothermal strain-controlled low-cycle fatigueFatigue (LCF) testing is utilized to assess the candidate alloys’ intrinsic fatigueFatigue capabilities. In addition, a low coefficient of thermal expansion (CTE) is desirable since the CTE largely controls, for a given thermal cycle, the magnitude of the thermally induced strains. The primary aim of this study was to compare the LCF performance of recently developed low CTE, high strength HAYNES® 244® alloy to that of WaspaloyWaspaloy, a legacy case alloy, from room temperature to 1400 °F (760 °C). A secondary aim was to shed light on the active deformation and damage mechanisms in these alloys. We observed that both alloys exhibited comparable fatigueFatigue lives, within experimental scatter, but the 244 alloy deformed by deformation twinning, whereas WaspaloyWaspaloy deformed by dislocation slip and precipitatePrecipitates shearing and looping.

To reveal the deformation behavior of a novel γ′-strengthened multi-component CoNi-based wroughtWrought superalloySuperalloys, the tensile propertiesTensile properties from room temperature to 850 °C and creep propertyCreep properties under 725 °C/630 MPa were studied. The results show that after hot forgingHot forging and sub-solvus solution plus two-step aging heat treatmentHeat treatment, the grain sizeGrain size of the investigated alloy is uniform, accompanied by the bimodal distribution of γ′ precipitatesPrecipitates. Elements such as Co, Cr, and Mo segregate within the γ phase, whereas Al, W, Ni, Ti, and Ta exhibited segregationSegregation in the γ′ precipitatePrecipitates. In comparison with typical wroughtWrought superalloysSuperalloys U720Li and WaspaloyWaspaloy, the investigated alloy exhibits enhanced yield strength at 750–850 °C, which may be associated with the alloy’s larger grain sizeGrain size and the increase in the intermediate temperature flow stress anomaly inherent to Co-based alloys. Meanwhile, the creep propertyCreep properties of the investigated alloy surpasses those of alloys U720Li and WaspaloyWaspaloy, which is primarily attributed to its larger grain sizeGrain size and relatively higher γ′ volume fraction. Additionally, the lower stacking faultStacking faults energy of the alloy facilitates the formation of stacking faultsStacking faults and micro-twinsMicro-twins, further enhancing its creep propertyCreep properties. The findings of this study will hold importance for the further optimization of CoNi-based wroughtWrought superalloysSuperalloys.

In this study, intermittent creepCreep tests and Electron Backscatter Diffraction (EBSD) analyses were conducted on GH4169GH4169 (IN718) at 800 °C to elucidate the change mechanism of its micro texture and the degradation mechanism of the strength of grain boundariesGrain boundary under mechanical loading at elevated temperature. Under creepCreep loading, the growth of δ-phase (Ni3Nb) precipitatesPrecipitates was found to be accelerated around grain boundariesGrain boundary. This growth of δ-phase precipitatesΔ-phase Precipitate caused the disappearance of the solute strengthened phase in the surrounding grains and, thus, the effective strength of grains decreased. Even though the precipitationPrecipitation around grain boundariesGrain boundary increased their effective strength at first, however, it started to decrease with time due to the acceleration of the growth and accumulation of vacancies and dislocations around the interface between the precipitatesPrecipitates and the matrix. This acceleration was attributed to the large lattice mismatch between the precipitatePrecipitates and the matrix, and the structural singularity around the edges of the needle-shape precipitatesPrecipitates. Finally, intergranular crackingIntergranular cracking was accelerated. The degradation of the strength of the material was validated by using a micro tensile test using a scanning electron microscope. The acceleration process was successfully explained by the concept of the stress-induced acceleration of diffusionStress-induced Acceleration of Diffusion, by applying the modified Arrhenius equation. The growth time of δ-phase precipitatesΔ-phase Precipitate around grain boundariesGrain boundary was also predicted quantitatively.

This study discusses the effect of the direct current heatingDirect current heating (DCH) on the creepCreep behaviour of Ni-based superalloysNi-based superalloys, in an attempt to converge on best practice and also reliability. Particular features are the use of an electro-thermal mechanical testing (ETMT)Electro-Thermal Mechanical Testing (ETMT) system which has allowed for a direct comparison of the DCH heating method with the more conventional resistance furnace heating (FH) approach. In addition, particular attention is paid to a testing protocol to confirm the accuracy of temperature measurement during DCH. WaspaloyWaspaloy, Alloy713CAlloy713C, and Alloy246Alloy246 are used in different geometrical configurations, thus enabling us to establish the difference in creepCreep behaviours caused by altering the sample sizes and the heating methods. The creepCreep rupture lives of the materials tested by the DCH method were consistently shorter than that tested by the FH method. Microstructural investigation revealed that there is no influence of the heating method on the coarseningCoarsening rate of the fine γ′ precipitatesPrecipitates and on the thickness of the oxide layer in WaspaloyWaspaloy during creepCreep test at 732 °C. Moreover, the presence of an axial temperature gradient did not influence the creepCreep life unduly. An analytical model calculation indicated that the radial temperature gradient is not sufficient for the short creep life. It is hypothesised that the diffusivity is increased by the direct current, and the dislocation climb is facilitated in the DCH testing for WaspaloyWaspaloy.

The Portevin–Le ChatelierPortevin-Le-Chatelier (PLC) effect is widely attributed to the partitioning of interstitial C atoms around dislocation cores in mild steels, to form Cottrell atmospheresCottrell atmosphere. However, limited empirical evidence exists to clarify how similar mechanisms extend to complex multicomponent alloy systems such as Ni-based superalloysNi-based superalloys. The presence of the PLC effect was demonstrated in coarse-grained RR1000 ranging 200–550 °C through digital image correlationDigital image correlations (DIC). The effect was also demonstrated in simple binary Ni-(20–25)Cr (wt.%) alloys, suggesting that the microstructural complexity of superalloysSuperalloys is not key to elucidating the atomistic origins. Probe-corrected scanning transmission electron microscopyTransmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy and atom probe tomographyAtom probe tomography were performed on interrupted specimens displaying the PLC effect. No evidence of long-range solute enrichment was observed in the vicinity of dislocation cores. Hence, alternative origins for the effect were considered; evidence for short-range orderShort-range order was assessed in Ni–Cr powder through neutron diffraction. A preference was observed for particular local configurations of Ni and Cr atoms within a unit cell, resembling, but not identical to, those in long-range ordered Ni2Cr. High-resolution STEM imaging demonstrated the presence of nanodomains in Ni–Cr and RR1000 containing diffraction contrast effects resembling superlattice fringes. STEM image simulations based on refined unit cell structures demonstrated that such features may relate to the local lattice distortion in short-range ordered (SRO) domains. The results do not support the presence of long-range Cottrell atmospheresCottrell atmosphere within the temperature range of the PLC effect, but instead of SRO, an alternate hypothesised origin of the PLC effect.

The present study assesses the microstructureMicrostructure of a new Ni-based alloyNickel-based alloys filler metal, Inconel® 680, at high resolution to understand the underlying mechanisms behind its remarkable mechanical propertiesMechanical properties. Investigations using aberration-corrected (scanning) transmission electron microscopyTransmission electron microscopy (TEM/STEM) were performed to complement the previous microstructural analysis. The results from TEM analysis confirmed Nb-rich C14-type Laves phase precipitationPrecipitation, besides different types of carbides in the interdendritic spacings. However, the advanced microstructural characterisationCharacterisation has also shown a heterogeneous pattern in terms of microstructureMicrostructure between the cellular-dendrite core and interdendritic region. A higher dislocation density was noticed in the interdendritic spacings accompanied by nanometric precipitationPrecipitation. These nanoprecipitates were identified as being tined globular-shape γ″-Ni3Nb phase. The γ″-Ni3Nb phase precipitationPrecipitation resulted from an intense microsegregation of Nb during the solidificationSolidification. This precipitationPrecipitation of the γ″-Ni3Nb phase plays an essential role in the enhanced yield strength ofAlloy 680 alloy 680.

A new model for the temperature-dependent yield strength of polycrystalline Ni-based superalloysPolycrystalline Ni-based superalloys at high strain ratesStrain rate (10−3 to 10−4 s−1) featuring multimodal precipitatePrecipitates microstructuresMicrostructure is presented. It extends existing mean field models based on the summation of particle strengthening and other strengthening contributions, which were originally created to describe behavior at room temperature. Temperature dependenceTemperature dependence is introduced for each contribution using existing models or datasets from the literature while maintaining physical validity and avoiding unnecessary fitting parameters. Further, the expected change in the precipitatePrecipitates distributions at high temperaturesHigh temperature is modeled using CALPHADCALculation of PHAse Diagrams (CALPHAD). By considering such microstructural changes, the model can predict the yield strength from room temperature to beyond the solvus temperature of the precipitatesPrecipitates. Due to the strategy for combining different strengthening contributions, the model can also predict the active deformation modes at any temperature and can be extended easily to incorporate additional deformation mechanismsDeformation mechanisms. The model’s sensitivity to microstructural parameters and its strengths and weaknesses are also discussed. Application of the model to three commercial alloys shows good agreement over the whole temperature range.

The anisotropyAnisotropy of deformation of a single crystal superalloySingle crystal superalloy is studied using a newly designed furnace arrangement which allows for videography from multiple apertures and subsequent digital image correlationDigital image correlations (DIC). Thus, for initially cylindrical specimens, the full field of surface strain is measured at a 25 μm spatial resolution. Our approach is tested for STAL15 along each of the <001>, <011>, and <111> crystallographic orientations at 800 °C. The time-dependent anisotropic deformation along each of these crystallographic directions is quantified. Crystal plasticityCrystal plasticity finite element (CPFE) calculations are used to rationalise the observations; it is demonstrated that the predictions are particularly sensitive—on account of the plastic anisotropiesAnisotropy—to the boundary conditions used for the loading. Implications for the testing of single crystal superalloysSingle crystal superalloy are discussed in detail, with a particular view towards improving testing protocols.

Single crystalSingle crystal (SX) nickel-based superalloysNickel-based superalloys have been developed specifically to withstand the high temperaturesHigh temperature that would engender severe creepCreep degradation in a lesser alloy. Use in gas turbine bladesTurbine blades has long been a driver for the development of ever more capable alloys that can survive higher temperatures without significant material damage (Hashizume et al. Proc Int Symp Superalloys 53–62 (2004)). The drive for increased financial efficiency has included a desire for increased duration, and longer time between overhauls, as well as reduced specific fuel consumption. The long-term behaviour of SX alloys at lower temperatures, such as that experienced at the blade root fixings, has not historically influenced alloy designAlloy design, but has become increasingly important as the target time between overhauls increases. This study looks at the behaviour of an example of a second, third, and fourth generation SX alloy at a temperature commensurate with that outside the gas path, in keeping with that around the blade root. The influence of dwell on both the predicted life and consequent fractography is examined. In addition, the dependence of crack growth rate with secondary orientation is assessed. Finally, the crack trajectory close to the crack tip is considered and the possible influence of different regions of the loading cycle is discussed.

Ni-based single crystalSingle crystal (SX) superalloysSuperalloys have a well-established history of application in high pressure turbine (HPT) blades and vanes. Nowadays, these components are increasingly being employed in the first stage of the low pressure turbine (LPT). As a result, aircraft engine manufacturers are facing new challenges arising from the designDesign, manufacturing processes, evolving service conditions, and refurbishmentRefurbishment requirements. Conventionally, the capability of Ni-based SX superalloysNi-based SX superalloy to withstand the harshest environments is related to a homogeneous cuboidal γ/γ′ microstructureMicrostructure. However, future new applications have considered bimodal γ/γ′ microstructuresMicrostructure (presence of fine tertiary γ′ precipitatesPrecipitates 10–100 nm inside the γ-channels) resulting from either the manufacturing processes or from the routine use. This study investigates the resistance to the viscoplastic deformation of a 3rd generation Ni-based SX superalloyNi-based SX superalloy with a bimodal distribution of γ′ precipitatesPrecipitates. TEM observations showed presence of dislocations after heat treatmentHeat treatment to achieve the bimodal microstructureBimodal microstructure. Even without plastic strain, dislocations were identified surrounding the secondary γ′ precipitatesPrecipitates. Stress relaxationStress relaxation and creep propertiesCreep properties at 750 °C and 850 °C were very sensitive to such bimodal microstructureBimodal microstructure. Specimens with a bimodal γ′ precipitationPrecipitation showed a creepCreep life five to six times lower than the reference samples. According to Norton’s type diagram, rate controlling deformation mechanismsDeformation mechanisms of the reference and bimodal microstructuresBimodal microstructure appear to be same at both temperatures and under different initial conditions (with and without prior plastic strain), but with a higher strain rateStrain rate for the bimodal microstructureBimodal microstructure.

Effect of tensile dwell on crack propagationCrack propagation under subsequent fatigueFatigue loading during dwell-fatigueDwell-fatigue crack propagationCrack propagation in Ni-base superalloysSingle crystal Ni-base superalloy was investigated using a single crystal superalloySingle crystal superalloy, CMSX-4CMSX-4, and a wroughtWrought superalloySuperalloys, IN718. Crack propagationCrack propagation tests with single tensile dwell introduced during pure fatigueFatigue loading under various conditions of stress intensity and dwell time were conducted at 900 °C and 650 °C for CMSX-4 and IN718, respectively. In CMSX-4CMSX-4, fatigueFatigue crack retardation occurred after the tensile dwell, which was attributed to stress relaxationStress relaxation induced by creepCreep deformation during the dwell and the resultant residual compressive stress during the subsequent fatigueFatigue loading. FatigueFatigue crack propagationCrack propagation rate after the tensile dwell was quantified by evaluating effective stress intensity factors based on the residual compressive stress field obtained by finite element analysisFinite element analysis. In IN718, acceleration of the fatigueFatigue crack propagationCrack propagation occurred after the tensile dwell at high Kmax values whereas the retardation occurred following temporary acceleration after the dwell at low Kmax values. The acceleration was attributed to grain boundaryGrain boundary (GB) damage caused by oxygen diffusionDiffusion along the GBs induced by high stress near the crack tip during the tensile dwell. The transition from the crack retardation to the acceleration in IN718 was rationally explained based on a size relationship between the stress relaxationStress relaxation area and the GB damage zone around the crack tip caused by the tensile dwell.

Computational designComputational design is increasingly being used by industry and research institutes for the development of new compositions of Ni-based single crystalSingle crystal (SX) superalloysSuperalloys. Alloy selection is often based on the mechanical propertiesMechanical properties of the fully heat-treated Ni-based SX superalloysNi-based SX superalloy. However, a more complex chemical composition leads to a more complex solution heat treatmentSolution heat treatment (ST). The development of a ST to reach optimum homogenization, requires time and resources, overall penalizing the time/cost savings provided by computational designComputational design approaches. Thus, the present study investigates several superalloysSuperalloys “as cast” mechanical propertiesMechanical properties to predict the creepCreep life of the solution heat treated alloy. Four commercial superalloysSuperalloys with distinct chemical composition were chosen. Each alloy was investigated in (i) the as cast (AC) and (ii) solution treated and aged (ST) states. The superalloysSuperalloys were creepCreep tested at 950 °C/390 MPa and 1050 °C/190 MPa. A creep life improvementCreep life improvement by a factor of 1.5–3 times has been observed after the ST, irrespective of the alloy chemical composition or creepCreep test conditions. Down-selecting alloys’ composition from the as-cast creep propertiesCreep properties can be a viable approach to speed-up alloy designAlloy design.

TMS-238 is a promising 6th generation superalloySuperalloys with impressive high temperatureHigh temperature creepCreep performance but it exhibits a particular tensile behavior at 650650°C-Tensile behavior °C with low yield strength, limited ductility and a non-classical hardening behavior. To study this behavior, TMS-238 specimens with different misorientationsMisorientation away from the perfect [001] crystallographic orientation were tensile tested at 650 °C. Mechanical tests revealed that highly misoriented specimens do not exhibit this atypical hardening behavior but a classical “plateau” behavior and keep a similar low yield strength. TEM investigations were conducted with post mortem observations in order to interpret these macroscopic tensile characteristics. Similar TEM experiments were also performed using other more classical alloys for reference. TEM investigations in TMS-238Dislocations in TMS-238 revealed a high density of dislocations and stacking faultsStacking faults in the γ phase and a homogeneous deformation in the perfect [001] oriented samples, contrarily to the heterogeneous deformation observed in the misoriented specimen. TEM energy disperse X-ray spectroscopy analysis confirmed significant amounts of Re and Ru in γ, two elements known to increase the γ/γ’ misfitMisfit and decrease fault energy, resulting in a lower mobility of dislocations in the matrix, the presence of extended stacking faultsExtended stacking-fault in the vertical γ-channels and a strong hardening.

Recently, the method called high-resolution imaging of misorientationsMisorientation in single crystalsSingle crystal has demonstrated its usefulness in accurately determining the single crystalSingle crystal quality in superalloysSuperalloys. This was made possible by obtaining high-resolution angular measurements of misorientationMisorientation (arc-sec) and high-resolution imaging (µm) combined with a relatively large measurement area. The technique combines traditional X-ray diffractionX-ray diffraction topography with high-resolution diffraction and advanced data post-processingProcessing, including color coding and 3D projections of diffraction images. It was demonstrated that the mosaic structure of superalloysSuperalloys is highly complex and varies at both the micro and macro levels. In the present work, the advanced high-resolution X-ray diffraction method for imaging misorientationMisorientation and mosaicity in single-crystal superalloysSingle-crystal superalloys was used to observe structural changes during uniaxial tensile loading. This research utilized single-crystalSingle crystal flat tension samples from CMSX-4CMSX-4 superalloySuperalloys prepared from the casting produced at a 3 mm/min withdrawal rate. The results indicate that the crystallographic orientation changes are within a few degrees up to the rupture. The evolution of the plastic deformation region was directly observed on the samples by contrast blurring.

Elemental segregationSegregation at stacking faultsStacking faults in the γ′ phase is a crucial part of the stacking-fault-based deformation of superalloysSuperalloys, and the local composition of a stacking faultStacking faults critically affects the resistance of γ′ to further shearing. Here, the process of elemental segregationSegregation at extrinsic stacking faultsStacking faults in γ′ is examined in Ni-base superalloySuperalloys CMSX-4CMSX-4 and Co-base superalloySuperalloys ERBOCo-4. By measuring fault compositions not only after deformation, but also after additional load-free annealing steps of different durations and temperatures between 700 and 900 °C, time- and temperature-dependent aspects of the segregationSegregation process are revealed. It is shown that elemental segregationSegregation continues to evolve toward an equilibrium composition after a fault has formed, indicating that fully formed stacking faultsStacking faults still provide a driving force for segregationSegregation processes and that their equilibrium composition can differ from that which they obtain during their formation. As the elemental segregationSegregation process is diffusionDiffusion-based, it is observed to be considerably slower at a lower temperature. In both alloys, the segregationSegregation trends observed after annealing depend on the annealing temperature: A lower temperature promotes a more γ-like fault composition associated with γ′ softening; conversely, at higher temperatures, the fault composition tends toward that of the η phase, which is considered beneficial in the context of local phase transformationLocal phase transformation strengtheningPhase transformation strengthening. As η-like segregationSegregation also involves the enrichment of slowly diffusing tungsten, kinetics likely play a role in the observed temperature dependenceTemperature dependence.

Evaluating the service conditions and the corresponding strain at high-pressure turbine bladesTurbine blades is crucial for the safe service and maintenance of aircraft engines. However, due to the harsh environment in turbines and the complex geometry of blades, it is difficult to directly monitor the variation of their service temperatures, stresses, and strains. In this work, an approach to predicting the equivalent service conditions and the local strainLocal strain of directionally solidified superalloysDirectionally solidified superalloys and turbine bladesTurbine blades was developed by integrating high-throughput creepCreep tests and machine learning tools. A large amount of experimental data was obtained using the flat specimens with the continuously variable cross-section and digital image correlationDigital image correlations technique. Then, the quantitative relationship between temperature, stress, strain, time, and the essential microstructureMicrostructure parameters was established with the help of machine learningMachine learning models. The established machine learningMachine learning models were then employed to predict the service conditions and the corresponding strain of a directionally solidified superalloyDirectionally solidified superalloys and a turbine bladeTurbine blades. Finally, the applicability and limitations of this method were discussed. The development of this method provides guidance for the service evaluationService evaluation of turbine bladesTurbine blades.

Single-crystalSingle-crystal (SX) nickel-based superalloysNickel-based superalloy are used as blade materials for gas turbine aircraft engines due to their superior mechanical and environmental performance. These properties of SX superalloysSuperalloys depend highly upon their crystallographic orientations. The SX superalloySuperalloys components contain no large angle boundaries, which excludes intergranular oxidationOxidation and rupture. However, the industrial manufacturing of SX superalloySuperalloys blades can still result in the formation of recrystallized (RX) grains which can significantly limit the life of these components. The RX grains can form anywhere within a SX blade but are most frequently observed in areas of high geometrical complexity. These are often areas where high thermal stresses occur during solidificationSolidification and subsequent cooling processes due to significant mechanical constraint between shell/core materials and airfoil and can occur on the interior walls of hollow configurations. Focused research efforts have been conducted to develop and demonstrate a non-destructive method for volumetric analysis and detection of RX grains in SX components. This method is being further developed and deployed via an integrated computational materials engineeringIntegrated computational materials engineering approach to further identify and control critical quality material and processingProcessing parameters to mitigate such features in the most complex production castings. Implementation of this advanced non-destructive evaluation process will be reviewed in terms of targeted locations based on probabilistic material and process modeling.

The heat-transfer dynamics in the Liquid Metal CoolingLiquid metal cooling (LMC) process, aimed at the production of directionally solidified single crystalSingle crystal, is intricate due to the involvement of two primary heat exchange pairs: the interaction between the mold and the casting metal, and the interaction between the mold and its environment including liquid metal coolant and the furnace. Present numerical simulationsNumerical simulation face challenges especially in accurately capturing the temperature and flow field evolution within the coolant, resulting in potential inaccuracies in calculating the solidificationSolidification process. In response to this, the current study proposes a coupled model designed to concurrently calculate the solidificationSolidification process in a single-crystalSingle crystal casting and simulate the temperature and flow fields within the coolant. This coupling is achieved through a mutual exchange of results, where the outcomes of each process serve as boundary conditions for the other. The computational results obtained from the coupled model are compared with experimental measurements taken within the casting and coolant (Sn), revealing a commendable level of agreement. Subsequently, the coupled model is applied to explore the influence of casting sizes and arrangements on temperature and flow of coolant. The simulations yield preliminary yet insightful findings, offering valuable information that may contribute to the optimization of the LMC process.

The recrystallizationRecrystallization of nickel-based single-crystal superalloysNickel-based single-crystal superalloy during solution treatment may be promoted by local deformation induced during the cooling stage of directional solidificationDirectional solidification and is related to alloy chemistry. In this study, the effect of Re additionRe addition on the sensitivity to recrystallizationRecrystallization of nickel-based single-crystal superalloysNickel-based single-crystal superalloy and the ways in which Re affects nucleationNucleation and growth of recrystallized grainsRecrystallized grains are investigated using the as-cast alloy without Re additionRe addition and those with Re additionRe addition of 2 and 4 wt.%. It is suggested that 4 wt.% Re additionRe addition can significantly increase the propensity to nucleate recrystallized grainsRecrystallized grains but significantly inhibit grain growthGrain growth of the as-cast SX superalloySuperalloys. The former is attributed to the increase of Re segregationSegregation at the dendrite scale, which leads to the increase of strength difference in γ matrix of the dendrite core and interdendritic region, as well as the increase in eutectics and casting pores, thus providing higher stored energy and more nucleationNucleation sites for recrystallizationRecrystallization nucleationNucleation. Meanwhile, the severe Re segregationSegregation in γ matrix of high-Re alloys increases the γ′ solvus temperature and reduces the precipitatePrecipitates-free zone, thus inhibiting growth of recrystallized grainsRecrystallized grains. These insights will be beneficial to alloy designAlloy design and process optimization and allow further improvement of the casting costs.

In determining the specifications for the production or recycling process of Ni-base single crystal superalloysNi-based single crystal superalloy, it is important to clarify the allowable trace impurityImpurity concentration. In this study, the effects of Pb and Sb on high-temperatureHigh temperature oxidation strength and oxidationOxidation resistance of 6th generation superalloySuperalloys TMS-238 were summarized. It was found that both Pb and Sb do not have a large effect on creepCreep rupture life, but they degrade oxidationOxidation resistance. The removal of impuritiesImpurity by melting in a CaOCaO crucible was tested and its mechanism was examined. It was found that Ca forms a compound with impuritiesImpurity within the alloy and prevents the segregationSegregation of impuritiesImpurity at the oxide film/substrate interface. The effect of CaOCaO was also confirmed in an experiment in which impurityImpurity S was removed from 1st generation Ni-base superalloySuperalloys TMS-1700 by adding CaOCaO particles in a 3-ton commercial melting furnace. Similar results were obtained for this alloy, where it was also found that CaS had formed inside the alloy, thus improving the oxidationOxidation resistance of TMS-1700.

A Super-Solidus Hot Isostatic PressingHot isostatic pressing, SSHIPSuper-Solidus Hot Isostatic Pressing (SSHIP), heat treatmentHeat treatment has been developed and applied to the third-generation Ni-based single crystal superalloyNi-based single crystal superalloy CMSX-4 PlusCMSX-4 plus. The aim of this new type of heat treatmentHeat treatment is to significantly reduce the solution heat treatmentSolution heat treatment time and improve the mechanical propertiesMechanical properties compared to those resulting from subjecting the single crystalSingle crystal to conventional heat treatmentHeat treatment routes. The partial melting of the regions with the lowest solidus temperatures during SSHIPSuper-Solidus Hot Isostatic Pressing (SSHIP) results in the acceleration of the diffusional processes, while the control of the cooling rate after SSHIPSuper-Solidus Hot Isostatic Pressing (SSHIP) results in the optimum precipitationPrecipitation of the fine γ/γ′ microstructureMicrostructure. This innovative type of heat treatmentHeat treatment, which can significantly reduce the economic penalties associated with lengthy high-temperatureHigh-temperature solution treatments while having a positive effect on the final mechanical propertiesMechanical properties, could be applied to other complex single crystalSingle crystal Ni-based superalloysNi- based superalloy containing large amounts of refractory elements (like rhenium, tungsten and tantalumTantalum), showing strong dendritic segregationSegregation and/or a large volume fraction of eutectics in the as-cast state.

The microporesMicropore and residual eutecticResidual eutectic in nickel-based single crystalSingle crystal (SX) superalloysSuperalloys can promote crack initiationCrack initiation in the SX turbine bladesTurbine blades during service, threatening the components’ safety. Hot isostatic pressing (HIPHot Isostatic Pressing (HIP)) treatment is an effective technique to reduce microporesMicropore and enhance the mechanical propertiesMechanical properties of the alloys. This work investigated the effect of two typical HIP implementation schemes on a nickel-based SX superalloySuperalloys compared with a conventional heat treatmentHeat treatment scheme (without HIP). The results show that the HIP-treated samples had fewer microporesMicropore, residual eutecticResidual eutectic and better mechanical propertiesMechanical properties than that of the non-HIP sample. The HIP treatment can increase the yield strength, elongation and reduction in area of tensile propertiesTensile properties at 760 °C compared with the non-HIPHot Isostatic Pressing (HIP) sample, but has no noticeable effect on that at 980 °C. The HIP treatment on the solution-treated sample enhanced the creepCreep rupture life more than that of the as-cast sample at 980 °C/250 MPa, but had no influence on it at 1100 °C/130 MPa. The HIP effect on the fatigueFatigue rupture cycles was closely related to the type of crack initiationCrack initiation site. The HIP treatment significantly prolonged the rupture cycles of low-cycle fatigueFatigue (LCF) at 760 °C and high-cycle fatigueFatigue (HCF) at 850 °C of HIP-treated samples by changing the crack initiationCrack initiation sites, but had no impact on that at 980 °C. Comparing different HIP implementation schemes on microstructuresMicrostructure and comprehensive mechanical propertiesMechanical properties, the HIP treatment on the solution-treated sample was better than that on the as-cast sample. This work can improve the mechanical propertiesMechanical properties and provide guidance for HIPHot Isostatic Pressing (HIP) implementation scheme of nickel-based SX superalloysSuperalloys.

To continue improving alloy performance in harsh service environments, the development of alumina-forming nickel-based superalloysNickel-based superalloy is essential. Current generations of these alloys heavily rely on the addition of refractory elements to enhance their mechanical propertiesMechanical properties at high temperaturesHigh temperature oxidation; however, a systematic understanding of how such additions affect the overall oxidationOxidation behavior is still not well established, particularly from the standpoint of predicting the transition from internal to external alumina formation. The present work seeks to better understand the intrinsic effects that common minor additions of Ta and Nb have on the oxidationOxidation behavior of alumina-scale-forming γ-Ni model alloysModel alloy. By combining a novel simulation approach with high-temperatureHigh temperature oxidationOxidation experiments and advanced characterization techniques, the present study provides insightful details on the differing effects that 2 at. % addition of Ta and Nb have on the alumina scale formation of Ni-based alloysNi-based alloy during 1100 °C oxidationOxidation.

Ni-base superalloysNi-base superalloys have excellent oxidationOxidation resistance, but impurityImpurity S drastically decreases their properties. This is due to the segregationSegregation of S to the oxide/substrate interface, but direct and quantitative measurements of the interfacial strengthInterfacial strength in relation to the S segregationSegregation level have not been widely conducted. The objective of this research is to quantitatively analyze the interfacial strengthInterfacial strength between the Al2O3 layer and Ni-base substrate, depending on the S segregationSegregation level, using nanoindentationNanoindentation. Ni-9.8 wt.% Al alloys were prepared by melting the material using either an Al2O3 crucible (high Sinterface alloy) or a CaOCaO crucible (low Sinterface alloy). NanoindentationNanoindentation tests using a 60 degree pyramidal diamond indenter were conducted, and the cross-sections of both specimens exposed the (100) plane. Indentation near the interface formed cracks at the boundary between the two layers, which can be observed as pop-in events in the load-depth curves. The amount of load at the initial pop-in most likely represents the interfacial strengthInterfacial strength between the Al2O3 layer and Ni-base substrate. A Weibull analysis of results showed that suppression of the S segregationSegregation level increased the critical β scale parameter for crack formation by 650 μN. This suggests that we were able to successfully compare the effect of S segregationSegregation on the interfacial strengthInterfacial strength between the Al2O3 layer and the Ni-base substrate quantitatively.

The impact of hot corrosionHot corrosion on the low-cycle fatigueFatigue (LCF) life of the AM1 Ni-based single-crystal (SX) superalloySuperalloys in as-cast (AC) and fully heat-treated (FHT) states has been studied. Microstructural features such as pores and chemical segregationSegregation have a significant impact on LCF fatigueFatigue strength at 950 °C. Surface pores act as stress concentration zones, influencing crack initiationCrack initiation and propagation. The reduction in the chemical dendritic segregationSegregation through solution heat treatmentSolution heat treatment improves the LCF resistance. Pre-corrosion lowers the LCF lives by increasing stress concentration and facilitating multiple crack initiationsCrack initiation with a brittle, spalled corrosion layer on the surface. The mechanical degradation occurs simultaneously to hot corrosionHot corrosion and oxidationOxidation, the latter being influenced by the distance from the layer undergoing hot corrosionHot corrosion. Heat treatmentHeat treatment still appears to be a positive factor in improving the service life of AM1.

Very High Cycle FatigueVery high cycle fatigue (VHCF) properties of thermally sprayed MCrAlY coated samples of CMSX-4 at high temperatureHigh temperature were studied. Nine fatigueFatigue tests up to failure were performed at 1000 °C, 20 kHz, and R = –1 on coated samples. These samples showed lower fatigueFatigue life when compared with the bare reference. Fatal crack initiationCrack initiation systematically occurred at the interdiffusion zone (IDZ) between the coatingCoating and the CMSX-4 substrate or at a casting pore in the vicinity of the IDZ. Failed samples showed a high density of secondary surface cracks. In order to better understand the mechanism leading to surface crackingCracking, one interrupted test was performed. It is shown that cracks nucleated from the very first fatigueFatigue cycles and propagated through the coatingCoating almost instantly. Surface cracks then spent most of the fatigueFatigue lifetime sitting at the IDZ/substrate interface. The fatal as well as the surface crack initiationCrack initiation mechanisms are discussed to better understand the durability in these VHCF conditions.

The brittleness of an aluminide diffusionDiffusion coating protecting a René 125 Ni-based polycrystalline superalloyPolycrystalline superalloy was investigated over a wide range of temperatures in its as-received and thermally aged form. Isothermal and thermal cycled aging were performed on the coated system at a maximum temperature of 1100 $$ ^{\circ }\text {C}$$ ∘ C . MicrostructureMicrostructure evolutions and damage initiation within the coatingCoating were characterized. Interrupted tensile tests and thermomechanical fatigueFatigue tests were conducted to document critical stress-strain conditions leading to the coatingCoating crackingCoating cracking and lifetime for the case of thermomechanical fatigue loading. Advanced digital image correlationDigital image correlations and acoustic emission techniques were used to detect coating crackingCoating cracking. Isothermal oxidationOxidation orCyclic oxidation cyclic oxidation led to improved strain-to-failure due to metallurgical evolutions and also longer fatigue life under thermomechanical fatigueFatigue conditions.

Thermal gradient mechanical fatigueThermal gradient mechanical fatigue (TGMF) tests were conducted on thermal barrier-coated (TBC) tubular specimens to investigate the damage behavior of the TBC under close-to-service conditions. Special attention was paid to the cracking behaviorCracking behavior of TBC under TGMF tests with different temperature ranges, mechanical strain ranges, and phase angles including in-phase (IP) and out-of-phase (OP) loading. Crack initiationCrack initiation, propagation, and coalescenceCoalescence within the ceramic top coat (TC) caused TBC failure during IP-TGMF tests with a 300–1000 ℃ temperature range and a mechanical strain range of 0.45–0.65%. Few cracks extend to the bond coat (BC) but did not further propagate into the substrate. Increasing the maximum temperature and mechanical strain range significantly changed the cracking behaviorCracking behavior of these TBCs. Cracks run straight through the TC and BC followed by partial penetration into the substrate. The OP-TGMF loading shortened the TBC cycles to failure and accentuated the TBC damage, with separation occurring between TC and BC as well as between BC and substrate. Severe delamination at the thermally grown oxide (TGO)/BC interface results in premature TBC failure when the TBC system reaches a certain critical thickness of TGO.

Due to the application of thermal barrier coatingsThermal barrier coating, the concentrations of Cr in turbine bladeTurbine blades alloys have been limited to low values (approximately 5 at.% or less) since the introduction of second-generation single-crystal superalloysSingle-crystal superalloys. Thermal oxidationOxidation-induced rumpling and swelling of coatingCoating could lead to coatingCoating spallation and inner alloy failure, especially in advanced thin-wall turbine bladesTurbine blades. The initial oxidized surface morphologies and elemental distributions were considered crucial to understanding the failure of superalloysSuperalloys. In this work, initial oxidationOxidation behavior in typical 1st- to 3rd-generation single-crystalSingle-crystal superalloysSuperalloys was systematically studied in situIn situ at nanoscale using an environmental transmission electron microscope from 20 to 800 ℃. With increasing oxygen pressure, the oxide nucleated at the γ/γ’ interface, expanded along the γ channel, and grew into the γ’ phase. In thin foil samples, oxidationOxidation prompted the diffusionDiffusion of base elements from the inner γ and γʹ phases to the γ/γʹ interfacesΓ/γʹ interface in all alloys. With increasing Re content, the oxidationOxidation resistance decreased due to the evaporation of Re2O7 at the γ/γʹ interfaceΓ/γʹ interface in the 3rd-generation superalloySuperalloys. This study provided technical guidance for optimizing the compositions of advanced single-crystalSingle-crystal superalloysSuperalloys to enhance their oxidationOxidation resistance.

PlatinumPlatinum is used to form NiPtAl coatingsCoating and improve the oxidationOxidation/corrosion resistance of nickel-based superalloysNickel-based superalloy operating under harsh conditions. The addition of 2 wt% (0.6 at%) Pt to the bulk composition of a single crystalSingle crystal nickel-based superalloyNickel-based superalloy (TROPEA) markedly increased the mechanical propertiesMechanical properties at high temperatureHigh temperature oxidation. However, the effects of Pt on the oxidationOxidation behavior of this new superalloySuperalloys have never been studied, which is the main goal of this paper. TROPEA and model γ and γ′ single-phase alloys were thus oxidized at 950 ℃ in synthetic air. TROPEA reaches a parabolic regime only after 70 h of exposure. The establishment of an α-Al2O3 protective layer is peculiarly delayed in comparison with the parent CMSX-4CMSX-4 superalloySuperalloys according to literature. TROPEA forms a multilayer of oxides, including a continuous layer of (TixTa1-x)O2 during the transient period. It appears that Pt promoted the formation of this Ta-rich oxide. The Ta-rich oxide layer may play a role hindering the lowering of the PO2 needed to selectively develop Al2O3 scale or its presence just outlines the difficulty of TROPEA to develop the protective Al2O3 scale in presence of platinumPlatinum in the substrate. The 0.6 at% of Pt addition thus hampers the formation of a continuous and protective layer of α-Al2O3 scale. In contrast, the model γ and γ′ phases are prompt to develop an adherent and even scale of duplex Cr2O3 + NiCr2O4 and of α-Al2O3 + NiO, respectively, when oxidised at 950 ℃ in air for 100 h.

Ni-based single-crystalSingle-crystal superalloysSuperalloys must endure high-temperatureHigh temperature oxidation applications where creepCreep is the main cause of failure. CoatingsCoating to protect against oxidationOxidation are not always applicable and oxidationOxidation should, therefore, be factored in. Thermogravimetric analysesThermogravimetric analysis were performed at 1000 °C on René N4 to obtain the oxidationOxidation rate $${K}_{p}$$ K p depending on the surface roughness. It was observed a strong nonlinearity attributed to a transition between chemisorption and physisorption. The evolution of the oxidationOxidation rate was then phenomenologically modelled combining Lennard–Jones-type and Gaussian-type equations. The obtained expression was then implemented in a crystal-plasticity model used to predict the lifetime variation depending on the initial value of the surface roughness. The predicted lifetimes were then compared to the creepCreep experiments at 1000 °C/200 MPa. The evolution of the lifetime depending on the surface roughness matches the evolution of the oxidationOxidation rate depending on the roughness and, therefore, the model predictions. Clearly, the results point to the importance of considering the contribution of surface roughness not only for fatigueFatigue loading but also to creepCreep, which will allow to better understand creepCreep-fatigueFatigue interactions.

This work focuses on the effect of partial and total replacement of Ti with Nb on the long-term oxidationOxidation at 800 °C up to ~ 2000 h of Ni-based superalloysNi- based superalloys for turbine discTurbine disc applications. The replacement of Ti with Nb leads to a lower mass gain of the specimens, in both pseudo and isothermal oxidationOxidation conditions. The alloys presented a relatively smooth oxide scale surface; however, the roughness of the Nb-free/high-Ti alloy is twice of that of the Ti-free/high-Nb alloy. From XRD data, the Nb-free/high-Ti alloy presented Cr2O3, TiO2, Al2O3, (Ni,Co)Cr2O4, and γ + γ′ (matrix) phases, while the Ti-free/high-Nb alloy presented Cr2O3, Al2O3, (Ni,Co)Cr2O4, γ + γ′ (matrix), and δ-Ni3Nb phases. The chromia layer thickness of the Ti-free/high-Nb alloy ranged from 1.0 to 1.5 µm, the thinnest among all the alloys for any given oxidationOxidation time and condition, with no mass change after approximately 280 h exposure. The exceptional oxidationOxidation behavior is attributed to the absence of Ti in this specific alloy composition. The semi-continuous alumina formation underneath the chromia scale as well as the formation of δ-Ni3Nb phase filling up the spaces between the alumina intrusions is probably the reason of delayed outward Cr diffusionDiffusion.

Hot corrosionHot corrosion dramatically lowers the life of superalloySuperalloys components by inducing pitting (Type II) or an extensive homogeneous attack (Type I), hence initiating cracks and decreasing the load bearing section. This degradation phenomena may occur in the coldest areas of high pressure turbine components, but they are commonly found in the low pressure turbine parts like in theDS200+Hf DS200 + Hf nickel-based superalloyNickel-based superalloy investigated in this work. Therefore, a Si-modified aluminide slurry coatingCoating was applied to the alloy, and its hot corrosionHot corrosion resistance was assessed under type I conditions with a Na2SO4 deposit at various temperatures and times. The comparison against simple slurry aluminide with and without Si and out-of-pack aluminide coatingsAluminide coating revealed for the first time in the literature that Si ties up W in the coatingsCoating thereby impeding acidic dissolution, which greatly improves the corrosion resistance of DS200 + Hf superalloySuperalloys. The W-free out-of-pack aluminide coatingAluminide coating forms a protective alumina scale that delays the initiation of hot corrosionHot corrosion unless the coatingCoating is damaged. In contrast, the simple slurry aluminide coatingAluminide coating without Si does not offer any protection in spite of the Cr segregationSegregation close to the surface.

Complex microstructureMicrostructure evolution has been observed in both bare and coated Ni-based single crystalSingle crystal superalloysSingle crystal superalloy. RaftingRafting and $$\upgamma '$$ γ ′ depletionDepletion are investigated in this study through a brief experimental analysis and a detailed phase fieldPhase field model to account for mechanical-diffusionDiffusion coupling. The proposed model has been implemented in a finite element code. As the main result, it is shown that raftingRafting, $$\upgamma '$$ γ ′ depletionDepletion close to free surface/oxide layer or $$\upgamma '$$ γ ′ coalescenceCoalescence close to coatingCoating layer, and mechanical behavior are strongly coupled. The local additional flux of Al explains this coupling to a large extent. Finally, a discussion of strain localization and local flux of Al paves the way for clarification of these cases that degrade the performance of superalloys.

Oxide inclusions and porosityPorosity are present in nickel alloysNickel alloys that are made using powder metallurgyPowder metallurgy. Both features can nucleate fatigueFatigue cracks. Surface oxide inclusions can significantly reduce fatigueFatigue lives. This is evident if lives are compared to those for crystallographic cracks that develop from slip bands, which are characterized by facets on fracture surfaces. Pores mainly occur from trapped gas in powder particles, whereas inclusions typically arise from melting the alloy and from powder manufacture. Since most inclusions are too small to be detected from ultrasonic inspection, probabilistic lifing assessment is required to ensure the risk of failure from “melt anomalies” in disk rotorsDisk rotors is acceptably low. Such calculations apply a mathematical description of inclusion size and frequency. This paper examines methods for characterizing size and frequency of oxide inclusions and porosityPorosity from powder and billet material, which includes material that contains oxide inclusions or “seeds” that were added intentionally to understand fatigueFatigue behavior. Large bar tensile (LBT) testing of billet material was found to be the most capable method for characterizing inclusion size. However, rate of occurrence information cannot be measured directly; it must be implied by fitting inclusion size data to probability functions. Given uncertainties with this approach, a method has been devised to measure rate of occurrence directly. Inspections of polished surfaces of billet material have been shown to be viable for production use. PorosityPorosity of a coarse powder fraction has been characterized by mounting powder in resin so that automated image capture and analyzes could be undertaken on polished surfaces. Finally, the sizes of the very low frequency of larger inclusions, which are detected by ultrasonic inspection of billet, have been characterized from electron microscopyElectron microscopy on polished surfaces.

The effect of strain rateStrain rate on microstructureMicrostructure evolution of a γ-γ′ nickel-based superalloyNickel-based superalloy during hot deformation has been examined in this study. The experiments were conducted below the γ′-phase solvus temperature, at 1050 °C with various strain ratesStrain rate ( $${10}^{-1}$$ 10 - 1 , $${10}^{-2}$$ 10 - 2 , and $${10}^{-3}$$ 10 - 3 $$\text{ s}^{-1}$$ s - 1 ) up to different macroscopic strain level (0.2, 0.8, and 1.3). An inverse effect of strain rateStrain rate on dynamic recrystallizationDynamic recrystallization has been observed with an increase in the recrystallized fraction as strain rateStrain rate increases, for a fixed macroscopic strain level. Scanning electron microscope and electron backscattered diffraction analysis were employed to investigate the deformed microstructuresMicrostructure in terms of γ phase evolution and γ′ precipitationΓ′ precipitation state. A constant average size value of recrystallized grainsRecrystallized grains at 1 µm is obtained for all tested conditions, suggesting that nucleationNucleation of new dynamically recrystallized grainsRecrystallized grains is favored at the expense of the growth of recrystallized grainsRecrystallized grains during hot deformation, which is inhibited. Furthermore, the complete absence of fine γ′ precipitationΓ′ precipitation within dynamically recrystallized grainsRecrystallized grains, reveals a strong interaction between the progress of recrystallizationRecrystallization front and the presence of fine γ′ precipitatesPrecipitates.

Udimet®720Li andAD730TM superalloy AD730®AD730® are γ-γ′ Ni-based superalloysNi- based superalloys manufactured through casting and wroughtWrought processingProcessing, i.e., using ingot-to-billet conversionBillet conversion (UDIMET is a registered trademark of Special Metals Corporation. AD730 is a registered trademark of Aubert & Duval). These alloys are intended for use in safety critical aeroengine components, and there are strict requirements on the microstructural characteristics they must achieve at the end of the conversion process. Conventional ingot-to-billet conversionBillet conversion is an expensive and complex process, requiring multiple open-die forgingForging operations and reheating steps to achieve a homogeneous microstructureMicrostructure. A main goal of this conversion process is to refine the as-cast grain γ structure, which comprises grains centimetres in size, down to a grain sizeGrain size of approximately 20 μm. The present work studies the microstructural evolutionMicrostructural evolution of Udimet 720Li and AD730 billet material at the “solvus transition” stage of the conversion process, i.e., the controlled cooling through the solvus temperature. The formation of γ′ prime precipitatesPrecipitates during controlled cooling from supersolvus temperatures, and their interaction with the γ grain structure is the focus of this work. It is shown that the size and morphology of the γ grains in both alloys are significantly affected by discontinuous precipitationDiscontinuous precipitation during cooling through the solvus temperature, and a method to exploit this during industrial ingot-to-billet conversionBillet conversion is suggested.

The presentFull-field modeling paper demonstrates great capability of the full-field finite element DIGIMUDIGIMU®® software included recrystallizationRecrystallization models to simulate the microstructureMicrostructure evolution during forgingForging an industrial part in René 65René 65, a γ/γ′ nickel-based superalloyNickel-based superalloy. The macroscopic forgingForging conditions simulated by the finite element FORGE® software were used as the thermo-mechanical inputs of the microstructureMicrostructure simulations in the DIGIMUDIGIMU®® software. The recrystallizationRecrystallization models in the DIGIMUDIGIMU®® software were calibrated from torsion tests on laboratory-scale samples at sub-solvus and super-solvus temperatures from 1000 to 1150 °C and at strain ratesStrain rate ranging from 10–2 to 0.75 s−1. The calibrated model is able to predict, correctly, the mean and distribution of grain sizesGrain size in different deformation conditions of laboratory-scale samples and, more importantly, for eight different positions of interest in an industrial part forged by multiple operations at sub-solvus temperatures and followed by a solution heat treatmentSolution heat treatment. The differences between the predicted and experimental average grain sizesGrain size are in the range of 0.5–1.0 ASTM (which is around 1.5–3.0 µm difference when comparing to the experimental grain sizeGrain size of 10 ASTM ~11 µm). The model will help with understanding and optimizing of forgingForging processes to achieve desirable microstructuresMicrostructure and, in turn, the mechanical propertiesMechanical properties of aircraft engine forged components.

The focus of this study is on the microstructural evolutionMicrostructural evolution of the γ-γ′ nickel-based superalloyNickel-based superalloy René 65René 65 under monotonic and incremental deformationIncremental deformation paths. Here, incremental deformationIncremental deformation is defined as the alternation of small deformations and short dwell times, representing the deformation path processed during a ring rollingRing rolling. Incremental and monotonic deformation paths are performed using hot torsion and hot compression experiments, and their resulting microstructuresMicrostructure are compared. For a given final strain, strain rateStrain rate and temperature, a monotonic deformation generates a higher fraction of recrystallizationRecrystallization than an incremental deformationIncremental deformation path does, for both torsion and compression experiments. Further experiments on interrupted incremental and monotonic compression deformation conditions are conducted to better understand the impact of dwell time on recrystallizationRecrystallization. Static recovery and γ′ precipitationPrecipitation evolution are two potential microstructural mechanisms discussed in this paper that could occur during dwell time and slow down the recrystallizationRecrystallization evolution for incremental deformationIncremental deformation conditions.

AD730®AD730® is a γ − γ′ polycrystalline superalloy which has been recently developed for future engine turbine discs. The γ′ precipitationPrecipitation influences mechanical propertiesMechanical properties at the service temperature but also the kinetics of recrystallizationRecrystallization during forgingForging process. Consequently, a deep understanding and monitoring of γ′ precipitationPrecipitation is required at each stage of the industrial process. The objective of this work is to understand the precipitationPrecipitation mechanism, determine the evolution of γ′ size during cooling and isothermal heat treatmentsHeat treatment, and calibrate a model for the nucleationNucleation and growth of γ′ during the ingot-to-billet conversionBillet conversion. The experimental analysis showed that water quenching at a cooling rate of 100 °C/s after a solution treatment at 1160 °C led to a fine and homogeneous γ′ precipitationPrecipitation for which the average circle radius ranges from 10 to 20 nm. Then, samples were heat treated between 1000 and 1080 °C with various holding times to characterize and quantify the size of precipitatesPrecipitates. The results were compared to the precipitationPrecipitation modeling using the in-house software PreciSo, a multi-class, Kampmann-Wagner Numerical precipitationPrecipitation model based on classical nucleationNucleation and growth theories. Even if the simulations of precipitationPrecipitation during isothermal heat treatmentsHeat treatment start with the correct initial radius, coarseningCoarsening appears to occur slower than predicted by the simulations. This discrepancy is attributed to one simulation parameter, the diffusivity of solute elements, which needs to be improved to obtain a better fit with experimentations.

In this study, a unique microstructureMicrostructure where $$\upgamma $$ γ grains contain multiple coherent primary $$\upgamma '$$ γ ′ particles is reported. We term these unique features “multiply coherent grainsMultiply coherent grains” (MCGs). While the $$\upgamma $$ γ grains containing coherent primary $$\upgamma '$$ γ ′ particles suggest theHeteroepitaxial recrystallization heteroepitaxial recrystallization (HeRX) mechanism, HeRX can only explain the presence of a single coherent primary $$\upgamma '$$ γ ′ within a given $$\upgamma $$ γ grain, not multiple. Other potential formation mechanisms of MCGs are assessed using combined energy dispersive spectroscopy and electron backscatter diffraction, then a likely formation mechanism is posited. The median grain sizeGrain size of MCGs surpasses other $$\upgamma $$ γ grains, which shows the potential impact of MCG formation on the grain size distribution.

Advanced electron backscatter diffraction (EBSD) and electron dispersive spectroscopy (EDS) techniques were used to systematically quantify meso-scale microstructural descriptors in an advanced powder processed polycrystalline Ni-base superalloyNi-base superalloys containing elevated levels of refractory alloying additions. The microstructural changes of the alloy as a function of effective strain were tracked and related to the subsequent heat-treated microstructuresMicrostructure. This emerging field of “microstructure informaticsMicrostructure informatics” extends beyond the conventionally used metrics of grain and precipitatePrecipitates sizes and distributions. Due to the multidimensional nature of the data, manual microstructure characterizationMicrostructure characterization becomes virtually impossible, especially when a multitude of different material states must be considered. This motivated the development of an automated microstructure characterizationMicrostructure characterization procedure, which extracts useful geometric, crystallographic, and chemical microstructureMicrostructure features through a batch process. These features provide a level of microstructureMicrostructure detail that has not traditionally been demonstrated at a statistically significant scale capable of effectively capturing the level of intrinsic heterogeneity that is present in polycrystalline Ni-base superalloysNi-base superalloys. In this study, microstructural descriptors from the deformed material were evaluated and used to understand the grain growthGrain growth response during super-solvus heat treatmentHeat treatment. Compared to traditional qualitative and semi-quantitative approaches for characterizing microstructuresMicrostructure, the innovative methodology used in this investigation provide insightful, quantitative microstructureMicrostructure metrics that lead to the generation of new knowledge and scientific understanding.

Additive manufacturingAdditive manufacturing (AM) has captured the imagination of many in the materials, manufacturing, and designDesign communities. While AM has shown significant potential to produce complex geometries from a wide range of materials, the stringent requirements of production components necessitate advances in the way components are designed, manufactured, and certified. Dynamic properties of AM components are known to be significantly hindered by the presence of build defects. An integrated computational materials engineering (ICMEIntegrated Computational Materials Engineering (ICME)) approach to AM has been pursued through which build defects can be accurately predicted on a component location-specific basis. This capability is leading to a model-based material definitionModel-based material definition (MBMD) approach to process designDesign and control, and subsequent component qualification and certificationCertification. The development and demonstration of component and process manufacturing designDesign examples of this approach for a nickel-base superalloyNickel-base superalloy powder case application will be provided.

Laser powder bed fusion (LPBF)Laser Powder Bed Fusion (LPBF) printing defects are investigated through multimodal 3D serial sectioningSerial sectioning data on a model CoNi alloy. Defect segmentation across three different LPBF prints with different scan strategies, in which interlayer rotation and the presence of a contour scan is varied, reveal fully dense microstructuresMicrostructure (>99.8% dense). Despite being in a density range commonly considered as fully dense material, these prints contain an array of small pores, lack-of-fusion defects, and cracks that can be highly anisotropic. Their size and number are compared to those found in conventional superalloy casting techniques (investment casting, single crystalSingle crystal Bridgeman casting). In the AM samples, most pores and cracks have a thickness on the order of 3–6 $$\upmu $$ μ m, beyond the resolution capabilities of most industrial non-destructive evaluation techniques. A comparison between 3D and 2D defects measurements is included, revealing significant variability between 2D measurements and the ground truth 3D data. A state-of-the-art machine learningMachine learning framework, U-Net, is implemented for defect segmentation within three TriBeam tomography datasets containing backscattered electron images with variable contrast conditions. U-Net results indicate high-fidelity defect segmentation within all three datasets where recall and precision are >85%. The 3D reconstructions of the CoNi alloy samples provide insight into the defect content that can be expected from high-quality fully dense LPBFLaser Powder Bed Fusion (LPBF) printed superalloy material.

Factors affecting strain-age crackingStrain-age cracking (SAC) have been quantitatively assessed in a range of Ni-base superalloysNi-base superalloys with differing γ′ contents. Differences in the amount of γ′ present in the as-built condition of HA282, STAL 15DE, CM247LC, and IN713LC are highlighted. In the as-built condition, γ′ are absent in HA282, but appear as nano-clusters in IN713LC. On heating, γ′ precipitatesPrecipitates coherently in the γ phase, increasing the yield strength. The kinetics of precipitationPrecipitation are dependent on the heating rate and precipitationPrecipitation terminates at different temperatures in different alloys. The propensity to SAC is assessed via volume changes accompanying precipitationPrecipitation, increase in elastic modulusElastic modulus accompanying γ′ precipitationPrecipitation, and a loss in ductility/grain boundaryGrain boundary cohesive strength with increasing temperature. A marked feature of additively built microstructuresMicrostructure is the dramatically low grain boundaryGrain boundary cohesive strength at ~800 °C, which is related to the segregationSegregation within the terminal liquid film at the grain boundaryGrain boundary. The most important factor contributing to SAC is the lack of ductility and reduced grain boundaryGrain boundary cohesive strength.

By making particular use of high-temperatureHigh temperature confocal laser scanning microscopy, static recrystallisationRecrystallisation is studied correlatively both in situ and ex situ in an as-fabricated superalloy made by laser-powder bed fusion (L-PBF). In this way, insights are gained into important recrystallisationRecrystallisation phenomena with direct observations made—for the first time in this class of material—of phenomena such as nucleationNucleation of recrystallisationRecrystallisation, subsequent grain growthGrain growth, jerky flow of boundaries due to pinning, and twin formation. The nucleation process—requiring strain-free lattice to be created by grain boundaryGrain boundary migration—is visualised, and its role in limiting the kinetics of recrystallisationRecrystallisation is elucidated. Moreover, it is demonstrated that boundary mobility is initially prevented by Smith-Zener pinning due to a fine dispersion of secondary phases but also with a role played by solute drag caused by cellular micro-segregationSegregation. With increasing annealing time, the retarding pressure reduces due to carbide coarseningCoarsening and/or dissolution as well as matrix compositional homogenisation, eventually allowing recrystallisationRecrystallisation to take place. Further work will allow rich quantitative datasets to be gained which will allow for the testing of recrystallisationRecrystallisation models.

Manufacturing of single-crystalline Ni-base superalloyNi-base superalloys blades is extremely complex due to the intricate geometry of the blade profile and cooling channels. Additive manufacturingAdditive manufacturing provides an excellent alternative to the classical Bridgman solidificationSolidification, as it significantly reduces the number of steps involved. However, since the solidificationSolidification conditions differ greatly, it is crucial to identify appropriate process parameters to ensure defect-free single-crystalline builds. In this paper, we present a complementary approach between experiments and numerical simulationsNumerical simulation of the melt pool shapes that enables the identification of the process parameter window for directional growth. Further, an incremental process optimization approach is developed that allows the gradual reduction of grains, creating conditions for epitaxialEpitaxial growth. Finally, it is revealed that the proposed strategy for achieving single-crystalSingle-crystal growth conditions leads naturally towards crack-free builds of rods with the CMSX-4CMSX-4 alloy.

Due to the small freezing range of eutectic alloys, the Cotac-type alloys might be viable alternatives to conventional Ni-based superalloysNi- based superalloy when processed through additive manufacturingAdditive manufacturing. Laser pass assessment reveals that both Cotac-74 and Cotac-744 display improved crackingCracking resistance when compared to the conventional Ni-based superalloyNi- based superalloy CM247LC. During laser powder bed fusionLaser Powder Bed Fusion (LPBF) Cotac-74 displayed the highest crackingCracking resistance, with no microcracking detected in the as-built or heat-treated microstructureMicrostructure. The promising results presented for Cotac-74 highlight the possible use of this alloy for the additive manufacturingAdditive manufacturing of high-temperatureHigh-temperature aerospace components.

Minor elementsMinor elements such as boronBoron, carbon, and zirconium have been used for many decades to improve the high-temperature properties of Ni-based superalloysNi- based superalloy. However, the advances in additive manufacturingAdditive manufacturing technologies and the resulting popularity have put these elements in a bad light since they have been identified to be the major cause of hot crackingCracking problems. This study covers the influence of these elements on hot crackingCracking, but its focus lays on their impact on strain-age crackingStrain-age cracking (SAC) and mechanical propertiesMechanical properties. The impact of these elements has been studied in four versions of a high tantalumTantalum-containing novel Ni-based superalloyNi- based superalloy that is being developed for Powder Bed Fusion-Laser Beam/Metals (PBF-LB/M). Increasing the boronBoron content from 0.007 to 0.019 wt.% leads to severe hot crackingCracking, but reduces SAC during the heat treatmentHeat treatment. The addition of 0.022 wt.% zirconium does not increase the hot crackingCracking susceptibility but increases the SAC susceptibility. The variation of minor elementsMinor elements does not affect room temperature tensile propertiesTensile properties, but an increased zirconium and boronBoron content increases the elongation at fracture at 850 °C. The alloys with a low boronBoron and medium boronBoron content show a high notch-sensitivity during stress-rupture tests, which leads to failure in the fillet of the sample. Only the boronBoron and zirconium alloys were able to achieve valid stress-rupture results.

Microcracking during processingProcessing and underperformance in creepCreep have limited wider adoption of high γ′-fraction superalloysSuperalloys in Additive ManufacturingAdditive manufacturing (AM). Certain processingProcessing issues are now understood to be related to solidification crackingSolidification cracking caused by elements such as B and Zr that are also essential for creepCreep performance, particularly with fine-grained AM microstructuresMicrostructure. A legacy γ′-strengthened polycrystalline superalloyPolycrystalline superalloy 738LC738LC was the subject of the current investigation. Printing trials conducted with the legacy composition and a modified version with 10 times the initial B content revealed extensive microcrackingCracking in the legacy composition, whereas the modified alloy produced a dense crack-free microstructureMicrostructure. CALPHADCALculation of PHAse Diagrams (CALPHAD)-based solidificationSolidification simulations and crackingCracking susceptibility index calculations were performed to attempt to rationalise these findings. After hot isostatic pressingHot Isostatic Pressing (HIP) (1120 °C, 200 MPa, 4 h) and ageing heat treatmentHeat treatment (850 °C, 24 h), stress ruptureStress rupture tests showed an improvement in the rupture life of the modified alloy. Samples perpendicular to the AM building direction (typically the weaker orientation) showed a 50% increase in rupture life compared to the conventional composition, and ruptureElevated-temperature ductility ductility was also enhanced. Elevated temperature tensile ductility in the perpendicular direction increased to ≈ 11 El% for the modified alloy versus ≈ 6 El% for the conventional composition. These improvements are attributed to the presence of fine boride precipitatesPrecipitates at grain boundariesGrain boundary of the modified alloy. The findings indicate that increasing the grain boundaryGrain boundary strengthening element content may be a potential solution for both processingProcessing and mechanical performance issues in this superalloySuperalloys.

This study aims to optimize the content of minor elementsMinor elements (C, B, Zr, Si, Hf) to enhance the manufacturability of γ/γ′ cobalt-based superalloysSuperalloys by additive manufacturingAdditive manufacturing without degrading hot mechanical propertiesMechanical properties or oxidationOxidation resistance. Results show that reducing the amount of minor elementsMinor elements improves resistance to liquid phase crackingCracking during additive manufacturingAdditive manufacturing. However, removing these elements leads to a degradation of grain boundaryGrain boundary strength or a reduction in the efficiency of the protective oxide layer at high temperaturesHigh temperature. To reconcile these different aspects, the contents of minor elementsMinor elements are adjusted in a γ/γ′ cobalt-based superalloySuperalloys following the study of its solidificationSolidification path. The chemical modifications enable the development of various crack-free microstructuresMicrostructure by directed energy deposition, which is unattainable before the chemical modifications. CreepCreep and cyclic oxidationCyclic oxidation testing show that these chemical modifications do not affect the initial properties of the superalloySuperalloys.

A series of new printable Ni and Co–Ni high γ′ superalloysSuperalloys designed for additive manufacturingAdditive manufacturing have been evaluated for strain-age crackingStrain-age cracking (SAC) resistance. Printability studies and heat treatmentHeat treatment experiments were conducted to identify processingProcessing windows and characterize the overall resistance to SAC. High-resolution synchrotron X-ray diffractionX-ray diffraction experiments were performed to measure the γ and γ′ lattice parametersLattice parameter as a function of the temperature. All the superalloysSuperalloys tested were found to have a positive γ/γ′ misfitMisfit at room temperature which decreases as the temperature increases. The misfitMisfit of a SAC-prone alloy, 247LC, decreases rapidly and turns negative at about 600 °C, whereas the misfitMisfit of superalloysSuperalloys with intermediate resistance to strain-age crackingStrain-age cracking remains slightly positive. In the three most SAC-resistant superalloysSuperalloys, the misfitMisfit remains larger than 0.05% until at least 883 °C. The results show that a critical aspect for designing SAC-resistant alloys is ensuring that the misfitMisfit between γ and γ′ remains positive at all temperatures to generate compressive stresses on grain boundariesGrain boundary. Furthermore, the work also highlights a critical positive misfitMisfit value of 0.05% to prevent crackingCracking.

A new high strength nickel-based superalloyNickel-based superalloy, XH67, has been fabricated by the laser powder bed fusionLaser Powder Bed Fusion (LPBF) process (LPBFLaser Powder Bed Fusion (LPBF)). Novel heat treatmentsHeat treatment via direct aging, solutionizing and aging, were carried out and compared with the as-printed alloy, to study the evolution of phase equilibria and bring out the optimum mechanical propertiesMechanical properties of high strength and good ductility. A γ′ strengthened alloy, XH67 shows a high strength of ~1040 MPa with a 25% ductility, after direct aging (γ′ size of 10–30 nm and a volume fraction of 25–30%). The as-printed structure with a cellular-dentritic morphology was retained during direct aging heat treatmentHeat treatment contributing to the strength by Hall–Petch strengthening along with precipitationPrecipitation hardening by γʹ precipitatesPrecipitates. Discontinuous precipitationDiscontinuous precipitation along the cell boundaries was a unique microstructural feature that are observed in the present alloy. Annealing twins were seen after solution and aging along with coarseningCoarsening of the γ′ precipitatesPrecipitates up to 50 nm. Thermodynamic calculation (Thermo-Calc) was used to validate the observed phase evolution as a function of heat treatmentHeat treatment.

Additively manufacturedMar-M 509 Mar-M 509, a cobalt based superalloyNi- based superalloy, was evaluated for its microstructureMicrostructure and tensile behavior (at room temperature and 650 °C) after short-cycle heat treatmentsHeat treatment, along the two orientations, longitudinal (L) and transverse (T) to the build direction. The microstructural evolutionMicrostructural evolution after single step heat treatmentsHeat treatment at 950, 1150, and 1250 °C for 3 h was characterized using transmission electron microscopyTransmission electron microscopy. The alloy comprises a columnar-cellular dendritic microstructureMicrostructure strengthened by MC carbides forming a network along the cell boundaries in the as-printed condition. On heat treatmentHeat treatment, the microstructureMicrostructure was characterized by the precipitationPrecipitation of M23C6 along with MC carbides. The T orientation showed higher yield strength and lower elongation than L for all the conditions. Amongst these, the 1150 °C heat treatmentHeat treatment showed the optimum combination of yield strength and elongation (~850 MPa, ~20%), attributed to the presence of fine MC carbides along the cell boundaries and coarse M23C6 carbides at the grain boundariesGrain boundary, with a carbide fraction of nearly 18%. At the test temperature of 650 °C, the optimum yield strength of ~740 MPa and elongation of 21% was seen in the 950 °C HT condition. This understanding of microstructureMicrostructure-mechanical propertyMechanical properties correlation for a palette of short-cycle ageing treatments thus allows for choosing the right combination for the desired application.

By controlling additive manufacturingAdditive manufacturing process, particularly Laser Powder Bed FusionLaser Powder Bed Fusion (PBF-L) process, significantly different microstructureMicrostructure and constituents’ formation and distribution can be achieved robustly for superior materials properties gain without altering the bulk materials chemistry. Furthermore, a supersaturated solid solution structure can be obtained without solution treatment and subsequent quench operation to attain optimized properties by forming a high-volume fraction and uniformly distributed fine strengthening phase. In this work, it is demonstrated through additive manufacturing, nano sized and temperature stable carbides (tungsten carbide) for Carbide Dispersive Strengthening (CDS)Carbide Dispersive Strengthening (CDS) were formed to improve not only general materials properties but more critically to provide strengthening mechanismsStrengthening mechanisms above traditionally γ′ solvus temperature range for extreme environment applications such as hypersonic leading edge and combustion devices. Uniaxial tensile data were obtained for Laser Powder Bed FusionLaser Powder Bed Fusion (PBF-L) of Haynes 230Haynes 230 at temperatures from 982 to 1177 °C. The data is analyzed in terms of the strain rateStrain rate sensitivity m and the stress dependence n. These two parameters are used to provide insight into the possible deformation mechanismsDeformation mechanisms controlling plastic flow in this alloy over the temperature–strain rateStrain rate range of interest. The values obtained suggest that under the present experimental conditions Haynes 230Haynes 230 deforms by a combination of dislocation slip and diffusionDiffusion mediated recovery within the grain interior. Stress–strain curves exhibit oscillations suggesting the material is undergoing dynamic recrystallizationDynamic recrystallization during the tensile test. Optical imaging of the gage sections confirms the presence of dynamic recrystallizationDynamic recrystallization.

Additive manufacturingAdditive manufacturing enables the fabrication of complex part geometries, and is attractive for advanced aerospace components. Laser powder bed fusion (LPBF)Laser Powder Bed Fusion (LPBF), specifically, is being assessed for manufacturing structural components of gas-turbine engines made from high- $$\upgamma ^{\prime }$$ γ ′ volume fraction superalloys. However, the formation of crack defects during LPBFLaser Powder Bed Fusion (LPBF) of nearly all superalloys within this class has undercut their mechanical performance greatly. This study builds on prior work examining the cracking susceptibility of high- $$\upgamma ^{\prime }$$ γ ′ volume fraction superalloys during LPBF by simplifying the LPBF process down to single track laser meltingLaser melting scans. The CoNi-base alloy GammaPrint-700 is utilized in this study, as the cracking resistance of the alloy can be controlled through the boronBoron content. A means of improving the cracking resistance of the alloy through homogenization treatments prior to laser meltingLaser melting was identified. Characterization of the single tracks reveals a possible mechanism of crack initiationCrack initiation via liquation cracking of grain boundariesGrain boundary in the substrate material, and propagation via solidification crackingSolidification cracking along grain boundariesGrain boundary in the melt pool. Additionally, a protocol for assessing the cracking resistance while developing new high- $$\upgamma ^{\prime }$$ γ ′ volume fraction superalloys for additive manufacturingAdditive manufacturing is discussed.

Ni-based superalloysNi- based superalloy processed by laser beam powder bed fusion (PBF-LB) additive manufacturingAdditive manufacturing (AM) are ideal for high temperatureHigh temperature structural applications in the aerospace and power generation industries due to the increased component complexity afforded by AM. However, conventional creep testingCreep testing limits the rate at which new materials can be produced with AM. To help accelerate the acceptance of AM Ni-based alloysNickel-based alloys for high temperatureHigh temperature applications, methods for high throughput creepCreep evaluation are needed. The stress relaxationStress relaxation test has potential to hasten the development and validation of PBF-LB Ni-based structural alloys by assessing a wide range of creepCreep rates relevant to service conditions with a single test. In this work, alloy Ni230, a gas atomized powder derivative of Haynes 230Haynes 230, and variant thereof containing added TiC are assessed. Each material was subjected to a limited subset of conventional creepCreep tests accompanied by stress relaxationStress relaxation tests. Following the calculation methodology described herein, stress relaxationStress relaxation tests predict creepCreep rates and rupture times that align well with conventional creepCreep test results. Stress relaxationStress relaxation tests also reveal features of microstructural characteristics and evolution which are not readily apparent with other experiments. Several advantages and challenges with stress relaxation testingStress relaxation testing are discussed.

Superior repairRepair technology is a principal driver for resource-effective operation in the aviation industry. Routine operation of aircraft engines exposes the turbine components to high stresses and high temperaturesHigh temperature. To withstand extreme operational conditions Ni-based superalloysNi- based superalloys are used to manufacture turbine components. A crucial factor in targeting the assurance of repairRepair reliability is improving the repairRepair braze gap strength. This study seeks to improve the braze repairRepair strength by optimising a novel superalloySuperalloys filler material. The superalloySuperalloys filler material acts as a complementary additiveAdditive that is blended with the braze alloy in powder form and improves the joint properties after brazing. The novel superalloySuperalloys filler was developed by materials simulationMaterials simulation using the CALPHADCALculation of PHAse Diagrams (CALPHAD) (CALculation of PHAse Diagram) approach. Phase fieldPhase field modellingModelling using MICRESS® was applied to study the brazing kinetics and microstructureMicrostructure evolution. The developed superalloySuperalloys filler was validated experimentally in respect to microstructureMicrostructure improvement and mechanical potential by tensile testing at service-equivalent temperature (871 °C). The application of the novel superalloySuperalloys filler shows an increase in ultimate tensile strength in comparison with a conventional braze blend.

RejuvenationRejuvenation treatments with integrated hot isostatic pressingHot isostatic pressing have been proven to re-establish the γ/γ′ microstructureMicrostructure of single crystalline nickel-based superalloysNickel-based superalloy after creepCreep deformation, close porosityPorosity, and recover creepCreep strength to a significant extent. Therefore, rejuvenationRejuvenation treatment is expected to be effective in extending the service life of components such as turbine bladesTurbine blades for gas turbines resulting in reduced costs and improved sustainability compared to replacing the component. Since such components are subjected to combined creepCreep-fatigueFatigue loading, this investigation is aiming to answer the question if a rejuvenationRejuvenation procedure, which has proved to recover single crystalline superalloysSuperalloys after creepCreep, is effective in rejuvenating these materials after low cycle fatigueLow cycle fatigue. For this purpose, test samples have been subjected after high temperatureHigh temperature low cycle fatigueLow cycle fatigue experiments to a rejuvenationRejuvenation procedure. The materials microstructureMicrostructure has been studied by non-destructive X-ray computer tomographyX-ray computer tomography before and after rejuvenationRejuvenation. Subsequently, metallographic sections were prepared from rejuvenated samples and investigated by scanning electron microscopyElectron microscopy. Pores and precipitatesPrecipitates detected in high resolution section images were identified in the reconstructed volume and used to fit the metallographic section images into this volume. Pores and cracks emanating at pores during fatigueFatigue testing were closed by rejuvenationRejuvenation as long as they were not connected to the surface. Electron backscatter diffraction performed on the sections revealed that, in contrast to creepCreep samples, the fatigueFatigue samples were recrystallized during rejuvenationRejuvenation. It is concluded that fatigueFatigue damage cannot be reversed by the same rejuvenationRejuvenation procedure that is effective for creep damageCreep damage.

Hybrid additive manufacturingAdditive manufacturing plays a crucial role in the restoration of gas turbine bladesTurbine blades, where, e.g., the damaged blade tip is reconstructed by the additive manufacturingAdditive manufacturing process on the existing blade made of a parent nickel-based alloy. However, inherent process-related defects in additively manufactured material, along with the interface created between the additively manufactured and the cast base material, impact the fatigue crack growthFatigue crack growth behavior in bi-material components. This study investigates the fatigue crack growthFatigue crack growth behavior in bi-material specimens of nickel-based alloysNickel-based alloys, specifically, additively manufactured STAL15 and cast alloy 247DS. The tests were conducted at 950 °C with stress ratios of 0.1 and −1. Metallographic and fractographic investigations were carried out to understand crack growth mechanisms. The results revealed significant retardation in crack growth at the interface. This study highlights the potential contributions of residual stressesResidual stress and microstructural differences to the observed crack growth retardation phenomenon, along with the conclusion from an earlier study on the effect of yield strength mismatch on crack growth behavior at a perpendicular interface in bi-material specimens.

In this study, a laser-based heat treatmentHeat treatment is applied on IN718 superalloySuperalloys substrate repaired by a minimally invasive direct energy deposition process. The focus is on the laser-based heat-treated microstructureMicrostructure, aiming to enable minimally invasive repairRepair processes to take place either “on-wing” or “near-wing”. Hardness measurements were performed on the as-repaired and heat-treated microstructureMicrostructure, while electron microscopyElectron microscopy, electron backscatter diffraction and atom probe tomographyAtom probe tomography analyses were performed to investigate the microstructureMicrostructure. After the laser-based heat treatmentHeat treatment, the hardness of the repaired part increased compared to the as-repaired and reached values similar to that of the substrate. Besides, microstructural analysis unveiled non-uniform γ″ precipitatePrecipitates formation, linked to observed micro-segregationSegregation from the repairRepair process persisting post-heat treatmentHeat treatment. PrecipitationPrecipitation-free areas were observed while co-precipitationPrecipitation of γ″ and γ′ in duplet and triplet particles was infrequent. Interdendritic areas exhibited Laves phases regions with needle-shaped δ precipitatesPrecipitates forming directly from the Laves phase. Carbonitrides coexisted with Laves phase, creating complex Laves regions. Although a single-step heat treatmentHeat treatment will not lead to a complete dissolution of the undesirable microstructureMicrostructure features, a solution treatment step using the same minimally invasive equipment can unlock the full potential of in-situ maintenance through laser-based heat treatmentsHeat treatment.

Nickel-based single-crystal turbine components used for aero-engine turbine bladesTurbine blades are exposed to cycles of temperature and stress during routine service. High temperatureHigh temperature exposure (exceeding about 950 °C) results in a transformation of the precipitationPrecipitation from cuboidal to coarsened morphology. The coarsened microstructureMicrostructure has a detrimental effect on the mechanical propertiesMechanical properties and, therefore, lifespan of the component. In the present study, simulated service conditions (approximated by non-isothermal creepCreep tests) were applied to SC2000 specimens. The effects of the non-isothermal creepCreep deformation and a rejuvenationRejuvenation heat treatmentHeat treatment on the alloy’s microstructureMicrostructure and tensile and creep propertiesCreep properties were analyzed. Non-isothermal creepCreep tests were performed with a base temperature of 900 °C and a peak temperature of 1100 °C. SEMScanning Electron Microscopy (SEM) analysis confirmed a quick deterioration of the microstructureMicrostructure exposed to the simulated service coupled with a marked dislocations activity around the precipitatesPrecipitates. In order to restore the initial properties, a super-solvus rejuvenationRejuvenation heat treatmentHeat treatment was applied after deformation. The rejuvenationRejuvenation process successfully restored the initial precipitatesPrecipitates morphology and the yield strength, while no improvement in creep propertiesCreep properties was observed. Hypotheses and investigations are presented to explain this effect.

The microstructureMicrostructure and mechanical propertiesMechanical properties of additively manufactured WaspaloyWaspaloy were characterized in the industrial context of refurbishmentRefurbishment. Two processes were studied to discuss their advantages and drawbacks considering the targeted application: Laser Metal DepositionLaser metal deposition with powder and Cold Metal TransferCold metal transfer. The resultant microstructuresMicrostructure were compared in the build-up and at the interface between the wroughtWrought base WaspaloyWaspaloy and the additively manufactured part, in the as-built condition and after post-weld heat treatmentsHeat treatment. Tensile and creep propertiesCreep properties were studied along three directions – horizontal, vertical and at the interface – to assess the anisotropyAnisotropy of the build-up and the strength of the interface/Heat Affected ZoneHeat affected zone induced by the metal deposition. Results show that both processes have limited anisotropyAnisotropy in the conditions studied, even though Cold Metal TransferCold metal transfer presents an important crystallographic and microstructural texture. Depending on the process used and heat treatmentsHeat treatment applied, the interface/Heat Affected ZoneHeat affected zone may not be the weakest point of the repaired part despite a neat interface. High temperatureHigh temperature creepCreep results also suggest that post-weld heat treatmentsHeat treatment may not be necessary if this is the primary designDesign criteria. Tensile propertiesTensile properties of Cold Metal TransferCold metal transfer WaspaloyWaspaloy are highly dependent on the deposit thickness from where the specimens are extracted, due to an in-situ heat treatmentHeat treatment that takes place during production of large deposits. The different results are discussed considering the limitations of industrial repairRepair operations such as metal costs, deposition rate, repaired volume and targeted application.

Brazing-repairRepair of nickel-based superalloyNickel-based superalloy precision casting has attracted a wide concern as the issues related to hot crackingCracking, dimensional distortion and loss of strengthening phase have not been solved for a long term. In this study, we proposed to use atomic clusterAtomic cluster powder filler which was manufactured through mechanical ball-mixing of Ni atomic clusterAtomic cluster nanoparticles with Inconel 718 powders to achieve a well-bonded joint. Spark plasma diffusion bondingDiffusion bonding was conducted to evaluate the atomic clusterAtomic cluster powder fillers and Inconel 718 powder fillers counterpart. The microstructureMicrostructure and mechanical propertiesMechanical properties of bonding joints were investigated to explore the effects of atomic clusterAtomic cluster nanoparticles on bonding of superalloysSuperalloys. It was found that the Ni atomic clusterAtomic cluster nanoparticles were uniformly distributed into Inconel 718 powders. The joint of atomic clusterAtomic cluster powder fillers exhibited a higher densificationDensification increased by 42% compared with Inconel 718 powder fillers. The tensile mechanical propertiesMechanical properties of bonded sample will be dependent on the bonding interface quality rather than microstructureMicrostructure of joint region. The atomic clusterAtomic cluster powder fillers resulted in a comparable yield strength of bonded sample with 200 K lower bonding temperature compared with Inconel 718 powder fillers.

Micro-twinningMicro-twinning is the major creepCreep deformation mechanismDeformation mechanisms in Ni-based superalloysNi-based superalloy at temperatures above 700 °C. Recent experiments suggest that superlattice stacking faultsStacking faults in γ′ phase may serve as the precursors to twin formation. SegregationSegregation of alloying elements to these precursors may have a significant effect on formation and extension of micro-twinsMicro-twins. Using atomistic modelingAtomistic modeling we investigate and explain the effects of Nb and Cr alloying additions on these processes. The simulation shows that Nb increases the creep resistanceCreep resistance which is mostly associated with impeding the reordering of the high energy double complex stacking faultStacking faults. Cr, on the other hand, promotes twin growth, degrading the high temperatureHigh temperature creep propertiesCreep properties. These results can help to understand the effects of elemental composition of the alloy on creep resistanceCreep resistance.

The precipitationPrecipitation strengthened alloy Haynes® 282 possesses very good strength, creep resistanceCreep resistance and corrosion resistance at high temperaturesHigh temperature. It is one of the best candidates for a range of high temperatureHigh temperature components for next generation of power systems as Advanced Ultra-Supercritical (A-USC) boilers and steam turbines, Supercritical carbon dioxide (sCO2) power cycles as well for aerospace gas turbine parts (combustor lines, nozzles, exhaust sections). Some of these components operate for very long time (30–40 years) during which they are subjected to high temperatureHigh temperature (up to 60% of the Liquidus Temperature, Tm) creepCreep as well as cyclic loading (cyclic temperature and stress variations). Assessing long term material behavior through combined accelerated testing and model predictions is an important step in the material qualification process. Towards this goal, a material constitutive model was developed capturing long term creepCreep as well as cyclic plasticity, with and without hold time at two temperatures: 593 and 760 °C. The model was calibrated on creepCreep and low cycle fatigueLow cycle fatigue tests performed on smooth specimens. The model predictions were compared with experimental data performed on smooth circular and on notched specimens, in terms of stress–strain response and number of cycles to failure.

Ni-based single-crystal superalloysNickel-based single-crystal superalloy, mostly used in turbine bladeTurbine blades applications, are inherently anisotropic and usually cast in the $$\langle 001\rangle $$ ⟨ 001 ⟩ direction. Any slight misorientationsMisorientation result in anomalies in the microstructural evolutionMicrostructural evolution, thereby, causing variation in the mechanical response. As the stability of the microstructureMicrostructure dictates the structural integrity of the blade, it is essential to understand the microstructural state as a function of the crystallographic orientation. Therefore, to predict the microstructural evolutionMicrostructural evolution of single-crystal superalloysSingle-crystal superalloys at any given orientation on the standard stereographic triangle, a crystallographic-sensitive phase-field model was developed. The phase-field simulations for the perfect $$\left[001\right]$$ 001 , $$\left[011\right]$$ 011 , and $$\left[111\right]$$ 111 orientations agreed well with the experimental characterizations. For the first time, a model also predicts the microstructuresMicrostructure for misorientationsMisorientation (10°) away from the main crystallographic directions. Finally, for a quantitative assessment of the macroscale performance of various orientations, the 3D phase-field microstructuresMicrostructure were employed to carry out crystal-plasticity finite-element (CPFE) micromechanical simulations for strain-controlled monotonic tensile tests at 1050 °C.

In this work, dynamic recrystallizationDynamic recrystallization kinetics for a nickel alloyNickel alloys are characterized during hot deformation. Gleeble compression testing within a temperature range from 927 to 1149 °C is employed to replicate typical forgingForging conditions with strain rateStrain rate varying from 0.1 to 10 s−1. Microstructural and flow stress results were analyzed via the Johnson-Mehl-Avrami-Kolmogorov (JMAKJMAK) model that describes the mechanisms triggering recrystallizationRecrystallization during forgingForging operations. This model considers the significant variables of hot deformation such as temperature, strain, strain rateStrain rate, and initial structure. The developed model illustrates the dynamic recrystallizationDynamic recrystallization fraction and dynamically recrystallized grainRecrystallized grains diameter within defined temperature ranges based on the JMAKJMAK configuration. Results indicate that deformation above 1093 °C yields high dynamic recrystallizationDynamic recrystallization. Conversely, at lower temperatures, grain structure is highly dependent on strain and strain rateStrain rate. Validation tests coupling Gleeble compression testsCompression test and FE modeling comparisons covering the forgingForging conditions are included in this study.

A finite element (FE) model was developed to simulate the densificationDensification of a nickel-base superalloyNickel-base superalloys powder during the hot isostatic pressingHot isostatic pressing (HIPHot Isostatic Pressing (HIP)) process. A unified material model which simultaneously captures the various deformation mechanismsDeformation mechanisms, such as plasticity and creepCreep, was used in this study. Elaborate experiments were carried out to generate thermal and mechanical propertiesMechanical properties and calibrate the model parameters for an aerospace industry relevant powder alloy. FE simulations of powder encapsulated in a stainless steel canister were performed to evaluate the model capability to capture the canister distortion as well as powder densificationDensification during the HIPHot Isostatic Pressing (HIP) process. The densificationDensification and shape change predictions from the FE model were verified using experimental data obtained from interrupted HIPHot Isostatic Pressing (HIP) runs performed at various temperatures and pressure ramp rates. Good agreement was found between the model predictions and the experimental results. It was found that including the creepCreep response of the canister in the simulation had a significant influence on the capability of the FE model to predict the powder densificationDensification behavior, especially during the hold time at peak pressure. The FE model provided critical insights on mechanical factors leading to non-uniform densificationDensification in the powder compact and canister deformation.