Superalloys 2020

LifingLifing is one of the main challenges for aeroengine manufacturers. For fatigue prediction, attention has been focused on the crack initiation mode depending on stress level and initial microstructure. Microstructure prediction during the component manufacturing, especially for final heat treatments and final forging operations, is required if it is to be included in fatigue analysis. Reliable tools are now available for basic nickel-based alloys such as Inconel 718Inconel 718. For other alloys, notably γ/γ′ alloys, research is still being performed in close partnership with academia. Globally, two main trends are emerging; first, one of our main interests is to develop the modelingModeling capability for the entire manufacturing processManufacturing process, including ingot conversion and billet forging. Second, new approaches are still under development by introducing more physical considerations through full-field models, which are very useful for a better understanding of specific issues such as heterogeneous grain growth. From a component lifingLifing point of view, the initial state of stress is also a key parameter to be considered. One method for the control of residual stressesResidual stresses is application of a pre-spinning process. Finally, a standard lifingLifing methodology is explained and improvements are proposed; in particular, size effect is used to model notch specimen life considering surface or internal initiation.

Two new alloy compositions for possible disk rotor applications have been examined. Both were intended to have higher $$ \gamma ^{{\prime }} $$ γ ′ content than the existing alloy, RR1000, and be produced using powder metallurgyPowder metallurgy and isothermal forging to enable forgings to show a consistent coarse grain microstructure. Small pancake forgings of the new alloys and RR1000 were made and from these, blanks were cut, solution heat treated, cooled at measured rates and aged. Results of screening tests to understand the tensile, creep and dwell crack growth behavior, oxidation resistance and phase stabilityPhase stability of these new alloys and coarse grain RR1000 are reported. The development alloys were similar in composition but exhibited different tensile and creep properties, phase stabilityPhase stability and resistance to oxidation damage. Despite attempts to minimize variation in microstructure from heat treatment, differences in $$ \gamma ^{{\prime }} $$ γ ′ size distribution were found to influence tensile and creep behavior. One of the new alloys (Alloy 2) showed improved yield and tensile strength compared to RR1000. Alloy 2 displayed similar initial creep strain behavior to RR1000 but superior resistance to subsequent creep damage, producing longer creep rupture lives. All of the alloys showed crack retardation at low stress intensity factor ranges (ΔK) from 3600 s dwell cycles at 700 °C in air. This occurred whilst crack growth was intergranular. Alloy 1 was found to precipitate C14 Laves phase from long term exposure at 800 °C. Like RR1000, σ phase was not detected in the new alloys after 750 h at 800 °C.

The new third-generation single crystalSingle crystal superalloySuperalloy AGAT has been developed for aircraft engine turbine blade applications. Alloy designAlloy design procedure is described and AGAT alloy properties are presented and compared with those of, respectively, first-, second-, and third-generation AM1, CMSX-4 and CMSX-10 alloys. AGAT alloy exhibits high creepCreep resistance at very high temperature (1200 °C) compared with first- and second-generation superalloysSuperalloy while maintaining moderate density (8870 kg m−3) and stable microstructure unlike the third-generation superalloySuperalloy. High cycle fatigue (HCF) and low cycle fatigue (LCF) properties of AGAT alloy are similar to second-generation CMSX-4 alloy. AGAT solution heat treatment allows suppressing the γ/γ′ interdendritic eutectic pools at a temperature 30 °C lower than for CMSX-10 with a shorter duration. OxidationOxidation resistance of AGAT alloy at 1150 °C is lower than that of second but higher than that of third-generation reference superalloysSuperalloy. AGAT shows low sensitivity to secondary reaction zone (SRZ) formation under β-NiPtAl bond coat (BC) and great spallation resistance of YPSZ EB-PVD thermal barrier coating (TBC)Thermal Barrier Coating (TBC) compared with reference alloys. Finally, single crystalSingle crystal turbine blades were successfully manufactured through industrial processes to be tested in engine conditions.

The interactions of solutesSolutes with crystal defectsCrystal defects at near-atomic-level were investigated in five single-crystal nickel-based superalloys deformed at temperatures between 850 and 1160 °C and various deformationDeformation conditions. These interactions, and consequently the composition of a particular soluteSolutes that segregates at a crystal defectCrystal defects , are controlled by the type of the crystal defectCrystal defects , the deformationDeformation conditions, i.e., temperature and stress, and the overall alloy composition. Atomistic phase-field simulations also reveal the effect of dislocation velocity on the amount of solutes that can segregate on dislocations. The observed plasticity-assistedPlasticity redistribution of interacting solutesSolutes phenomena results in microstructural and chemical alterations, which are associated with recrystallization, rafting, and the formation of topologically close-packed phases. Deciphering these interactions by enabling quantitative three-dimensional imaging of solutesSolutes at crystal defectsCrystal defects with high sensitivity and spatial resolution will allow to develop a solute–defect database that can be used as a key-design parameter for advanced superalloys.

Ni–Co-based γ + γ′ superalloys were explored in a wide range of Co/Ni ratios for designing a wrought superalloy with a combination of superior high temperature strength and environmental resistance at temperatures above 927 °C (1700 °F). While alloys with high Co/Ni ratios possess superior creepCreep resistance, reductions of the Co/Ni ratio and additions of Fe and B are effective in improving ductilityDuctility in the intermediate temperature range and hold-time low cycle fatigueFatigue resistance. A formation of fine, discrete Co2W Laves precipitates covering a large fraction of grain boundariesGrain boundary is responsible for the improvements. The experimental alloys exhibited excellent oxidationOxidation resistance at 982 °C (1800 °F) by forming a protective alumina scale during cyclic exposure.

In the present work, three Ni-based single-crystal superalloys (SXs) were investigated, aRe- Re-containing alloy ERBO/1 (CMSX-4 type) and two Re-freeRe-free superalloy single crystals SXs referred to as ERBO/15 and ERBO/15-W, which differ in W content. The microstructural evolution of the three alloys during heat treatment and their creepCreep behavior is investigated. When one applies one heat treatment to all three alloys, one obtains different γ/γ′-microstructures. Subjecting these three alloys to creepCreep in the high-temperature low-stress creepCreep regime, ERBO/15 outperforms ERBO/1. In order to separate the effects of alloy chemistryAlloy chemistry and microstructure, the kinetics of the microstructural evolution of the three alloys was measured. The results were used to establish similar microstructures in all three alloys. Comparing ERBO/15 with ERBO/15-W, it was found that in ERBO/15-W particles grow faster during the first precipitation heat treatment and that ERBO/15-W creepsCreep significantly faster. At constant microstructures, ERBO/15 and ERBO/1 show similar creepCreep behavior. In the high-temperature and low-stress creep regime, ERBO/15 shows lower minimum creep rates but ERBO/1 features a slower increase of creep rate in the tertiary creep regime. It was also found that in the high-temperature low-stress creepCreep regime, ERBO/1 shows a double minimum creep behavior when particles are small.

TROPEA, a new platinumPlatinum -containing single crystal (SX) superalloy, has been developed for high-temperature components of aircraft engines. Criteria used for the alloy design procedure to target specifications are presented. TROPEA properties have been characterized and compared to reference alloys. First casting showed good castability of the alloy with no tendency to freckles formation. TROPEA exhibits excellent yield stress up to 800 °C compared to second- or third-generation SX superalloys and resistance above 800 °C similar to second-generation superalloys. This new alloy shows high low cycle and very high-cycle fatigue properties, particularly at low temperature. TROPEA exhibits high creepCreep resistance at temperatures above 1200 °C, stable microstructure (no TCP phases) with a density (8.83 g.cm−3) intermediate to that of second- and third-generation SX superalloys. Atome probe tomography measurements show that platinumPlatinum preferentially partitions to γ′ precipitates. PlatinumPlatinum additions significantly stabilize γ′ phase near solvus temperature and consequently increase alloys properties at (very) high temperature despite a low rhenium content. Thus, platinumPlatinum emerges as a promising element to enhance high-temperature properties of SX superalloys in addition to or as an alternative to rhenium. The high creepCreep properties at very high temperature combined with excellent fatigue properties at low temperature make TROPEA a promising SX superalloy for highly cooled turbine components.

The cast and wrought nickel base superalloy M647 has been developed with high mechanical performance and deformability. This study describes an overview of the microstructure and the properties of M647. A full scale forging process has been developed with uniform fine grain distribution. The detrimental effect of thermal exposure at 700 °C ~ 800 °C on mechanical propertiesMechanical property was evaluated. M647 showed good mechanical properties and equivalent thermal stabilityThermal stability to conventional turbine diskTurbine disk alloys. M647 offers good balance of deformability and mechanical properties with lower cost than powder metallurgy alloys.

A new cast and wrought Ni-base superalloyNi-base superalloy , M647, has been developed for turbine diskTurbine disk application with high mechanical properties at elevated temperatures and reasonable hot deformability. Conventional Ni-base superalloysNi-base superalloy are known to be strengthened by primary, secondary, and tertiary γ′, but optimal heat treatment is dependent upon specific chemical composition. For optimal mechanical properties, microstructural evolutions including austenitic γ grain size and γ′ morphology were identified for M647. To evaluate these properties, a full-scale low-pressure turbine diskTurbine disk was manufactured. This was subjected to a sub-solvus solution heat treatment, such that primary γ′ remained to prevent grain growth, enabling required mechanical properties, especially proof stress and low-cycle fatigue, to be achieved. As for secondary γ′, precipitation behavior was controlled intentionally by changing cooling rate after solution treatment, and this also resulted in modifying the precipitation behavior of tertiary γ′ during aging. In this study, microstructural evolution and tensile properties at 650 °C were investigated to clarify the relationship between them and to identify the strengthening mechanisms. From the result of SEM observation, size distributions of secondary γ′ had a clear relationship with cooling rate. Moreover, the mean diameter of secondary γ′ was unchanged during aging and this suggested that only tertiary γ′ was precipitated during aging. Finally, critical resolved shear stressCritical resolved shear stress was calculated using both weakly and strongly coupled dislocation models, and it was clear that M647 was strengthened by a high volume fraction of secondary γ′ and a small volume fraction of fine tertiary γ′ precipitates.

There is a constant push for higher efficiencies, lower cost, and increased power in power-generating and propulsion gas turbines. In order to meet these requirements, hot section materials with higher temperature capabilities are needed. Ni-base superalloys are selected for these applications. In this study, commercially available and model Ni-based superalloy compositions were simulated with thermodynamic calculations using Thermo-CalcThermo-Calc software, which were then experimentally evaluated. Previously, alloy development campaigns have relied heavily on preparing many heats of alloys to examine the effect of various alloying additions and various levels to down-select a single alloy. Methods like PHACOMP have used understanding of partitioning behavior for alloy design. More recent studies have utilized regression analysis of empirical data to inform new alloy design. Physical models can be used to improve upon these methods. The ability to use computational materials science approaches to reduce the number of heats processed in an alloy development program was explored. By validating database sensitivity to compositional changes, future alloy development work can be performed precisely, leading to faster alloy development, validation, and implementation. Model alloys were optimized for phase stability, cost, and density. Continued experimental validation of thermodynamic prediction databases will create a more robust system for alloy property prediction and development.

To provide precipitation hardening of nickel-base superalloysNickel-base superalloys, γ′ and γ″ phases can be partially replaced by phases like eta (η) and delta (δ), which may be stable up to temperatures higher than 800 °C. Nevertheless, there is still a lack of information about these phases in terms of crystal structure, composition, thermodynamic stability, and precipitate morphology. Therefore, the present study focuses on the composition and temperature stability of η and δ phases∆-phase in alloys with high Nb and Ta contents. Various experimental nickel-base alloys were designed using literature and thermodynamic calculations. After solution and aging treatments, they were characterized using SEM to determine precipitate morphology and distribution, and TEM-EDS to determine crystal structure and composition of precipitated phases. Combined in situ HE-XRD experiments followed by metallographic analysis were performed to determine the solvus temperature of η phaseΗ-phase in several alloys. Results on the nature of the precipitated phases show that thermodynamic calculations (TCNI7 database) and composition criteria are not always consistent with experimental data. The investigation also reveals that Ta is more favorable to form η phase than Nb and that other elements than Nb, Ta, Al, and Ti have an effect on η and δ phases∆-phase, such as Cr, Co, and Fe. Hence, the composition criteria for the formation of η and δ phases in alloys with high Ta content are discussed.

This paper describes the advanced alloy designAlloy design program (AADP) and improvement of a sixth-generation single crystal superalloy, TMS-238TMS-238, using this AADP. CreepCreep rupture life prediction equations for five different creepCreep conditions, 800 °C/735 MPa, 900 °C/392 MPa, 1000 °C/245 MPa, 1100 °C/137 MPa, and 1150 °C/137 MPa, were obtained with excellent determination coefficients from 0.95 to 0.98. Using the AADP, we successfully developed three alloys, TMS-238mod-A, TMS-238mod-B, and TMS-23mod-C, which have 10–22 °C higher-temperature capabilities than those of the TMS-238TMS-238 alloy. The AADP successfully predicted that the creepCreep life was maximized when the volume fraction of γ′ phase was approximately 65% at low-temperature and high-stress conditions, such as 900 °C/392 MPa, compared to that of approximately 60% under high-temperature and low-stress conditions, such as 1100 °C/137 MPa. This prediction enables us to precisely optimize the creepCreep property. Moreover, the prediction equation of the weight change after 1100 °C/1 h oxidationOxidation was also updated. The determination coefficient of this equation was R2 = 0.90.

Phase equilibria among the A1 (γ-fcc), Ni2Cr (oP6) and TCP phases in Ni–Cr–Mo system at temperatures above 973 K have been investigated, in order to evaluate the possibility for utilizing a novel microstructure design principle for Ni-based alloys having TCP phase at grain boundaries and GCP phase other than γ′ phase within grain interiors. Unlike the phase diagram calculated based on commercially available thermodynamic databases, the Ni2Cr phase in the binary system becomes stabilized by the presence of Mo solute atoms in solution at temperatures greater than 200 K, and the Ni2(Cr, Mo)-oP6 single-phase region exists as an island at around the composition of Ni–20Cr–15Mo (at.%) at temperatures above 973 K. The oP6 phaseOP6 phase decomposes to γ and P (oP56) phase at temperatures above 1073 K. Two distinct three-phase regionsThree-phase region of γ + oP6 + P and γ + oP6 + NiMo (oP56) were found to exist around the oP6 single-phase region. In case of the decomposition of high-temperature γ phase to the three-phase mixture comprised of γ + oP6 + P, very fine coherent particles of oP6 phaseOP6 phase that are only a few hundred nanometers in size form in the γ matrix with a tweed-like morphology. These precipitates possess an orientation relationship of $$\left\{ {1\bar 10} \right\}$$ 1 1 ¯ 0 γ//(100)oP6, <001>γ//[010]oP6, just like precipitation behavior of γ′ particles in Ni-base superalloys. In contrast, the TCP phase preferentially precipitates at the γ grain boundaries. The novel phase transformations and microstructures occurring in this class of alloys may potentially lead to advances in the design of novel Ni-based alloys.

A new Ni-based cast-and-wrought alloy designed for high temperature strength, stabilityStability , notch ductility, and minimal elevated temperature dwell fatigue crack growthDwell fatigue crack growth rate has been laboratory developed and scaled up in the mill. Based on studies of a relatively wide compositional space, a Co-free composition was selected. Other properties, including tensile strength, stress ruptureStress rupture , creep, low cycle fatigue, oxidation, and sulfidation resistance, as well as hot workability were also studied. Several heat treatments were developed to achieve property balances as appropriate for various end-user applications such as those limited by damage tolerance, creep, fatigue, or tensile strength. Microstructural stability after extended exposure at temperatures ranging from 704 to 871 °C was studied. Multiple 12-ton heats have been successfully processed as VIM/VAR and VIM/ESR ingots converted into forged billets, rolled bar, and strip products. Potential applications for this alloy include turbine disks, gas turbine engine casings, high temperature fasteners, heavy duty diesel engine exhaust valves, and high temperature gaskets. This paper discusses alloy and process development as well as microstructure--property relationships of the alloy.

Additive manufacturing trials are carried outGhoussoub, Joseph N. on two new nickel-based superalloys designed specifically for this processing methodTang, Yuanbo T.. Their performance—with emphasis on theirChinnapat, Panwisawas capability to resist cracking—is assessed by comparing with the two legacyNémeth, André alloys IN939 and CM247LC. The two new alloys are found to have demonstrably superior printability. Thermophysical testing andReed, Roger C. quantitative characterization, particularly via stereology, are used to help rationalize the physical basis of the improved manufacturability displayed.

Alloying effects on the competition between discontinuous and continuous precipitation of δ-Ni3Nb (D0a) phase and η-Ni3Ti (D024) phase were investigated in model Ni-Cr-Fe-Nb and Ni-Ti-based alloys, respectively, to aim at designing polycrystallinePolycrystalline superalloys Ni based superalloys with better temperature capability. Discontinuous precipitationDiscontinuous precipitation tends to occur at lower temperatures, while continuous precipitation dominates at higher temperatures in the two alloy systems. The addition of Mo promotes continuous precipitation with respect to discontinuous precipitationDiscontinuous precipitation while that of Ti promotes discontinuous precipitationDiscontinuous precipitation rather than continuous precipitation in δ phase precipitation alloys. A replacement of Ti with Mo promotes continuous precipitation with respect to discontinuous precipitationDiscontinuous precipitation in η phase precipitation alloys at 800 °C. The observed alloying effects are discussed in terms of chemical driving force, interfacial energy between the matrix phase and the δ(η) phases, and coherency strain caused by the formation of the metastable phases prior to the formation of δ/η phases.

Alloy 263 is a candidate Ni-based superalloy for use in advanced ultra-supercritical (AUSC)Advanced ultra-supercritical (AUSC) power plants that are targeted to operate above 700 °C. Exposure times in this type of environment are considerable with target creep properties specified at 100,000 h making phase stability an important parameter in the alloy selection process. In this investigation, the microstructureMicrostructure of alloy 263 and two modified compositions in both castCast and cast/wroughtWrought forms were investigated after exposure at 800 °C for various times ranging from 1000 to 10,000 h. Tension and creep properties were assessed for all alloys in both forms. Modification to the Ti and Al concentrations successfully slowed down the γ′ to η transformation while doubling the γ′ fraction. Thin elongated η needles or platelets formed in the wroughtWrought products, while thicker, plate-like, η precipitates were found in the castingsCast. The effect of the various microstructuresMicrostructure on the creep properties was determined using isothermally aged specimens aged at 800 °C for 3000 and 10,000 h prior to creep screening. Strong precipitation of the η phase was found to considerably decrease creep life and increase minimum creep rate. By modifying the Ti and Al content, the creep life and ductility of the new formulation tested on specimens isothermally aged for 3000 and 10,000 h were near that of the nominal alloy in its standard aged condition.

High-temperature fatigueFatigue of Ni-based single crystal superalloysSuperalloy is studied at 1000 °C in a wide range of loading conditions (−1 ≤ R ≤ 0.8) and number of cycles (103 − 109). Under fully reversed conditions, a competition between crack initiations from the surface—assisted by oxidation—and from internal metallurgical defects—mostly large casting pores—is observed. Increasing the testing frequency shifts the competition to a higher number of cycles. Conversely, decreasing the casting pore size or coating the specimen promotes surface initiations. When a positive mean stress is added (R ≥ 0), a creepCreep deformation/damage mechanism mainly controls fatigueFatigue life, despite fracture surfaces presenting a variety of initiation types. Fatigue life can be predicted by a simple creepCreep law if the contribution of the alternating stress is considered. A linear damage summation method that considers pure fatigue and pure creepCreep damage is used to predict the fatigueFatigue lives, and Haigh diagrams for different alloys are presented.

Fretting fatigueFretting fatigue (FF) lives of the CMSX-4 single crystal alloy have been measured, via some different counterpart materialsConterpart materials in similar and dissimilar material combinations at room and elevated temperatures. The CMSX-4 specimens were machined so that the [001] crystallographic orientation was within 5° along the longitudinal axis of the SC specimen. In this work, the work special attention was paid to find some useful methods to extend the FF lives by controlling the surface texture of contacting pad, as well as to get the metallurgical and mechanical aspects in the FF damage. The experimental works demonstrated that the texturing was effective to improve the FF lives. The stress field equations along the contacting interface have also been derived employing contact mechanics theories so that the FF lives and the failure characteristics can be rationally estimated, on the basis of mechanical backgrounds. A new method proposed successfully provided reasonable predictions in agreement with the experimental results.

Fatigue tests under a series of stress amplitudesStress amplitude at 760 °C of a single-crystal nickel-based superalloySingle crystal Ni-based superalloy were conducted to investigate the relationships between loading conditions and initiation sites of fatigue cracksFatigue crack initiation. It was found that initiation sites of the dominant fatigue crack gradually transferred from pores to MC carbides with an increase of the stress amplitudeStress amplitude. At low stress amplitude, the fatigue crack that causes the final fracture initiated at pores and the fatigue life depended on size and shape of pores. Although the carbides on surface cracked as a result of oxidation and cyclic loading, the cracks did not penetrate the carbide/matrix interface. However, at high stress amplitude, the cracks in the carbide can penetrate the carbide/matrix interface rapidly under applied stress, and following crack propagation resulted in decisive fatigue fracture. Compared to blocky carbides on the surface, script carbides at subsurface were more likely to induce fatigue crack initiationFatigue crack initiation.

Understanding the long-term creepLong-term creep behavior (creep lives longer than 5,000 h) of nickel-based single-crystalSingle crystal (SX) superalloys is of great interest for the service safety of industrial gas turbine (IGT) blades. However, understanding the influence of various factors on long-term creep behavior of nickel-based SX superalloys has always been a challenge. In this study, the creep behavior of nickel-based SX superalloys with or without 2 wt.% Re additionRe addition were investigated at 900 °C and 200 MPa. Microstructural characterization was systematically conducted to elucidate the microstructural evolution of the experimental alloys after creep rupture tests. The results indicated that the creep lifetime was increased significantly by 2 wt.% Re addition, which dramatically reduced the creep strain rate and caused the lattice misfitLattice misfit of γ/γ′ phases to inverse from positive to negative, leading to N-type rafting exhibiting a stronger barrier to dislocations during creep. The evolution of lattice misfit of the γ/γ′ phases was closely associated with the elemental partitioningElemental partitioning behavior between the γ/γ′ phases, which was determined as a consequence of “time accumulation effect” and corresponding elemental diffusion special for prolonged time. This study provides a new insight in the effect of Re on the long-term creep life and microstructureMicrostructure evolution of nickel-based SX superalloys. Such knowledge will be helpful to provide the guideline of superalloys design for large-scale IGT blades.

The creep behavior under non-isothermal and isothermal creep conditions was studied for a single crystalSingle crystal, a directionally solidifiedDirectionally solidified, and a polycrystallinePolycrystalline superalloy. Regardless of the alloy type, the non-isothermal creepNon-isothermal creep life of the alloy was much lower than that under isothermal conditions, yet the samples ruptured at higher plastic strains. Irregular interfacial network formation and early (but homogeneously distributed) shearing of the γ′ phase was identified as the main source of damage for all three alloys. Additionally, secondary precipitates with thermal expansion coefficients different from the surrounding matrix led to the formation of creep voids, especially under thermal cycling. In that respect, surface carbides were seen as particularly damaging due to their selective oxidation, which resulted in the extension of secondary cracks. The higher plastic strains exhibited by the non-isothermal creepNon-isothermal creep samples were correlated to the more homogeneously distributed damage accumulation, in comparison with the more localized deformation during isothermal creep. This study provides an overview of the damage accumulation sources in the three types of available superalloys and highlights areas which need to be further considered when designing future alloys for the non-isothermal creepNon-isothermal creep regime.

Unintentional plastic deformation during production of turbine blades made from Ni-based single-crystal superalloyNi-based single crystal superalloy has a huge risk on its durability. In the present study, a plastic deformation was applied at various temperatures to AM1 alloy and its effect on the alloy’s microstructure and creepCreep properties were analyzed. Microstructure evolution during aging treatment was different for plastic deformation at lower temperature (≤750 °C) and higher temperature (950 °C) because of different deformation mechanisms. AM1 with mild γ′ directional coarsening after plastic deformation at 950 °C and following aging treatments performed well in creepCreep test at 1050 °C/140 MPa, but poorly at 850 °C/500 MPa. Rhenium containing CMSX-4 Plus was tested similarly to AM1. Pre-deformationPre-deformation has huge impact on creepCreep durability of CMSX-4 Plus at 1150 °C/110 MPa; however, the creepCreep life debit at 1050 °C/200 MPa was minor. In order to restore properties of pre-deformed single-crystal superalloys, rejuvenationRejuvenation heat treatment process was added after pre-deformationPre-deformation . RejuvenationRejuvenation treatment successfully restored microstructure after room-temperature plastic deformation, and creepCreep properties of rejuvenated specimens are equivalent to that of original AM1 and CMSX-4 Plus.

TEM characterization of the deformation micromechanisms after interrupted creep test at 980 °C is proposed in the case of two single-crystal Ni-based matrix alloys without rheniumRhenium (Re) and with 9 wt% Re. This paper is aimed to propose an identification of the physical parameter at the origin of the strengthening effect of Re. The TEM observations carried out in the alloy with 9 wt% Re indicate that the deformation proceeds through the propagation of long dislocation pile-upsDislocation pile-ups . A leading pair is observed within these pile-ups. As this dislocation configuration is a signature of short-range orderShort range order (SRO), a quantitative approach is used to evaluate the SRO degree and thus the resistance due to SRO. The experimental positions of the dislocations are used to evaluate the total elastic force experienced by the dislocations. The strength associated with the SRO is evaluated to: γ0 = 50 − 60 mJ/m2. The observation of the matrix alloy without Re indicates the presence of both individual dislocations and some short dislocation pile-upsDislocation pile-ups . It is also associated with a lower SRO degree. It can be concluded that the Re effect leads to an increase of the SROShort range order degree. SRO is still present at high temperature (930 °C), whereas it has been shown to disappear in similar matrix alloy with a lower amount of Re. RheniumRhenium appears then to promote SROShort range order . It can be at the origin of its well-known strengthening effect.

The peculiar atomicBarba, Daniel structure of $$\gamma '$$ γ ′ precipitates [Ni $$_3$$ 3 (Al/Ti)-L1 $$_{2}$$ 2 ] in Ni-based superalloys produces high-energy faults when dislocations glide them, giving their significant strength at highEgan, Ashton J. temperatures. The mechanisms behind the strength failure of these alloys above 700–800 $$^\circ $$ ∘ C are still controversial. Recent advances in atomic resolution microscopy have allowed to study these mechanisms with unprecedented detail. In our study, we have characterised in a careful systematic study a SX-[001] superalloy from RT to 1000 $$^\circ $$ ∘ C. MultiscaleGong, Yilun microscopy (TEM and SEM) has been combined with physical metallurgy and atomistic modelling to fully understand the correlation between the strength drop and the observed changes in the $$\gamma '$$ γ ′ shearing mechanism. Our results show that, far from previous beliefs, the initial failing of alloy strength is not a consequence of the activation of dislocationMills, Michael J. climbing. Instead, there is a transition between three different mechanisms: ( $$T<750~^\circ $$ T < 750 ∘ C) continuous planar stacking faults below, (T = 750 $$^\circ $$ ∘ C) APB shearingReed, Roger C. at the strength peak anomaly and ( $$T>800~^\circ $$ T > 800 ∘ C) extensive twin deformation after the yield drop. Local chemical changes around the $$\gamma '$$ γ ′ shearing dislocations boost these changes, thus producing the sudden drop of strength.

Chemical inhomogenities due to dendritic solidification of Ni-based superalloys result in different local microstructures with varying mechanical properties. New indentation creepIndentation creep test methods allow probing of the local creep properties at the dendrite scale at high temperatures. The as-cast single crystalline Ni-based superalloyNi-base Superalloy ERBO1A (a derivative alloy of CMSX–4) was investigated and electron-probe microanalysis (EPMA) measurements revealed strong segregationDendritic segregations of, e.g., Re and W in the dendritic region and, e.g., Ta in the interdendritic region. Indentation creepIndentation creep experiments at 750 °C and micropillar compressionMicropillar compression tests at 785 °C were conducted in both regions, and a higher creep strength was found in the dendritic region compared to the interdendritic region. Theoretical models for solid solution hardening as well as γ′ precipitation hardening confirm these results, since they predict a higher strength in the dendritic region than in the interdendritic region. Compared with the fully heat treated state, a smaller difference in the local mechanical properties or even a reverse strength ratio of the dendritic and interdendritic region can be expected.

A new constitutive equation predicting the evolution of oxide thickness is proposed and implemented in a crystal plasticityCrystal plasticity framework with a hardening-based damageDamage density function. The oxidationOxidation model depends on the initial surface roughnessSurface roughness as well as the amount of accumulated plastic strain and stress triaxiality. The effect of oxidationOxidation on the mechanical behavior and damageDamage is considered at the flow stress level by modifying the amplitude of the kinematic hardening. The oxidationOxidation-altered kinematic hardening is able to predict the effect of oxidizing environments on the mechanical behavior, specifically the plastic strain rate in the secondary creepCreep stage, and lifetime. In addition, the oxidationOxidation model has also been tested in 3D by performing a finite-element simulation on a notched specimen subjected to a creepCreep load. It revealed that the model is able to predict surface roughnessSurface roughness and oxide thickness distributions in 3D and for multiaxial stresses.

Directional coarsening of γ′ phase (raftingRafting) in Ni-based single-crystal superalloysNi-based single crystal superalloy during tensile creepCreep at 1273 K is simulated by the phase-field (PF) method. A number of PF simulations are performed with various values of PF model parameters. The obtained results are used to train a neural networkNeural network (NN) to enable fast and accurate prediction of the raftingRafting time ( $$ t_{\text{raft}} $$ t raft ) from the values of model parameters. Material parameters of first-, second-, third-, and fourth-generation superalloys are estimated from their chemical compositions for predicting $$ t_{\text{raft}} $$ t raft using the trained NN. The $$ t_{\text{raft}} $$ t raft of several practical superalloys are predicted in the tensile stress range of 130–190 MPa. The NN prediction results show that $$ t_{\text{raft}} $$ t raft tends to be longer along with the order of alloy generation. Furthermore, creepCreep rupture time ( $$ t_{\text{rup}} $$ t rup ) of practical superalloys is estimated based on the Larson–Miller parameter method. It is found that there is a positive correlation between $$ t_{\text{raft}} $$ t raft and $$ t_{\text{rup}} $$ t rup , and the correlation becomes stronger with increasing the magnitude of external tensile stress.

This paper has studied the anisotropyAnisotropy in the creepCreep properties of DD6DD6 alloy under different temperatures and applied stresses. The results reveal that the anisotropic creepCreep of DD6DD6 alloy near the [001] orientationOrientation is strongly influenced by the temperature in the range of 650–980 °C. The anisotropyAnisotropy in the primary creep strain and rupture lifetime at an intermediate temperature of 760 °C is dependent on the applied stress. Compared with the specimens oriented close to the [001]–[111] boundary, the specimens oriented close to the [001] direction and the [001]–[011] boundary exhibit lower primary creep strains and longer rupture lifetimes at intermediate temperatures and high applied stresses. With the increase in the testing temperature or the decrease in the applied stress, the anisotropic creep behavior of the alloy near the [001] orientationOrientation disappears. The mechanism of anisotropic creep is attributed to heterogeneous γ′ precipitate deformation by <112> {111} slip.

Strength of Ni-base single-crystal superalloys under high temperature and low stress creep usually is enhanced by formation of γ/γ′ raft structureRaft structure and larger aspect ratio of γ′ phase in the γ/γ′ raft structureRaft structure. Elastic misfitElastic misfit between γ and γ′ phases is one of the most important factors to control the aspect ratio of the γ′ phase in the γ/γ′ raft structureRaft structure formed under external stress. The aspect ratio of the γ′ phase is controlled by kinetics for the γ/γ′ raft structureRaft structure formation, which is affected by a strain inhomogeneity caused by this elastic misfitElastic misfit between the γ and γ′ phases under external stress. To realize a new alloy designAlloy design approach to control the aspect ratio of the γ′ phase in the γ/γ′ raft structureRaft structure, this research aimed to obtain the regression equations which can predict elastic modulusElastic modulus of the individual γ and γ′ phases for multi-component Ni-base single-crystal superalloys based on measurements of elastic modulusElastic modulus of Ni-base single-crystal alloys. Elastic modulusElastic modulus of the individual γ and γ′ phases of various kinds of Ni-base single-crystal alloys was measured by using rectangular parallelepiped resonance (RPR) method. Using the analyzed and referenced elastic modulusElastic modulus, regression equations for predicting <100> longitudinal elastic modulusElastic modulus of the individual γ and γ′ phases and its temperature and composition dependence were obtained. Detailed analysis of the elastic modulusElastic modulus and its composition dependence was executed to clarify the contribution of each element on the elastic modulusElastic modulus. At 900 °C, Re, Ta, Ti, Al, and Mo reduce the <100> longitudinal elastic modulusElastic modulus in the γ phase. On the other hand, Ru, Re, Ta, Ti, Al, W, and Mo enlarge the elastic modulusElastic modulus in the γ′ phase.

Temperature-dependent fatigue crackFatigue crack propagation in a Ni-based single crystal superalloy was experimentally and numerically investigated in a single crystal Ni-based superalloySingle crystal Ni-based superalloy. Fatigue crackFatigue crack propagation tests at room temperature 300, 450, and 700 °C were conducted using four types of compact specimens with different combinations of crystal orientationsCrystal orientation in loading and crack propagation directions. It was revealed in the experiments that the crack propagated along slip planes in crystallographic cracking manner at room temperature, while the cracking mode transitioned from the Mode I to crystallographic cracking at 300, 450, and 700°C. Mode I stress intensity factor range ΔKI values at the transitions depended on the testing temperature as well as crystal orientationCrystal orientation. To interpret these temperature-dependent crack propagation, a crystal plasticityCrystal plasticity finite element analysisFinite element analysis was conducted by taking into account the 3D inclined crack plane and the activity of slip planes in front of the crack. Slip plane activity, proposed as a damage parameter, could rationalize the fatigue crackFatigue crack propagation rates both during the crystallographic and Mode I cracking. It has been found that crack propagation resistance for crystallographic cracking is more or less the same at low temperature, while that for Mode I cracking decreases with the increase of the temperature. This damage parameter also provided an explanation of the critical condition that induces the transition from Mode I to crystallographic cracking.

Ni-based single-crystal superalloysSingle-crystal superalloys are high-temperature materials used for turbine blades in jet engines. FatigueFatigue damage can pose a major threat to the integrity of such components in operation. Traditionally, TEM-based studies on the fatigue behaviour of superalloys has been studied by investigating cyclic plasticity in the bulk of the material. When the cyclic loads are nominally elastic, however, such investigation may not contribute to the understanding of the alloy’s fatigueFatigue behaviour, since plastic micro-strains are confined to regions near stress raisers such as microstructural defects and are therefore randomly distributed. In turn, the plastic micro-strains near the ‘critical’ stress raiser, i.e. the one that acts as nucleation site for the dominant crack, govern fatigueFatigue life by inducing early crack initiation, and are therefore the key to capture the material’s fatigueFatigue behaviour. Hence, this work is concerned with the experimental characterisation of cyclic plasticityCyclic plasticity at the initiation site in a Ni-based single-crystal superalloySingle-crystal superalloys at 800 °C tested with nominally elastic cyclic loads. Such investigation was carried out by focused ion beam (FIB) lift-outs and subsequent transmission electron microscopy (TEM) studies. It is shown that deformation is significantly more pronounced near the ‘critical’ stress concentrationStress concentrations; in addition, deformation is rather homogeneous across large regions surrounding the stress raiser, with remarkably different deformation modes compared to those observed in the bulk of the specimens and from those expected in superalloys tested in similar conditions. The investigation of local cyclic plasticityCyclic plasticity at stress concentrationsStress concentrations promises therefore to provide new insight into fatigueFatigue crack initiation in Ni-based superalloys.

Tensile and fatigue life variabilities are investigated for new-generation single crystalSingle crystal Ni-based superalloysNi-based superalloys: the 3rd generation CMSX-4 Plus, the 6th generation TMS-238, and a newer Ni-based superalloy, TROPEA containing Pt. Consistently from the results of previous research, very high cycle fatigueVery High Cycle Fatigue (VHCF) (VHCF) properties at the chosen condition of T = 1000 °C/Rε = −1/f = 20 kHz are mainly influenced by the solidification/homogenization pore size and position. TROPEA alloy has the best low cycle fatigue (LCF)Low Cycle Fatigue (LCF) life among all tested alloys at 650 °C and Rσ = 0.05/f = 0.5 Hz. To better understand the influence of chemical composition on the LCF endurance, tensile propertiesTensile properties were investigated using nine different single crystalline alloys at 650 °C with the strain rate of 5 × 10−4 s−1. The yield stress is directly affected by the chemical composition of the Ni-based superalloysNi-based superalloys, and alloys with high contents of Ti and Ta have a higher yield stress, due to an increased shearing resistance of γ′ precipitates. Hence, the yield stress is the main control parameter of LCF at the selected condition. No influence of chemical composition on VHCF life durability has been observed, in good agreement with previous studies.

Effect of creep deformation at crack tip on fatigue crack propagation behavior in a single crystal and a directionally solidified superalloys at 900 °C was investigated. Creep-fatigue crack propagationCreep-fatigue crack propagation tests with single tension hold introduced into cyclic fatigue loading were conducted. In specimen extracted from the single-crystal superalloy, ICMSX-4, when the cyclic fatigue loading was restarted after the tension hold, nascent crack was immediately initiated followed by significant crack retardation. This crack propagation behavior was ascribed to mechanisms based on two different concepts of residual compressive stress and crack closureCrack closure. From the viewpoint of mechanism based on the residual stress concept, material degradation at crack tip induced by the tension hold was investigated using scanning electron microscope, while stress relaxation and the resultant residual compressive stress at crack tip were quantified by elastic-plastic-creep finite element analysisFinite element analysis coupled with digital image correlationDigital image correlation technique. Finally, crack propagation behavior in polycrystalline specimen extracted from the directionally solidified superalloy, MGA1400, was investigated focusing on effect of grain boundaryGrain boundary. An insight into criteria of a transition from crack retardation to accelerated intergranular cracking was suggested based on a relative grain sizeGrain size to “creep affected zone.”

This work demonstrates on flat test specimens that orbital friction weldingFriction welding is a suitable process to join a typical disk alloy 718 and a typical blade alloy 713LC. The capability to join nickel-based blades on disks enables the manufacture of blisks instead of the typically used bladed disks. Blisks have the potential to save weight within an aero engine. Orbital friction weldingFriction welding is an alternative process to linear friction weldingFriction welding to enable blade and disk joints. The advantage of orbital friction weldingFriction welding is that the movement is homogenous and the amplitude (or eccentricity) is smaller compared to linear friction weldingFriction welding. This allows joints in areas which are geometrically challenging and impossible to join for linear friction weldingFriction welding. Due to the solid state joining process, there is basically no material mixing within the weld, but within both alloys, recrystallizationRecrystallization can be seen near the weld line. The result is a fine-grained microstructure within both alloys adjacent to the bond line. While this means only a small microstructural change for alloy 718, it is a major microstructural change for the cast coarse grain alloy 713LC. Consequently, the mechanical properties are changed compared to the base material due to the friction weldingFriction welding process. The microstructural changes of orbital friction weldingFriction welding and their influence on the material properties are presented within this work.

The rotating turbine disks, spools, and seals in the hot section of gas turbine engines are typically made of nickel-base superalloys. These components operate at high temperatures resulting in dirt accumulation, oxidationOxidation, and hot corrosion, and must be cleaned during overhaul to enable inspection and repairRepair. It was found that the typical cleaningCleaning solutions used may dissolve the fine carbides present in some superalloys with potential impact on low cycle fatigue (LCF), but that prior shot peeningShot peening can mitigate this effect. It was also found that, for the highest operating temperatures, grain boundary oxidationOxidation may occur in some superalloys and the resultant very fine grain boundary oxides near the surface may be removed by the cleaningCleaning process with potential impact on LCF, but that post-cleaning shot peeningShot peening can address this effect. Studies also showed that more aggressive cleaning solutions may be possible but their impacts on surface condition must be carefully assessed. Finally, it was shown that ultrasonic energy can improve the productivity of the cleaning solutions but must be introduced with proper care to prevent unintended resonance vibration risk to the part.

The HIPHot Isostatic Pressing (HIP) (Hot Isostatic Pressing) +  ITF (IsoThermal Forging)IsoThermal Forging (ITF) process was carried out for SS-PREP® (Super Speed Plasma Rotating Electrode Process) powder of nickel-based superalloy FGH4097. Powder made via SS-PREP® shows fine and uniform particle size, high sphericity and purity. The HIPed compact has good hot workability during isothermal forging to obtain refined grains. The heat treatment tests were performed at different solution temperatures, and mechanical properties were measured. Compared with the established as-HIP process, SS-PREP®+ HIP + ITFIsoThermal Forging (ITF) with 1180 °C solution heat treatment resulted in higher strength, ductility, and low cycle fatigue (LCF) life, with acceptable increase in accumulated creep strain.

The heat-treatment of a second generation single crystal Ni-base superalloy was implemented in a hot isostatic press providing fast quenching rates. Thus, it is possible to homogenize chemical heterogeneities, close porosity, and to set a fine and uniform γ/γ′-microstructureMicrostructure via fast quenching and subsequent aging in one processing step. The microstructural evolution in dependence of parameters such as temperature, pressure, and quenching is investigated on different length scales using diverse characterization methods. A virtually defect-free microstructureMicrostructure is the outcome of this unique integrated supersolvus HIPHot Isostatic Pressing (HIP) heat-treatment.

In order to investigate the applicability of a direct and complete recycling method to commercial-scale ingot production, 3 tons of a Ni-base single crystalNi-base single crystal superalloys (SC) superalloy, TMS-1700(MGA1700) was melted and desulfurized by CaO during the melting. Sulfur content in the molten alloy was reduced from about 23 ppm to about 2 ppm within 60 min after adding granular CaO to the molten alloy. Microstructural observations using SEM and EPMA showed no presence of inclusions caused by CaO addition. CreepCreep rupture lives of the recycleRecycle-simulated TMS-1700 were equivalent to that of the standard TMS-1700 under the conditions from 800 °C/735 MPa to 1100 °C/137 MPa. The recycleRecycle-simulated TMS-1700 exhibits even better oxidation resistanceOxidation resistance compared with the standard TMS-1700, which has better oxidation resistanceOxidation resistance than CMSX-4TM. Thus, it became clear that the CaO desulfurizationCaO desulfurization improves the oxidation resistanceOxidation resistance of Ni-base superalloysNi-base single crystal superalloys. High cycle fatigue (HCF) properties of the standard and the recycleRecycle-simulated TMS-1700 were equivalent. From the results described above, it has been suggested that the application of direct and complete recycling method to commercial-scale ingot production is feasible.

Ring-rolling has proven notoriouslyAdziman, Fauzan difficult to model, control, andTakai, Ryosuke optimise accurately. We have studied a multi-objective optimisation for the manufacturability of aero-engine turbine disc by integrating (1) geometrical constraint, (2) alloy design, and (3) process parameters. AccurateTang, Yuanbo T. computational models based on accurate experimentation which allow for knowledge-based choice of theIshikawa, Shigehiro manufacturing variables represent an important step towards an optimised ring-rolling process. Optimal ring-rolling formability is found by (1) minimising the adiabatic heating effect and stress intensityBarba, Daniel feature (geometrical constraint); (2) developing a new C&W M647 nickel-based superalloy with an optimal ring-rolling formability at temperatures from 950 to $$1050\,^\circ $$ 1050 ∘ C at strain rates between 0.1 /s and 0.001 /s, whilst enduring a good formability between 850 to $$950\,^\circ $$ 950 ∘ C and 1050 to $$1100\,^\circ $$ 1100 ∘ C over strainAlabort, Enrique rates between 10/s and 0.1/s (alloy design); and (3) implementing a feedback framework to control and optimise the process parameters. Within the optimal regime, mechanical behaviour is characterisedNemeth, Andre by stable and homogeneous deformation. The findings are used to construct a processing map, on which the dominant deformation mechanismsKanno, Naoya are identified and used to develop a high-fidelity numerical tool, deducing a range of safe operating windows for an optimal crack-free result using a reduced processing time. The resultsReed, Roger indicate a major improvement of ring-rolling manufacturability by cutting more than 80 per cent of processing time whilst maintaining a crack-free condition.

In the present work, the novel high-resolution X-ray diffractionX-ray diffraction imaging technique for imaging misorientationMisorientation and mosaicityMosaicity in single crystalSingle crystal superalloys is introduced. The technique is based on classical X-ray diffraction topography geometries combined with high-resolution diffraction and post-processing of obtained data including color coding and 3D projections of each diffraction images. For the investigations, the single crystalSingle crystal rods were produced from CMSX-4 superalloy at a withdrawal rate of 1, 3, 5, and 7 mm⋅min−1. The high spatial and angular resolution of the method allows to visualize the complex nature of mosaicityMosaicity present in single crystal superalloys. It was observed that mosaics blocks differ in size and misorientation on the multi-scale level from the small present in dendrite arms, through single dendrites, group of dendrites, up to subgrains. Measurements of misorientation were done on cross and longitudinal sections of the castings. It was proved that increasing withdrawal rate influences the mosaicity structure and mechanisms of its evolution. Solidification at withdrawal rates from 1 to 5 mm⋅min−1 possesses higher misorientation between dendrites. With higher misorientationMisorientation between dendrites, when the widening of solidification front occurs, the mosaicityMosaicity spreads across the casting in the form of misoriented dendrite lines as the solidification progress by growing long secondary dendrite arms. Also, it was stated that 3–5 mm⋅min−1 rates possess a higher possibility of creation of subgrains or selector grains defects in the casting.

The present study assessed the mechanical propertiesMechanical properties and metallurgical features of a new solid-solution Ni-based alloy called Inconel 680, which has been developed for high strength applications, including dissimilar weldingWelding for high strength steels. Steel plates were buttered with Inconel 680, to prevent any dilution with the steel. In sequence, the buttered steel plates were joined. Samples from the test plates were extracted for chemical composition analysis, metallography preparation, uniaxial tensile testing, and hardness and microhardness evaluations. Microstructural characterization was performed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results of the mechanical propertiesMechanical properties showed that the Inconel 680 alloy filler metal provides an excellent mechanical strength, including a high yield strength, with an average value superior to 600 MPa, and good ductility, reaching an elongation of at least 40%. In addition, this study provides valuable insight into the microstructureMicrostructure of the Inconel 680 alloy. The overall evaluation indicated that this filler metal is an excellent option for many dissimilar weldingWelding applications, such as API 5L X65 pipes internally cladded with Inconel 625.

Nickel superalloys are used to manufacture high temperature rotating engine parts such as high pressure discs in gas turbine engines. Alloy 720LiAlloy 720Li (720Li) is a precipitation-hardened Ni-based superalloy commonly produced by cast and wrought processes. Conventional ingot-to-billet conversion is an expensive and very complex process, requiring multiple open die forging operations and reheating steps in order to achieve a homogeneous microstructure. The present work studies the microstructural evolution of 720Li billet material with the presence of large unrecrystallized structures. The interaction of γ′ precipitates(’ precipitates with recrystallizationRecrystallization during hot forging at subsolvus temperatures was investigated. Double truncated cones were forged at subsolvus temperatures following two forging approaches: single blow and double blow with an intermediate heat treatment. Combined EBSD-EDX analysis was employed to characterize the microstructural evolution of 720Li during hot forging operations. Primary γ′ precipitates(’ precipitates promote hetero-epitaxial recrystallizationRecrystallization during slow cooling, whereas secondary precipitates, formed during slow cooling are not dissolved during reheating, prior to the forging operations. The presence of undissolved secondary γ′ promotes strain accumulation and the occurrence of continuous dynamic recrystallizationRecrystallization (CDRX). Intermediate heat treatment plays an instrumental role on the recrystallizationRecrystallization behaviour for the alloy 720LiAlloy 720Li. Dissolution of the secondary γ′ precipitates(’ precipitates results in a strong preconditioning of the 720Li microstructure prior to the second blow of deformation, promoting the formation of fully recrystallized structures and removing the undesired large unrecrystallized regions. The coalescence of intragranular γ′ precipitates(’ precipitates into clusters of coalesced γ′ precipitates represents a clear transition from the apparent unrecrystallized regions to the fully recrystallized structures.

The need of developing new high temperature materials has increased significantly in the last decades owing to the demand of higher engine operating temperatures. This requires improved microstructural stability of the polycrystalline nickel-based superalloys used for turbine disks. The microstructure of VDM Alloy 780 consists of γ′ strengthening precipitates in addition to the needle/plate-shaped particles (of δ and/or η phase) to pin the grain boundaries. The present article aims at discussing the recrystallization behavior of the new VDM Alloy 780 in the supersolvus domain. Both dynamic and post-dynamic microstructuralDynamic and Post-dynamic recrystallization evolutions are reported. The dynamic recrystallization (DRX) kinetics was found to be rather sluggish. For a plastic strain of 1.3 at 1050 °C applied at a strain rate of 0.01 s−1, the microstructure of VDM Alloy 780 is only 50% recrystallized. The DRX grain sizes are quite close for the two applied strain rates—0.01 and 0.1 s−1. Despite slow DRX kinetics, the fast post-dynamic evolutions allowed to achieve fully recrystallized microstructures with a grain size of 26 μm within 5 min of post-deformation holding at 1050 °C. The post-dynamically recrystallized grain sizes were predominantly temperature dependent and were not sensitive to strains and strain rates within the applied range. Deformation followed by 5 min holding at temperatures below 1050 °C but still in the single-phase domain could eventually generate finer grain sizes (<20 μm). The low sensitivity of the grain size obtained after post-dynamic evolution to the applied strain and strain rates is an advantage for the industrial forging routes wherein the deformation conditions can vary over the piece. Another advantage of this alloy is the relatively slow grain growth kinetics which makes possible to obtain homogenous and reasonably fine microstructures after supersolvus forging.

The major part of all material and microstructural data used for the modelling of nickel superalloy forgings is obtained from uniaxial laboratory tests with limited plastic strain and very simple thermo-mechanical history. At the same time, new challenges in near net shape industrial forging require a high level of reliability of modelling prediction of metal flowMetal flow , for predicting the risk of defects and microstructural transformation. A few recently conducted benchmarking studies have shown that despite the availability of various material models (including microstructural ones) embedded in commercial FE software, in many cases, the level of prediction remains unsatisfactory. This is especially true for fast industrial forging processes (like screw press or hammer forgings). This paper suggests a methodology for processing the results from industrial forgings for obtaining robust data for calibration, validation, and improvement of material and microstructural models. This also can provide additional information on the material science behind the microstructural phenomena, which are problematic to capture and study using simple uniaxial tests.

ThermomechanicalCharpagne, M. A. processing routes areStinville, J. C. used to produce microstructuresPolonsky, A. T. that minimize plastic strain localization at the sub-grain scale in aEchlin, M. P. polycrystalline $$\gamma -\gamma $$ γ - γ ’ nickel-basedMurray, S. P. superalloy. This novel approachChen, Z. is made possible by the useBozzolo, N. of innovative experimental tools and statistical data analysis that captureCormier, J. slip events over large representative fields of view. Results are correlated to conventional observations of fatigue crack initiation and early stage of propagation. The effect of coherentValle, V. twin boundaries and primary $$\gamma '$$ γ ′ precipitates onPollock, T. M. fatigue properties and plastic localization is detailed.

M647, a Ni-based superalloy with excellent mechanical properties and good hot deformability, was recently developed for application in airplane engine disks. In airplane engines, fine-grained superalloys are required to improve high-temperature fatigue properties. Methods to control fine grains have been extensively studied, including a refined method of applying the pinning effect of a coarse γ′ phase. However, previous reports focused on the mechanism of nucleation and abnormal grain growth, and reports on modeling to predict grain size are rare, making it difficult to optimize the forging process. The present study proposes a modeling method of M647 recrystallization with a coarse γ’ phase and compares modeling and experimental results. Recrystallization was experimentally observed in M647 with a coarse γ′ phase originating from the grain boundaries of the prior γ phase. Moreover, recrystallization is promoted by heating at a high temperature, applying a high strain, and maintaining the heat for a long duration. Grain growth is restricted by the pinning effect of the coarse γ′ phase. The area fraction of the coarse γ′ phase changed with the heating temperature, and the γ′ grain size increased with heating. Electron backscatter diffraction analysis shows that the kernel average misorientation increased with increasing forging temperature. These trends indicate that the pinning and driving forces of recrystallization fluctuate continuously during forging and reheating. The microstructure is predicted by applying Avrami-type equations, but the accuracy is insufficient because the fluctuation effects are not considered.

A process–microstructure–propertyProcess–microstructure–property relationship database on forgingForging and heat treatment of superalloy 720Li was established by high precision large-scale 1500 ton forgingForging simulator and laboratory-scale forgingForging simulator. The database was utilized to determine the parameters of flow stress, microstructure, and strength prediction models. The models were integrated to CAE software to predict process–microstructure–property relationships. In the integrated model, the stress, strain, and temperature distributions and their temporal development are calculated by using flow stress model and thermophysical properties. The calculated stress, strain, and temperature data are inputted into the microstructure model. The microstructure model considers grain growth, recrystallization, and precipitation of γ′ and calculates the temporal evolution of microstructural features. The strength model considers solution, grain boundary, and precipitation strengthening and calculates high-temperature 0.2% tensile proof stress, which is related to creep and low-cycle fatigue properties, from the calculated microstructural features. The integrated model successfully predicted the load, microstructure, and strength distribution of a prototype forgingForging experiment conducted by the Hitachi Metals 6000 ton forgingForging machine. The integrated model is a promising tool to design the forgingForging and heat treatment process of the alloy.

The goal of an agingAging heat treatment in Ni-based superalloys is to control the volume fraction and size of the strengthening precipitates. We predict the precipitate size evolution of primary L12 and D022 precipitates in the industrially relevant alloys CMSX-4, René80, and IN718 using a one-dimensional phase-field model with an artificial Gibbs–Thomson driving force considering the shape of non-spherical precipitates. In comparison with the classical theory of precipitate ripening, the presented phase-field model considers all alloying elements, non-isothermal cooling, and heating stages and off-equilibrium volume fraction. We implicitly consider elastic effects between precipitate and fcc solid solution matrix by adjusting the mobility parameters and considering size dependent non-spherical D022 precipitate shapes. Literature data reveals the strong predictive potential of this multi-scale method for integrated computational materials engineering. We identify individual agingAging stages during which precipitate growth is dominated by either ripening or precipitation from the supersaturated matrix phase.

The characteristic flow behaviour of the γ + γ′ duplex structure during the thermomechanical processing (TMP) of highly alloyed disc alloys was comprehensively investigated. Based on the TMP of the γ + γ′ duplex, a series of promising techniques for the billet conversion and microstructure customization can be developed. The superplasticitySuperplasticity of γ + γ′ duplex was confirmed by tensile tests, and the optimum combination of temperature and strain rate was determined. For a certain alloy, the maximum elongation up to more than 1000% can be achieved at the temperature of equilibrium γ′ solvus minus 100 °C. Near net shape forgings can be produced within this temperature region. During conversion of the VAR ingots, a proper TMP can trigger a discontinuous precipitation instead of the dynamic strain ageing effect which usually leads to a drastic degradation of plasticity. Therefore, the processing window of ingot conversion can be greatly broadened. Producing the dual microstructure disc forging using TMP from a γ + γ′ duplex billet exhibits the inherent advantages over standard heat treatment techniques. The grain size of coarse-grained region can be continuously modulated by controlling the local plastic strain, and the location of the interface between the coarse-grained region and fine-grained region can be precisely determined by controlling the strain distribution. The multi-cycle TMP techniques can play a constructive role in the conversion of the as-HIP billets of P/M material. By producing the γ + γ′ microduplex, prior particle boundaries (PPBs) and inclusions can be efficiently broken up during TMP. As a result, the hot workingHot working ductility and the quality of the ultrasonic inspections can be greatly improved.

The occurrence of abnormally large overgrown grainsOvergrown grains upon sub-solvus annealing of certain hot-worked structures has been reported for several cast and wrought and powder metallurgy superalloys. These overgrown grains typically feature a high density of annealing twins and a precipitate distribution similar to the one in the adjacent fine-grained areas. While these special hot working conditions can be established experimentally for a given alloy, it is desirable to provide some general guidance for the avoidance of these features. Following a recently published framework by Bozzolo’s group in France, i.e., Zener pinningZener pinning pressure versus stored energyStored energy driving force, abnormal grain growth (AGG) maps were generated for two commercial HAYNES alloys. Thermo-mechanical processing paths resulting in AGG in these alloys are studied in detail. The generality and limitations of aforementioned framework are discussed.

Alloy 720Li with a polished surface and with a 1000% coverage shot peened surface has been tensile tested at 200–650 °C and two loading rates. Shot peened specimens had lower ductilityDuctility than unpeened specimens across the temperature range, but the effect was greatest at 450 and 500 °C. This reduction in ductility resulted in reduced tensile strength (TS)Tensile Strength (TS) or fracture stress at 300-500 °C. The shot peeningShot peening effect was present at both loading rates. Shot peened specimens showed surface micro-cracking all over the gauge length of tested specimens and at the point of fracture which explains their reduced ductility. The micro-cracks started at the surface and propagated along strain bands in the material. In contrast, unpeened specimens showed no surface micro-cracks. Interrupted testing on peened surfaces at 500 °C 300 MPa/sec showed that the micro-cracks formed first at 2.3% plastic strain. The application of further strain caused new cracks to form and existing cracks to grow deeper until the specimen fractured at ~9% elongation. Further specimens were tested with different peening coverage levels of 125%, 200%, 400%, 600%, and 1000%. Peening coverage of 125% caused a large reduction in ductility and small reduction in TS relative to the unpeened state. Higher coverage caused further reductions in ductility and TSTensile Strength (TS) . The reduced ductility of shot peened specimens is attributed to the low ductility of the shot peened layer which is heavily strained and work hardened.

Fine microstructural analyses have been performed to identify the microstructural mechanisms controlling stress relaxationStress relaxation during aging heat treatment of AD730TM disk superalloySuperalloys. Morphological evolution of the hardening γ′ precipitates and plastic activity occur during relaxation tests. For a 500 MPa initial stress, the relaxation test shows atypical behavior with sluggish relaxation in the first hours and then a faster one. To understand this atypical behavior, isothermal dilatometry tests were used to decouple the effects of stress and temperature. The latter revealed a contraction of the specimen when subjected to a constant temperature. This contraction induces a tendency for an increase in stress during the relaxation test to meet the imposed condition of constant total deformation. Relaxation is then controlled by the competition between the classical relaxation mechanisms (vacancy diffusion and/or dislocation gliding) which tend to lower the stress and the contraction of the specimen which tends to increase the stress during the test.

Performing the double aging thermal treatment on Inconel 718Inconel 718 forged components directly after forging instead of the standard thermal treatment sequence including a solution annealing step before the double aging is known to be beneficial to mechanical properties. In this work, this so-called direct agingDirect aging (DA) effect has been assessed for tensile propertiesTensile properties at room temperature and for creep propertiesCreep properties at 650 °C/750 MPa. Mechanical characteristics obtained on direct aged specimens are compared to those obtained on specimens submitted to the standard thermal treatment sequence (i.e., including a solution heat treatment) and tested in the same conditions. The classical effect of direct agingDirect aging thermal treatment is obtained; better tensile and creep propertiesCreep properties are reached for DA specimens. However, contrary to what is generally assumed, fine microstructural characterizations in specimen threads reveal that the direct agingDirect aging effect cannot be attributed to a higher residual strain hardening level preserved in the microstructure. It is actually shown that its beneficial effect on mechanical properties is due to a lower δ phase content obtained in microstructures having undergone such a treatment rather than the standard sequence. On the one hand, it has indeed been demonstrated that, counter-intuitively, the δ phase content actually increased during the “solution” treatment performed at 955 °C for 1 h. On the other hand, the relationship between mechanical properties and microstructural features (grain size, δ phase content, residual forging strain hardening level) has been analyzed in detail, and differences in mechanical behavior are mainly controlled by this increased amount of δ phase which deteriorates the mechanical properties. On the contrary, no significant effect of residual forging strain hardening on tensile and creep propertiesCreep properties has been found for the investigated conditions.

The mechanical properties of nickel-based superalloys depend strongly on their microstructure, namely the grain size and the state of precipitation. Main design criteria in aeronautical turbine disks are the resistance to disk burst and low cycle fatigue in the bore, but also to creep in the rim part due to higher temperatures. The chosen microstructures result from a compromise between these contradictory requirements. Indeed, creep durability is improved using a coarse grain microstructure while the increase of static and fatigue strength requires a fine grain microstructure. Moreover, the volume fraction and the size distribution of γ′ precipitates are the predominant parameters controlling mechanical properties at lower temperatures. A spatial optimization of the microstructure is reachable using specific technologies, e.g. dual-microstructure heat treatment. The development of microstructure-sensitive models is thus a major concern for the optimal design of these components including gradient of grain size and/ or precipitate size. Full-field finite element simulationsFinite elements simulations may be employed to predict the macroscopic behavior of polycrystalline aggregates using crystal plasticityCrystal plasticity constitutive equations whose parameters depend explicitly on the microstructural attributes. In this framework, the present study is devoted to the evaluation of the yield stress of polycrystalline AD730TM nickel-based superalloy, chosen as a model material. This work includes microstructural characterization and mechanical tests carried out on single crystals and polycrystalline specimens with well-controlled microstructures. Predictions of the macroscopic yield stress are provided by preliminary simulations carried out in the elastic regime combined with a specific post-processing.

The stress relaxationStress relaxation behavior of two experimental and one commercial powder-processed Ni-base were assessed using a servo-hydraulic frame under strain control at 700 °C. In addition to quantifying the effect of composition on the stress relaxationStress relaxation behavior, the effect of microstructure and initial strain were also evaluated. The magnitude and rate of stress reduction for the various samples was measured during testing and an apparent activation model was used to normalize the magnitude of the stress drop with the initial stress. Stress relaxationStress relaxation tests with an initial strain of 0.6% exhibited characteristic behaviors that could be correlated to alloy chemistry and microstructure. The extent of stress relaxationStress relaxation in highly alloyed RRHT5P samples possessing high volume fractions and a high number density of intragranular γ′ precipitates was limited. Although P additions were not observed to exert any significant effect, processing of these alloys with lower cooling rates from solution to coarsen the γ′ precipitates was shown to effectively increase the degree of stress relaxationStress relaxation. Reducing the degree of alloying and maintaining a lower overall fraction of γ′ precipitates effectively confers a higher degree of stress relaxationStress relaxation at 700 °C. Stress relaxationStress relaxation testing and the application of an apparent activation volumeActivation volume model may be effectively used for characterizing the notch sensitivity and crack growth behavior of high temperature structural materials.

The non-metallic inclusions (NMIs)Non-Metallic Inclusions (NMIs) population of two versions of AD730™ (the standard one and a high carbon-doped one) alloy were studied in terms of oxidationOxidation and low-cycle fatigue behavior at 450 and 700 °C. Cracking of NMIs was observed in the early stages of oxidationOxidation without any applied load, even at intermediate temperature. It is assisted by volume expansion and thermal mismatch between the NMIsNon-Metallic Inclusions (NMIs) and the surrounding γ/γ′ matrix. It was observed that the cracking took place in the Nb-rich part of the inclusions. However, the presence of pre-cracked inclusions does not have a detrimental effect on the fatigue lifetimes. The mechanical behaviors and mechanisms were investigated. Moreover, addition of carbon may lead to a debit in LCFLCF life depending on the loading direction with respect to the NMIs cluster alignments. The observation of the fracture surfaces showed that no cracks initiated within inclusions in the high carbon content material despite a much higher density of NMIsNon-Metallic Inclusions (NMIs). The inclusions and grain size distributions, the particles alignment orientations as well as the environment strongly contribute to the crack initiationCrack initiation mechanisms.

The high temperature dwell fatigueFatigue crack growth behaviours of 30% cold-drawn and direct-aged ATI 718PlusTM718Plus were studied. This was compared to an annealed and direct aged variant of the same material. This was in order to rationalise the effects of cold work. Tests were conducted at 650 °C using different dwell times, in air and in relative vacuum, at R = 0.1. It was found that the cold-worked and direct-aged condition produced slower dwell fatigueFatigue crack growth rates in all tests. This has been linked to the larger elongated grain morphology and the dislocation structure retained after cold working. Increased crack tortuosity and roughness-induced crack closure are considered to enhance the cycle-dependent crack growth properties. Superior resistance to environmentally assisted intergranular failure is related to greater crack tip stress relaxation and more homogeneous grain boundary deformation.

TEM characterization of the deformation micromechanisms in the case of AD730TM disk superalloy has been performed in order to identify the relevant parameters controlling its creep behavior at 700 °C under 600 or 850 MPa. The creep behavior has been investigated for different microstructures resulting from different heat treatments: a coarse grain (CG) and a fine grain microstructures (FG). The specific influence of the primary γ′Primary γ′ precipitates precipitates, which are only present in the fine grain microstructure, is of main focus. TEM observations indicate that, in the first stage of the creep deformation, primary γ′ precipitatesPrimary γ′ precipitates may act as dislocation sources. The stability of this phase was confirmed using samples aged at 850 °C for several hundreds of hours. TEM spectroscopy has been used to characterize the local chemical composition after aging. A clear evolution of these primary γ′ precipitatesPrimary γ′ precipitates has been evidenced and a dissolution of the secondary γ′ precipitates during aging. The presence of these primary γ′ precipitatesPrimary γ′ precipitates induces a strong localization of the deformation. Its detrimental effect on the creep properties in the case of polycrystalline Ni-based superalloy at high temperature may be concluded.

The ultra-high strength of multiphase (MP) alloys for fastener applications is endowed by cold deformation, which induces deformation twinning or phase transformation in the materials. In the present work, MP159 alloy as well as NiCoCr medium-entropy alloy are investigated to assess the effectiveness of a torsional pre-straining as well as surface mechanical grinding treatments to further improve their strength. For the pre-torsion route, microstructural analysis shows that with the activation of different twinning systems and stacking faults, the sequential torsionTorsion and tension tests lead to the observed hierarchical microstructure, which gives rise to the significant increase in the yield strength in the investigated alloys while retaining a good ductility. Further studies reveal that aging treatment of the pre-torsion bars provides additional strengtheningStrengthening to MP159 alloys, with the synergistic strengtheningStrengthening of nanotwinsNanotwins and nanoprecipitates. After the surface mechanical grinding treatment of NiCoCr alloys, a nanocrystalline structure is formed in a region extending about 150 μm from the edge. Considering the high applied strains, a high density of deformation twins is expected in the nanocrystalline region. Micropillar compression of single crystal and nanocrystalline NiCoCr alloy shows that with the refinement of grain size from 200 μm to 40 nm, the yield strength could increase from 900 to 2500 MPa. This reveals that the refinement of grain sizes can significantly increase the yield strength of NiCoCr alloys with a low stacking fault energy.

Non-metallic inclusionsNon-metallic inclusions (NMIs) (NMIs) and slip bands parallel to and slightly offset from twin boundaries are observed to be preferential sites for fatigueFatigue crack nucleation in wrought superalloys. Potential interactions between NMI cracking and slip activity within neighboring grains or at twin boundaries were investigated under monotonic tensile loading (up to 1.3% total strain) at room temperature. High resolution- and Heaviside-digital image correlation measurements were performed during interrupted tensile loading to identify strain localization, associated slip systems, and damage initiation. Different mechanisms and scenarios were identified: (1) Microplasticity generally starts at twin boundaries even at stresses as low as 70% of the macroscopic yield strength, (2) transgranular slip activity intensively develops above the macroscopic yield stress, (3) intense slip activity develops near and parallel to 21% of the twin boundaries intercepting NMIs, (4) 7% of the twin boundaries intercepting NMIs lead to slip-assisted NMI cracking, (5) no transgranular slip activity participates in NMI cracking, (6) the fraction of cracked NMIs progressively increases with the load, and (7) within the NMIs that initiated cracks, 67% cracked below 90% of the macroscopic yield strength without the presence of slip activity in the neighboring grains. While slip-assisted NMI cracking was evidenced in the present study, most NMI cracking is due to strain incompatibility between NMIs and neighboring grains at the high end of the elastic regime without slip interaction.

This work investigates two nominally similar polycrystalline alloys, with a subtle difference in Nb content, intended to elucidate its effect on local phase transformationLocal phase transformation strengthening during high temperature creepCreep . Tests were conducted at 750 °C and 600 MPa to target the creep regime dominated by superlattice intrinsic and extrinsic stacking faults, as well as microtwinning. Alloy A, with higher Nb and lower Al, was found to be superior in creep strength to Alloy B, with lower Nb and higher Al, as well as previously investigated ME3 and LSHR. Atomic resolution scanning transmission electron microscopy and energy-dispersive spectroscopy found that this increased creep strength was due to a novel local phase transformation occurring along microtwinMicrotwin boundary interfaces as a result of the Nb increase. Complementary density functional theory calculations helped to confirm that this was χ phase formation. It is hypothesized that this transformation was the cause of the increased creepCreep strength exhibited by Alloy A.

Previously reported strength prediction models for polycrystalline Ni-base superalloys tend to underestimate the overall strength. This is primarily due to neglecting the rule of mixtures for hardening by γ/γ′ and inaccurate estimation of other strengthening factors such as solid solution hardening of γ matrix and grain boundary strengthening. To address these issues, a series of single-crystal tie-line modeling alloys with γ′ size suitable for validating the strong pair-couplingStrong pair-coupling and Orowan loopingOrowan loop mechanisms were prepared, in an attempt to develop more reliable γ′ hardening models. Critical resolved shear stressCritical resolved shear stress values were measured on these modeling alloys at 650 °C using compression tests, which allowed the distinction between strength contribution from rule of mixture and interfacial strength. Compared with the experimentally obtained interfacial strengths, the predicted results by classical models could not reflect the strength increment expected at higher γ′ volume fraction. Hence, a modified version of the classical equations has been proposed here for improved predictability.

A microstructurally informed crystal plasticityCrystal plasticity model was developed for polycrystalline alloy René 88DT (R88DT) at a high temperature (650 °C). R88DT is a γ′ precipitation-strengthened Ni-based superalloy used in gas turbine engine disks due to high tensile strength, superior creep resistance, and high resistance to fatigue crack growth. It was experimentally observed [1] that crack initiation in polycrystalline superalloys is highly dependent on the grain size, orientation, and microstructure, as well as on the inelastic subgrain properties. Thus, understanding the fundamental deformation mechanisms at the subgrain level is a necessary step for development of fatigue life models. For this purpose, single crystalsSingle crystal with microstructures representative of R88DT were first created by the investment casting process. Specific crystallographic orientations were mechanically tested in tension and compression to investigate dominant deformation mechanisms as activation of various slip systems, stacking faults/microtwin formation, and precipitate shearing. These mechanisms were incorporated into a physics-based viscoplasticViscoplastic model constitutive model. The model was calibrated using data from tests on single crystalsSingle crystal, and it was then applied for each grain in polycrystalline R88DT to predict the macroscopic stress–strain response as well as heterogeneous local stress and strain fields.

Alloy 718 is widely used in gas turbine engines. Even though recent studies have been focusing on the unique deformation mechanisms of the tetragonal γ″ phase as compared to those of the cubic γ′ phase, γ′ and γ″ often co-precipitate and form composite particles. The deformation mechanisms of these composite particles have not been investigated in detail. In this work, we use a combination of ab initio and microscopic phase field methods to study shearing of a dual-lobed type γ″/γ′/γ″ composite particle by various dislocations. Complicated fault structures within both γ′ and γ″ phases are predicted, and some of them have been observed in the experiment. The difficulty associated with experimental characterization of the fault structures in the co-precipitates is also discussed.

A high-throughput approachHigh-throughput approaches for collecting microstructure and mechanical properties were developed to model the process–structure–property (PSP) correlationsProcess–structure–property correlation modeling in polycrystalline Ni-based superalloy ME3. The semi-automated image processing algorithm captured the area fraction and size distribution of secondary and tertiary γ′ particles from scanning electron microscopy (SEM) images of polished samples. The yield strength and elastic modulus were calculated with an automated algorithm using load-time-displacement data generated by microindentationMicroindentation. Thirty heat treatments were conducted to create various γ′ distributions which are the primary strengthening mechanism of Ni-based superalloys. The PSP correlations among the predictor and response variables were evaluated with regression models and validated with adj-R2 and residual standard error statistics. The PSP statistical models built by using high-throughput protocols align with the previous statistical and theoretical models.

The evolution of gamma primeGamma prime precipitates generated from supersolvus solution heat treatment and controlled continuous cooling of two powder metallurgyPowder Metallurgy (PM) (PM) disk superalloys are characterized and modeled. Variation in cooling rate for these alloys shows a tendency for unstable precipitate growth with slower rates. The degree to which this variation is observed in terms of its effect on particle spacing and deformation-induced defect interaction has not been well characterized previously. To better understand this effect, a series of laboratory heat treatments of varied cooling rates have been carried out and the mechanical response quantified via elevated temperature tensile tests followed by characterization. A phase-field-based approach is used to simulate the growth instability of gamma primeGamma prime precipitates during slow continuous cooling; the simulated particle morphologies are compared to post-mortem characterization of laboratory-scale coupons using Focused Ion Beam (FIB) serial sectioning and volumetric reconstruction. Phase-field modeling is then used to interrogate the interaction of the particle morphology with planar dislocationDislocation evolution. It was determined that the incipient stages of particle evolution are dictated by interface growth instability more so than elastic anisotropy effects. Planar deformationDeformation in the presence of these larger, more evolved particles tended to promote Orowan looping, while smaller particles with smaller gamma channel widths showed a tendency for stacking fault formation under the conditions characterized. These observations suggest that particle morphology is a secondary effect to the overall alloy strengthening mechanism.

Steam turbine and boiler materials for Advanced Ultra-Supercritical (A-USC)Advanced Ultra-Supercritical (A-USC) coal-fired power plants are required to withstand steam conditions up to 760 °C and 35 MPa for a typical service life of over 100,000 h. Long-term creep evaluation of these materials is challenging and relies on extrapolation of data from accelerated tests under steady-state conditions and increased temperatures and/or stresses. This study is focused on improving the accuracy of creep property predictions by using a microstructurally informed constitutive creep modelCreep modeling. The model was recently developed within the framework of continuum damage mechanics (CDM) for γ′ precipitation strengthened alloys Haynes 282Haynes 282, Inconel 740HInconel 740H, and Nimonic 105Nimonic 105 at stresses between 100 and 400 MPa and temperatures between 700 and 850 °C. The model incorporated physical mechanisms including (1) growth of micro-voids and cracks to macroscopic cavities, (2) accumulation of dislocations toward excessive inelastic deformations, and (3) microstructural changes (e.g., γ′ coarsening). The model was validated by comparing creep strain evolution in time and rupture times to data from uniaxial creep tests. As damage mechanisms are captured by specific damage variables, the model aims at making predictions beyond the secondary creep regime, well into the tertiary regime by tracking the evolution of the damage variables, eliminating the typical need of linear extrapolation.

The stress-induced variant selectionStress-induced variant selection (SIVS) of γ″ phase caused by thermo-mechanical couplingThermo-mechanical coupling condition is studied systematically. The SIVS index f is devised and utilized to quantify the degree of SIVSStress-induced variant selection via SEM image post-processing within <001> oriented grains. Applied stress condition does not affect the volume fraction, average size, and crystallographic orientation of the γ″ but changes the proportion of the three variants. The SIVSStress-induced variant selection behavior of γ″ phase is dependent upon the applied stress direction and grain orientation. By considering the variation of the SIVSStress-induced variant selection index with thermal aging and creep parameters, a causal relationship between degree of SIVSStress-induced variant selection with temperature, stress, and time is elucidated. The yield strength and tensile strength at 650 °C decrease with the increase of SIVSStress-induced variant selection degree. Contour map for the degree of SIVSStress-induced variant selection is presented to evaluate the service temperature and stress of an Inconel 718Inconel 718 component. The formation mechanism of SIVSStress-induced variant selection of γ” is proposed: the orienting effect of stress condition is appreciable only during Ostwald ripening, due to the change of the γ″/γ lattice mismatch.

A new disk superalloy has been developed by NASA to improve high-temperature creep performance utilizing the recently discovered local phase transformation strengthening mechanism. Creep tests were performed at 760 °C and 552 MPa, to approximately 0.3% plastic strain, a regime where the formation of γ′ shearing modes such as superlattice extrinsic and intrinsic stacking faults are active. The new alloy exhibited superior creep performance over the current state-of-the-art superalloys, ME3 and LSHR. High-resolution characterization confirmed the formation of the strengthening η phase along superlattice extrinsic stacking faults and χ phase along superlattice intrinsic stacking faults. In addition, creep deformation analysis via scanning transmission electron microscopy appears to show a significant reduction in microtwin formation as compared to LSHR and ME3. This improvement in creep performance was also accompanied by an improvement in both room temperature and high-temperature strength.

The addition of an air plasma sprayed (APS) “flash” layer on top of a high velocity oxygen fuel (HVOF) bond coating has been shown to extend the lifetime of thermal barrier coatingsThermal barrier coating . A series of furnace cycle testsFurnace cycle testing (FCTs) has been conducted at 1100 °C in air + 10% H2O to study the benefit of flash coatings on rod and disk alloy 247 specimens and provide a better mechanistic understanding of their benefit. Flash coatings of NiCoCrAlY and NiCoCrAlYHfSi both improved the FCTFurnace cycle testing lifetime of rod specimens tested in 100-h cycles and disk specimens tested in 1-h cycles. In 1-h cycles, the NiCoCrAlY flash coating significantly outperformed an HVOF-only NiCoCrAlYHfSi bond coating and a NiCoCrAlYHfSi flash coating. Both flash coatings increased the bond coating roughness compared to HVOF. During exposure, the flash layer formed an intermixed alumina-metal layer that appeared to inhibit crack formation. Using a time series of observations, the lower Y + Hf content in the Y-only flash coating appeared to reduce Al consumption. The HVOF layer acted as a source of Al for the adjacent mixed zone. A second series of specimens included a fully APS bond coating where oxide had penetrated through the entire coating to the substrate after only 100, 1-h cycles and lifetime was similar to an HVOF-only bond coating. The inner HVOF layer with the outer APS flash coatingAPS flash coating prevented this complete penetration from occurring.

Hot corrosionHot corrosion behavior at 700 °C (Type IIType II) and 900 °C (Type IType I) and creepCreep property from 800 to 1100 °C were investigated for representative alloys from the first generation to the sixth generation of Ni-base single-crystalSingle crystal superalloys. From the metal loss in the corrosion test and creep rupture life, it is found that corrosion resistance and creep strength are improved with generation advances. Regression analysis was performed to the results of the corrosion test at both 700 and 900 °C. Equations for predicting corrosion characteristics were constructed, and alloying elements contributing to corrosion resistance were estimated.

Although evidence exists of the potential impact of stress, co-incident with corrosive environments at high temperature, for single-crystal turbine blades, the mechanism responsible is not fully understood. This work explores the effect of CaSO4, Na2SO4 and sea salt on the scale formation and crack initiation of CMSX-4CMSX-4 at 550 °C in 50 ppm of SO2 and synthetic air under a static stress of 800 MPa. The cross-sectional analysis showed that the CaSO4 and the Na2SO4 salted specimens did not undergo a significant degree of corrosion degradation, and no cracks were detected after 400 h of exposure. However, sea salt caused significant degradation to the scale and cracks were detected by X-ray CTX-ray Computed tomography scanning after 400 h of exposure. The findings from this study suggest that the sulphation of chlorine-containing species in sea salt led to the formation, vaporisation, and re-oxidation of metal chlorides, and this mechanism was found to play a key role in the formation of a non-protective scale. An active oxidation mechanism has been proposed to interpret the results. In conclusion, it is hypothesised that due to the synergistic effect of stress and the formation of a non-protective scale, fast diffusion paths for sulphur, oxygen, and chlorine ingress were formed. Further work is currently being undertaken to understand the effect of these species on the local embrittlement of CMSX-4CMSX-4 that ultimately led to the initiation of cracks in the specimen.

The great potential of Rapid Thermal Annealing (RTA)Rapid Thermal Annealing (RTA) , a widespread technique in semiconductor technology, for studying high-temperature properties of superalloys is demonstrated. As an application example, early stages of high-temperature oxidationOxidation of Co/Ni-base superalloysCo/Ni-base superalloys are investigated. Model alloys with different Co/Ni ratio are oxidized in synthetic dry air at 900 °C to understand the differences between the oxidationOxidation behavior of Ni- and Co-base alloys. Upon short exposure times (below 64 s), the two-phase γ/γ′ microstructure has a pronounced influence on both the morphology and growth kinetics of Al2O3 precipitates. Depending on the base element, Al2O3 nucleates in either the γ′ phase in the form of needles (Ni-base superalloy) or in the γ matrix as lath- and plate-like precipitates (Co-base superalloy) with the latter showing much slower formation kinetics. Observed differences in oxidationOxidation behavior can be directly correlated to changes in the partitioning of W, which acts as a γ former in Ni- and a γ′ former in Co-base superalloys. We propose that a significant amount of W in γ′ has an inhibitory effect on the diffusionDiffusion of Al, suppressing the formation of Al2O3 in the γ′ phase of Co-base superalloys. Our approach proved to be very successful for oxidationOxidation studies and opens up new opportunities for superalloys research.

Gas turbines blades are required to operate at high temperatures while being subjected to stress and corrosive environmentsCorrosive environments . These demanding conditions have led to the need to better understand the interactions between corrosion and loading in order to improve lifing algorithms used for service interval predictions. A new crack growthCrack growth measurement technique involving direct current potential difference (PD)Potential Difference (PD) has been developed for use in these harsh conditions. A good correlation between PD signal and crack area has been achieved. Estimations of the crack depth have been made based on fracture surface imaging, these experimentally measured crack depth propagation rates have been compared with Paris law predictions. A stress intensityStress intensity factor (SIF) interaction between multiple cracks was found, where the SIF is enhanced when cracks become close. It was found that both the fatigue cycle rate and the crack shape appear to influence the SIF magnitude and the crack depth at which specimens fail, or initiate into crack propagation which is consistent with fatigue.

Additions of reactive elements are known to improve the oxidation resistance of alumina-forming materials. The critical thresholds for the reactive element to be beneficial are however dependant on the overall metallurgical environment. Therefore, this paper investigates the influence of increasing amounts of Hf on the cyclic oxidationCyclic oxidation behaviour of AM1 first generation single crystal Ni-based superalloy at 1150 °C in air. Insufficient doping and overdoping effects are clearly identified. The former occurs because of insufficient blockage of cation outward diffusion to establish and maintain protective scales of alumina. The overdoping effects arise on the one hand from the well-known internal oxidation of Hf that produces bulky oxides. On the other, an indirect effect is reported for the first time whereby the excess Hf destabilizes the Ta and Ti carbides. Therefore, Ta and Ti diffuse outwardly to form bulky and non-protective oxides that provoke the spallation of the protective layers.

This study investigated the linkages between deposit chemistry and degradation morphology of field-exposed aero-turbine parts and the associated development of effective laboratory-scale replication testing procedures. Inductively coupled plasma-optimal emission spectroscopy (ICP-OES) analysis indicated that the water-soluble deposit constituents were mainly Na-rich, Ca-rich or a mix of these, with the relative amounts having a dependence on geographic region in which a given part was mainly exposed. Degradation of the field-exposed parts was characterized using scanning electron microscopy (SEM) and revealed unconventional hot-corrosionHot corrosion attack. Microstructures included duplex oxide scales, Ca-containing oxide, internal oxidation, sulfidation, and nitridation. Degradation mechanisms elucidated by replication testing were deposit-induced selective depletions at high-temperature and low-temperature oxidationHigh temperature oxidation . Field degradation was replicated when considering the thermal dependences of both stages. Replication of a chromide-coated field part required calcium chromate liquid formation during high-temperature testing. Electron probe microanalysis (EPMA) of the coating subsurface revealed extensive Cr depletion after exposure to CaO above the calcium chromate eutectic temperature. Oxidation temperature also contributed to replicating field degradation microstructures. Specifically, matching degradation was observed after the depleted superalloy or chromide coating was oxidized in air at an intermediate temperature.

It is known that a single-crystal Ni-base superalloySingle crystal Ni-based superalloy containing a trace amount of sulfurSulfur has reduced high-temperature oxidationOxidation resistance. Our previous study has shown that melting of Ni-base superalloys in a CaO crucible can eliminate the effect of sulfur on oxidation resistance, but the mechanism was not completely clear. In this study, we finally succeeded in detecting the segregationSegregation of sulfur at the oxide/substrate interface of 6th generation single-crystal superalloys using STEM-EDS analysis and confirmed that segregation could be suppressed by melting in the CaO crucible. As a result, it was found that melting in the CaO crucible can improve the oxidation resistance while maintaining the creepCreep characteristics of sixth-generation single-crystal Ni-base superalloySingle crystal Ni-based superalloy.

A new coating has been developed for single-crystalSingle-crystal (SX)-alloy-coating system (SXS) (single-crystalSingle-crystal (SX) (SX) superalloySuperalloy/γ/γ′ bond coatBond coat (BC) (BC)/thermal barrier coatingThermal barrier coatings (TBC) (TBC)) for aircraft engine turbine blade applications. Design procedure of the new SXS, mechanical, and environment characterizations is presented and compared with other reference systems composed of AM1, CMSX-4 SLS, and CMSX-4 Plus superalloysSuperalloy with β-NiPtAl (BC) and TBCsThermal barrier coatings (TBC). Targeted coating compositions were optimized for extending TBC lifetime and avoiding secondary reaction zoneSecondary reaction zone (SRZ) (SRZ) formation while maintaining satisfactory level of oxidationOxidation resistance. In order to avoid the formation of SRZSecondary reaction zone (SRZ), the strategy chosen was to develop new γ/γ′ structural coatings close to the substrate composition. This new SXS exhibits high creepCreep resistance at high temperature compared with the other reference systems while exhibiting high oxidationOxidation resistance. Indeed, oxidation resistance of the new SXS with CMSX-4 Plus alloy (CMSX-4 Plus/γ/γ′ coating) at 1100 °C is similar to that of the other reference systems. Otherwise, oxidation resistance of the new SXS with AGAT-3 alloy (AGAT-3/γ/γ′ coating) at 1100 °C is higher than that of the other reference systems. The new coating (γ/γ′) does not generate SRZSecondary reaction zone (SRZ) even after aging or creepCreep testing at high temperature and ensures high lifetime of thermal barrier coatingsThermal barrier coatings (TBC) (TBCs) (YPSZ-electronElectron beam physical vapor deposition (EBPVD) beam physical vapor depositionPhysical vapor deposition (PVD) (EB-PVD) and YPSZ-suspension plasma sprayingSuspension plasma spraying (SPS) (SPS)). Finally, turbine blades with such a new system were successfully manufactured and will be tested in engine conditions.

Thermal barrierMaurel, Vincent coating (TBC) systems are currentlyMahfouz, Lara often tested by thermal cycling with or without temperature gradient on cylinder coupons. As a major drawback, edge effect associated with this geometryGuipont, Vincent induces large scatter in TBC life at spallation. Thus, we promoteMarchand, Basile the use of a laser shock to induce an artificial defect located at the top-coat/oxide interface. This methodGaslain, Fabrice enables firstly to monitor damage evolution by means of non-destructive methods from this defect during thermal cycling at homogeneous temperature and for burner rig testingKoster, Alain with superposed thermal gradient across the TBC. Secondly, by the knowledge of artificial defect location, an accurate 3DDennstedt, Anne reconstruction of the crackBartsch, Marion tip was performed based on serial sectioning by focus ion beam and viewing by scanning electron microscopy. Founded on these observations, a sensitivity analysis of the measurementCoudon, Florent uncertainties with respect to the energy release rate of propagating cracks and to the process zone where damage elaborates is proposed.

Oxidation resistant overlay coatingsOverlay coatings protect the underlying superalloy component in industrial gas turbinesGas turbines from oxidation attack. Rate of depletion of the Al-rich β-phase in the bond coat governs the lifetime of these coatings. The applicability of a computational method in accelerating the development of corrosion resistant coatings and significantly reducing the extensive experimental effort to predict coating lifetimes and microstructural changes in three-coated Ni-based superalloys for real operational durations (20–40 kh) was undertaken in the present study. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and electron microprobe analysis (EPMA) were employed to characterize MCrAlY-coated superalloy substrates (1483, 247 and X4) after exposure at 900 °C in air + 10% H2O for up to 20,000 h. The model predicted the longest coating lifetime for the coating on X4 substrate. Precipitation of γ′ in the coatings was correctly predicted for all three coating systems. Additionally, the model was able to predict the formation of topologically close packed (TCP)-phases in the investigated coating systems.

Fatigue cracking of nickel disk superalloyDisk superalloy fatigue specimens subjected to hot corrosionHot corrosion exposures prior to and during fatigue testing at 704 °C has been investigated. Pre-corroded notched fatigue specimens were exposed to a mixture of sulfates at 677 °C prior to fatigue testing, while in situ treated specimens were only exposed to the sulfate mixture during the elevated temperature fatigue test. Both the pre-corroded and in situ corrosion treatments resulted in a significant reduction in fatigue life compared to uncorroded specimens tested at the same temperature and stress conditions. Investigation of fracture surfaces by scanning electron microscopy and electron microprobe equipped with an energy-dispersive spectrometer allowed for the determination of fatigue crack initiation sites, as well as morphological and chemical evaluation of the alloy microstructure damaged by hot corrosionHot corrosion. Fatigue crack initiations originated from pit features and microstructural degradation caused by hot corrosionHot corrosion. For pre-corroded specimens, cracks initiated along grain boundaries near the pit depth and propagated intergranularly into the alloy. For in situ treated specimens, cracks formed near the surface oxide above growing pits and propagated into the pit as it grew, eventually propagating in a transgranular nature further into the alloy. Sulfur-rich precipitates were detected along grain boundaries below the hot corrosionHot corrosion pits of both pre-corroded and in situ treated specimens. Sulfur was also detected along crack surfaces and grain boundaries that were 50–100 μm below crack initiation sites in the in situ treated specimens.

A new γ′-strengthened Co–Ni-base superalloy was developed using a combination of computational thermodynamics and lab-scale experimental heats. Multiple 18 and 180 kg ingots were melted under vacuum and forged to study the composition–process–property relationship. Several heat treatments were developed to evaluate the balance between strength and creep resistance. In the fine-grain condition, the superalloy exhibits a yield strength of almost 1300 MPa at room temperature, 1071 MPa at 704 °C, and 766 MPa at 816 °C. In the coarse-grain condition, the creep resistance is superior to that of Waspaloy and comparable to that of alloy 720. SulfidationSulfidation and cyclic oxidationOxidation tests show better resistance to environmental damage than Waspaloy and alloy 720 due to the formation of a protective alumina layer. We also show that this superalloy can be atomized and processed via powder metallurgy. Potential applications include disks and casings in land-based and jet engine turbines, fasteners, bolts, and studs in automotive exhaust systems and exhaust valves in internal combustion engines.

Gamma-prime strengthened Co–Al–W-based superalloys offer a unique combination of weldability, mechanical strength, creep resistance, and environmental resistance at temperature—leading many to consider the system as an alternative to nickel-base superalloys for future generation turbine engine hardware. However, little information exists regarding the deformation processing required to turn these novel alloys into useable product forms with appropriate microstructure refinement. SupersolvusSupersolvus thermomechanical processing sequences were successfully demonstrated using right-cylindrical upset specimens for two wrought γ′-strengthened cobalt-baseCobalt-base superalloys at industrially relevant temperatures and deformation rates. Hot flow behavior and microstructure evolution were quantitatively characterized and compared to available information on a legacy nickel-base system, Waspaloy. Further, density functional theory was used to explore the compositional dependency of the intrinsic material properties influencing single-phase hot working behavior of model Ni–Al binary and Co–Al–W ternary systems. The apparent similarity in the supersolvus thermomechanical processing behavior of Co–Al–W-base systems and their two-phase γ–γ′ Ni-base counterparts suggests conventional pathways, models, and equipment may be leveraged to speed transition and implementation of wrought Co–Al–W-base alloys for components where their properties may be advantageous.

The effects of Al, Cr, and Ti on the oxidation behaviors of multi-component γ/γʹ CoNi-based superalloysCoNi-based superalloys, Co–30Ni–(6–8)Al–3W–1Ta–(3–6)Ti–(12–14)Cr, were investigated at 800, 900, and 1000 °C for 100 h. The results show that Al has a superior effect for improving the oxidation resistance of the alloys in comparison with Cr, particularly at 900 and 1000 °C, while Ti was detrimental to the oxidation performance of the alloys. Oxidation resistance of alloys containing 6 at.% Al were primarily provided by Cr2O3Cr2O3 layer, which was not sufficiently protective at 900 and 1000 °C. Continuous Al2O3Al2O3 layers could form in alloys containing 8 at.% Al at 800 and 1000 °C, but failed at 900 °C. Higher content of Cr can assist the formation of continuous Al2O3Al2O3 layer. For the target service temperature of 800 °C or below, alloys with lower Al and higher Ti content may be suitable since higher solvus temperature and volume fraction of the γʹ phase could be achieved with sufficient oxidation resistance provided by dense Cr2O3Cr2O3 layer. While for a higher target service temperature above 900 °C, higher content of Al (≥8 at.%) is required to form continuous Al2O3Al2O3 layer. The current study is helpful to understand the oxidation behavior of multi-component CoNi-based superalloysCoNi-based superalloys and provide guidance for alloy composition design and optimization.

Successful application of selectiveMurray, Sean P. electron beam melting to a novelPusch, Kira M. CoNi-based superalloy named SB-CoNi-10 is demonstrated. Crack-free as-printed microstructures exhibit excellent ductilities above 30% andPolonsky, Andrew T. ultimate tensile strengths above 1.1 GPa at room temperature in tension. Conventional post-processing consisting of a super-solvus hot isostatic pressing (HIP), a solutionTorbet, Chris J. heat treatment (SHT), and a low-temperature aging has been applied to remove microstructural inhomogeneities present in the as-printed microstructure. The microstructures of the as-printed and HIP+SHT+Aged alloysSeward, Gareth G. E. have been investigated to determine the effect of post-processing heatNandwana, Peeyush treatments on the nanoscale $$\gamma $$ γ / $$\gamma ^{\prime }$$ γ ′ microstructure and theKirka, Michael M. mesoscale grain structure. Tensile testsDehoff, Ryan R. have been conducted at room temperature and elevated temperatures above 850 $$^{\circ }$$ ∘ C to investigateZhou, Ning mechanical properties in both the as-printed and HIP+SHT+Aged conditions. The high-temperature ductilityForsik, Stéphane A. J. and strength are strongly affected by the microstructure, with a mostly columnar-grained microstructure in the as-printed condition exhibitingSlye, William superior ductility to the fully recrystallizedPollock, Tresa M. microstructure in the HIP+SHT+Aged condition.

In our previous study, Co-Al-W-Ta-Ti quinary alloys were found to show excellent creep resistance at elevated temperature. In order to clarify the influence of the initial microstructureInitial microstructure on the creep behaviorCreep behavior of a Co-base single crystal superalloy, the tensile creep behaviorCreep behavior of the Co-base alloyCo-base alloys with different initial microstructuresInitial microstructure was investigated at 900 °C and 420 MPa and the relationships between the initial microstructuresInitial microstructure and the creep properties, such as the rupture life and the minimum creep rate, were analyzed. It is suggested that the γ′ size increased with increasing aging time or temperature, while the volume fraction of this Co-base alloyCo-base alloys remained similar through heat treatments at 900 and 1000 °C for 50–1000 h. Moreover, the initial dislocation density also increased with increasing aging time but varied negligibly with higher aging temperature. On the other hand, four distinct creep stages were observed during the creep process, and the rupture life exhibited an optimum peak as a function of γ′ size, which was also influenced by the initial γ/γ′ lattice misfit. Meanwhile, the minimum creep rate increased with an increase of the initial dislocation density. This study can serve as a guide for future microstructural design and optimization of Co-Al-W-base superalloys.

CastingCasting of single-crystalline superalloys is known to be difficult due to a variety of challenges. Co-based superalloysCo-base superalloys, however, promise to show quite good castability compared to Ni-based superalloys, as documented in this manuscript. Therefore, an alloy series, which is designed by changing only the Co-to-Ni ratio, is used to address this topic. In addition to the castingCasting behavior, segregationSegregation and recrystallizationRecrystallization behavior were also investigated, since these are closely related to castingCasting challenges. It was found that in the as-cast state, all alloying elements distribute more homogeneously in the Co-rich alloys which is beneficial for castingCasting. CastingCasting of directionally solidified tubes revealed that the Co-rich alloys show a lower susceptibility to hot tearing and predominantly develop cold cracks. Since cold cracking can be addressed by component design, Co-base superalloysCo-base superalloys are suggested to show a better castability compared to Ni-based superalloys. RecrystallizationRecrystallization, however, is more pronounced for the Co-rich alloys. This might become a problem during castingCasting of single-crystalline Co-base components, if high deformation is introduced in the solidified component.

The system Co–Ti–Cr was recently identified as a very promising base for low mass-density Co-based superalloys. This study presents the influence of quaternary alloying additions on the thermophysical and mechanical properties of a Co–11Ti–15Cr model superalloyModel superalloy. Al, Ta, Re, and W were selected as a starting point for alloy developmentAlloy development as they represent important alloying elements in both Ni-based and Co–Al–W-based superalloys. Microstructure analysis reveals that the addition of only 1 at.% of either of these elements causes the formation of different undesired intermetallic phases in addition to γ and γ′ phases, which is in agreement with thermodynamic calculations. However, a diffusion couple between the Co–Ti–Cr and a Co–Al–W–Ta alloy suggests that the tolerance for alloying elements without destabilizing the γ/γ′ two-phase microstructure is higher when they are present in a certain ratio. Energy-dispersive X-ray spectroscopy shows that Cr and Re are enriched in the γ phase, whereas Ti and W preferentially partition to γ′. All alloying elements increase the yield strength at room temperature, but are disadvantageous at 1000 °C. Re and W retain their strengthening effect up to 900 °C.

We report atomic structures and chemical compositions of defects associated to planar faults in a creepCreep deformed Co-base superalloyCo-base superalloy and discuss their formation and contribution to plastic deformation. The multinary single crystalline Co-base superalloy was creep deformed under tension along $$ \left[ {001} \right] $$ 001 -direction at 850 °C and 400 MPa. The creep microstructure comprises a high density of planar defects. Solute segregationSegregation to superlattice intrinsic stacking faultsStacking faults (SISF) is characterized via EDXS analysis of a statistically relevant number of faults and compared at different creep stages. The amount of solute segregation shows negligible difference at different creep stages indicating that segregation directly occurs during planar fault formation and does not significantly evolve afterward. Based on the observation and analysis of Frank partial dislocationsFrank dislocation with $$ {\raise0.7ex\hbox{$a$} \!\mathord{\left/ {\vphantom {a 3}}\right.\kern-0pt} \!\lower0.7ex\hbox{$3$}}{\langle111\rangle} $$ a / 3 ⟨ 111 ⟩ Burgers vectors terminating SISF, we discuss a new route to SISF formation via dislocation climb. Additionally, two more complex fault structures are analyzed, and potential formation mechanisms are discussed. The first of these structures is a terminating end of an SISF where an $$ {\raise0.7ex\hbox{$a$} \!\mathord{\left/ {\vphantom {a 3}}\right.\kern-0pt} \!\lower0.7ex\hbox{$3$}}{\langle112\rangle} $$ a / 3 ⟨ 112 ⟩ partial dislocation splits up into two closely spaced $$ {\raise0.7ex\hbox{$a$} \!\mathord{\left/ {\vphantom {a 6}}\right.\kern-0pt} \!\lower0.7ex\hbox{$6$}}{\langle112\rangle} $$ a / 6 ⟨ 112 ⟩ partials separated by an SESF. The second structure consists of two parallel SISFs connected by an anti-phase boundary (APB). All deformation mechanisms described in this study show an involvement of solute segregation directly affecting formation and propagation of creep defects by changing planar fault energies and chemical environments of dislocations. Solute segregationSegregation is therefore expected to be a key to future alloy design by enabling control of creepCreep deformation mechanisms in specific temperature and stress regimes.

In the present work, we show the feasibility of microstructural control by additions of Ti and Cr to a γ/γ′ Co-30Ni–10Al–5Mo–2Nb superalloy. Solutioning at 1300 °C followed by aging at 900 °C leads to homogenous distribution of L12 ordered cuboidal γ′ precipitates in face-centered-cubic (fcc) γ matrix. Compositional measurements show Al, Mo, and Nb partition to γ′ that indicates the γ′ can be described as (Co, Ni)3(Al, Mo, Nb). An addition of 2 at.% Ti leads to increases in the γ′ volume fraction(′ volume fraction from 56 to 70% and solvus temperature from 990 to 1030 °C. Ti strongly partitions to γ′ with respect to γ matrix. Similarly, an addition of 10 at.% Cr to the base alloy leads to a morphological transition of γ′ precipitates from cuboidal to near spherical shape, indicating a direct influence on the γ/γ′ lattice misfit. Unlike Ti, Cr partitions to γ matrix, and additionally, Cr influences Mo to partition into γ matrix. A combined addition of Ti and Cr leads to high γ′ volume fraction(′ volume fraction ~76% and an increase in γ′ solvus temperature to 1045 °C, while maintaining the spherical γ′ morphology(′ morphology. These superalloys show 0.2% proof strength comparable to those of Co–Al–W-based superalloys. At 870 °C, 10 at.% Cr and 2 at.% Ti added alloys show higher specific 0.2% proof stress than Co–Al–W-based superalloys. The obtained results show the microstructural sensitivity of these Co-based superalloys toward their designing for better performance.

As a new class of promising high-temperature materials, Co–Al–W-base alloys have been developed by alloying additions to improve the microstructure stability and other properties. However, the optimization of Co–Al–W-base alloys becomes more complicated with increasing variety and content of alloying elements. In this study, an accelerated approach to design γ′-strengthened Co–Ni-base superalloys with well-balanced properties was developed, by integrating the diffusion-multiple approach and machine-learning tools. A large amount of experimental data was obtained using the diffusion-multiple approach and fed into machine learningMachine learning tools to establish the relationship between alloy compositions and important thermodynamic and microstructural parameters such as the phase constituent, the γ′ phase fraction (Fγ′) and the γ′ solvus temperatureγ′ solvus temperature (Tγ′). The established machine-learning models were then employed to predict the characteristic parameters of multicomponent Co-Ni-base superalloys containing up to nine elements (Co, Ni, Al, W, Ta, Ti, Cr, Mo, Nb), even though most of the collected compositions from experiments were quinary to septenary alloys. Using the predicted results from the models and the computational thermodynamics tools, a multicomponent Co–Ni-base superalloy aimed at the application as single crystal blades was designed and characterized to test the reliability and robustness of the novel design approach.

A new computational approach to model precipitate compositions and properties in the CoNi-design space for yield strength anomalyYield strength anomaly prediction is presented. The antiphase boundary (APB)Antiphase boundaries energies on {111} and {010} and the degree of elastic anisotropy are known to influence the yield strength anomaly. APB energies were estimated by a diffuse multi-layer fault (DMLF) model using the structural energies of proximate structures: L12, γa′, and D023. The elastic moduli were calculated via the energy-strain approach using density functional theory calculations. (Ni1−xCox)3(Al1−yWy) was chosen as the model system, and Yoo’s criterion is evaluated over the entire range of compositions to identify regions that exhibit the yield strength anomalyYield strength anomaly . (Ni0.65Co0.35)3(Al0.5W0.5) has the maximum APBAntiphase boundaries (111) and any two-phase alloy (γ + γʹ) in Ni–Co–Al–W system with this precipitate composition might exhibit higher strength upon shearing by a matrix dislocation. ThermoCalc was employed to identify stable L12 regions in (Ni1−xCox)3(Al1−yWy). The effect of Co on the yield strength anomalyYield strength anomaly was investigated experimentally in three (Ni1−xCox)3Al alloys with L12 structure. All three alloys exhibited the yield strength anomalyYield strength anomaly , validating the computational approach. The addition of Co provides solid solution strengthening to Ni3Al at room temperature; however, this contribution to the overall strength diminished as a function of temperature. CoNi-alloys displayed strengths similar to Ni3Al at elevated temperatures with Co addition resulting in a marginal increase in strength at the peak temperature. The present study elucidates that APBAntiphase boundaries energies from a DMLF model combined with elastic moduli can be employed to predict yield strength anomalyYield strength anomaly using Yoo’s criteria.

Additive manufacturingAdditive manufacturing has enabled the production of highly complex designs that are not producible using traditional manufacturing techniques. While superalloysSuperalloys such as IN718 have been used in these processes for the manufacture of turbine engine structural components, applications requiring higher service temperatures necessitate the development of alloys with increased capability. Rene 65Rene 65 was developed as a cast and wrought alloy with increased capability relative to wrought IN718, and characteristics of that alloy, including temperature stability and thermal crack-resistance, made Rene 65Rene 65 an appealing candidate to withstand the extreme temperature gradients that are characteristic of direct metal laser melting (DMLM) additive manufacturingAdditive manufacturing . The as-built DMLM microstructure is very different from as-forged microstructure, and this work will examine the effect of heat treatments both below (sub-) and above (super-) the gamma prime (γ′) solvus on the grain and precipitate structure of AM Rene 65Rene 65 material. Tensile, fatigueFatigue and creepCreep behavior of the alloy in these different heat treatment conditions is reported. Relative to AM IN718, AM Rene 65Rene 65 shows the desired improvement in temperature capability analogous to that in the cast and wrought versions of the alloys. Differences in the balance of properties are noted between AM Rene 65Rene 65 and cast and wrought Rene 65Rene 65 , which are attributable to the differences in grain size and precipitate distribution and which may provide benefits for certain applications.

The occurrence of hot crackingHot cracking is a significant problem during welding processing of highly heat resistant nickel-base superalloys. Hot crackingHot cracking is most often associated with liquid films that are present along grain boundaries in the fusion zone and the partially melted zone and can only be suppressed to a very limited extent. The latter is the case despite remarkable studies and analyses of the phenomenon. In this work, a new approach is presented which intends the suppression of hot crackingHot cracking by using a non-contact method to influence the solidification process. It is based on the idea of a modification of the laser-induced melt pool convection (Marangoni convection) using customized magnetic fields. As a consequence, special system technology is derived on the basis of theoretical considerations while the effectiveness to be expected is estimated on the basis of the information available in the literature. The implemented system technology is described in detail. The focus of this description is on the magnetic flux density distribution or the temporal change, respectively, with respect to the laser-induced melt pool. The presented experimental results provide a comparative view of samples welded with and without the influence of a magnetic field while a significant difference is evident. The outlook of this work describes key data of a test stand specially developed for examining the identified topic in in-depth investigations.

This article presents the effect of carbideCarbide inoculantsInoculants additions on microstructure evolution and mechanical propertiesMechanical properties of Inconel 718 superalloy fabricated by selective laser meltingSelective Laser Melting (SLM) (SLM) process. Flakes of titanium carbideCarbide (TiC) and niobium carbide (NbC) were mixed with the Inconel 718 powder and acted as nucleating agents to induce heterogenous nucleation in order to eliminate anisotropic grain structure. By increasing fraction of carbideCarbide inoculantsInoculants , more isotropic grain texture was detected. Furthermore, significant improvements on creep properties have been observed with minor carbide additions. With 0.5 wt% TiC and NbC addition, creep rupture life could be increased from 198.5 h to 449.5 h and 371.8 h, respectively. Moreover, creep strain rate was dramatically decreased from 0.513 × 10−8 to 0.12 × 10−8 s−1 by 0.5 wt% TiC additions. This study has demonstrated that minor carbideCarbide addition can have profound impact on the microstructure and property of Inconel718 processed by SLMSelective Laser Melting (SLM) .

Additive manufacturing (AM) providesPolonsky,Andrew T. enormous processing flexibility, enabling novel part geometries and optimized designs. Access to a local heat source further permits the potential for localRaghavan, Narendran microstructure control on the scale of individual melt pools, which can enable local control of part properties. In order to design tailored processing strategies for target microstructures, models predicting theEchlin, McLean P. columnar-to-equiaxed transition must be extended to the high solidification velocities and complex thermal histories present in AM. Here, we combine 3D characterization with advancedKirka, Michael M. modeling techniques to develop a more complete understanding of the solidification process and evolution of microstructure during electron beam melting (EBM) of Inconel 718. Full calibrationDehoff, Ryan R. of existing microstructure prediction models demonstrates the differences between AM processes and more conventionalPollock, Tresa M. welding techniques, underlying the need for accurate determination of key parameters that can only be measured directly in 3D. The ability to combine multisensor data in a consistent 3D framework via data fusion algorithms is essential to fully leverage these advanced characterization approaches. Thermal modeling provides insight on microstructure development within isolated solidification events and demonstrates the role of Marangoni effects on controlling solidification behavior.

This study investigates the anisotropic mechanical and microstructural behavior of the laser powder bed fusionLaser powder bed fusion (LPBF) manufactured Ni-based superalloy Hastelloy X (HX) by using slow strain rate (10−5 and 10−6s−1) tensile testing (SSRT) at 700 °C. LPBF HX typically exhibits an elongated grain structure along the building direction (BD) and the texture analysis from the combination of neutron diffractionNeutron diffraction and EBSD discloses a major texture component <011> and a minor texture component <001> along BD, and a texture component <001> in the other two sample directions perpendicular to BD. Two types of tests have been performed, the horizontal tests where the loading direction (LD) is applied perpendicular to BD, and the vertical tests where LD is applied parallel to BD. The vertical tests exhibit lower strength but better ductility, which is explained by the texture effect and the elongated grain structure. A comparison of the mechanical behavior to the wrought HX shows that LPBF HX has better yield strength due to the high dislocation density as proved by TEM images. Creep voids are observed at grain boundaries in SSRT for both directions and are responsible for the poor ductility of the horizontal tests. The vertical ductility in SSRT maintains the same level as the reference tensile test at the strain rate of 10−3s−1, due to the extra deformation capacity contributed by the discovered deformation twinningDeformation twinning and lattice rotation. The deformation twinningDeformation twinning, which is only observed in the vertical tests and has not been found in the conventionally manufactured HX, is beneficial to maintain the ductility but does not strengthen the material.

The microstructure and properties of alloy Haynes 282Haynes 282 produced through laser powder bed fusionLaser powder bed fusion were investigated as a function of the post-deposition heat-treatmentHeat-treatments . Scanning electron microscopy and X-ray diffraction were utilized to characterize the microstructure, whilst electro-thermal mechanical testing was used to evaluate the tensile and creep properties at 900 °C. In the as-deposited state, the initial microstructure consisted of the γ and γʹ phases along with M6C and M23C6 carbides. These carbides were observed to govern the recrystallizationRecrystallization behaviour of the material and resulted in a minimum recrystallizationRecrystallization temperature of 1240 °C. Following post-deposition heat-treatmentsHeat-treatments , the microstructures consisted of a monomodal distribution of γʹ with M6C and M23C6 carbides along the grain boundaries. Tertiary γʹ particles were found to form in the vicinity of carbides in samples that employed a γʹ super-solvus step prior to ageing at 788 °C. The tensile properties were found to be similar in all heat-treated states, consistent with the minimal differences observed in the microstructures. In contrast, significant differences in the creep behaviour of the alloy were observed following the different heat-treatments, although no correlation with the microstructures was observed.

Mass lossMass loss due to vaporizationVaporization induced by the high energy heat source during powder-bed fusion additive manufacturingPowder bed fusion additive manufacturing (AM) is one of central issues which concerns the compositional variations across the AM build. Potentially, defects can be initiated where local chemistry in the AM build is not homogeneous. In this work, the evaporation effect of powder-bed fusion AM has been first developed in a binary alloy Nitinol (NiTi) and then applied to nickel-based superalloysNickel-based superalloys via physics-based integrated modelling framework and experimental investigation. Volatile species which depart from the nominal composition result in significant mass lost up to 2 at% of Ni in the binary alloy system Nitinol, consistent with energy-dispersive X-ray analysis. As for the multicomponent alloy system of nickel-based superalloys, inductively coupled plasma optical emission spectroscopy (ICP-OES) results reveal further that Al, Co, and Cr are preferable to vaporize first with the loss up to 1.2 at%; this affects the thermal fluid behaviour given that the thermophysical property is altered by composition variations. Hierarchical microstructures have been characterized to rationalize the process-structure-property relationship. The mathematical tool validated with targeted experimentation can be further developed for AM materials design, particularly for taking care of vapourization.

In this study, we provide novel in situ monitoringMonitoring of strainStrain during laser metal deposition of Inconel 718Inconel 718 by neutron diffractionNeutron diffraction . Thermal, phase and stress-related contributions to the lattice parameter evolution are addressed for the representative regions during processing: melt pool, near melt pool, and far-field. The evolution showed a strong dependency on the build height and distance to the melt pool, i.e., time and temperature gradient, as expected. The different regions of interest reached at different moments the processing stable regime, which is contrasted with microstructural characterization. A homogeneous microstructure of coarse epitaxial dendrites and Laves phase was found from the third printed layer on, with fine globular delta phase precipitation at the grain boundaries. In comparison, neutron diffractionNeutron diffraction strainStrain monitoringMonitoring highlighted an offset of process stabilization after 24 layers for the melt pool and near-melt pool regions.

A new high-γ′ volume fraction Ni-base superalloy for additive manufacturingAdditive manufacturing was developed using a CALPHAD-based approach. Selective laser melting followed by hot isostatic pressing produces a microstructure that is free of microcracks and fusion defects. Post-processing includes a solution-and-age heat treatment to precipitate about 55 vol.% of γ′. Two chemistries with different levels of grain boundary strengthening elements were tested to evaluate the balance between printability and high-temperature properties. After heat treatment, the alloy exhibits 980 MPa yield strength and 1400 MPa ultimate tensile strength at room temperature and maintains that level of yield strength up to 800 °C. Cyclic oxidation tests show good resistance to environmental damage due to the formation of a protective alumina layer.

The heat treatment response of the new superalloy ABD-900AM, designed specifically for additive manufacturingAdditive manufacturing (AM), is studied. The as-fabricated microstructure is characterised at multiple length-scales including by X-ray synchrotron diffractometry and transmission Kikuchi diffraction imaging. The very high cooling rates arising during the process suppress γ′ precipitation; thus the details of heat treatment are shown to be important in establishing properties. The yield stress and tensile strength developed are marginally improved by super-solvus rather than sub-solvus heat treatment, but the ductility is then compromised. The tensile behaviour is superior to the heritage alloy IN939 which has a comparable fraction of γ′; this is due to the larger refractory content of ABD-900AM and its finer scale precipitation. The internal strains developed during processing are sufficient to promote recrystallizationRecrystallization during super-solvus heat treatment which breaks down microstructural anisotropy and promotes grain growth; however, this effect is absent for the sub-solvus case.

The recent advances in additive manufacturingAdditive Manufacturing (AM) have led to printing of complex structural components. The highly non-equilibrium processing conditions encountered during direct metal laser melting (DMLMDirect Metal Laser Melting (DMLM)) frequently lead to micro-cracking in high-temperature capable Ni-superalloysSuperalloys, irrespective of processing conditions, limiting their current applicability. This paper aims to develop a general criterion to assess printability of a Ni-superalloySuperalloys solely based on its composition. Thirty-four Ni-superalloysSuperalloys spanning a wide range of alloying elements were printed, each with twenty-four process conditions, and their crack densities were measured in order to have a consistent set of experimental data for building a model. The models available in literature for predicting cracking susceptibility were evaluated against the experimental data. Finally, a hybrid model, based on physics-based quantities, was built with the most significant input features (x’s). This model correlates well with the experimental data and is applicable across a wide range of Ni-superalloySuperalloys compositions.

Additive manufacturing processesKirka, Michael M. are becoming increasingly utilized throughout industry. These technologies enable novel design and manufacture of complex components, reduce material waste streams, and potentiallyFernandez-Zelaia, Patxi reduce development cycles via rapid prototyping. The high- $$\gamma ^{'}$$ γ ′ Ni-base superalloys are of particular interest to the gas turbine engine industry due their high-temperature resistance. However, as a result of the high $$\gamma ^{'}$$ γ ′ volume fraction, these alloys are traditionally termed non-weldable due to a propensity to crack during welding fromLee, Yousub a host of mechanisms including hot-tearing and strain-age cracking. These mechanisms are subsequently found in fusion-based additive manufacturing processes. Furthermore, there is a long historyNandwana, Peeyush of employing traditionally processed components in gas turbine engines, and hence, the performance of materials produced by additive manufacturing needs to be closely investigated. In this work is presented the tensile, high-temperature fatigue, and creep deformation behavior for Inconel 738 fabricated through electron beam melting (EBM) additive manufacturing. Under tensileYoder, Sean and fatigue conditions, the material was observed to perform in an isotropic manner when tested parallel and transverse to the build direction. AlthoughAcevedo, Obed under creep conditions, the material exhibited an anisotropy between material tested parallel and transverse to the build direction. With the difference attributed to the fine columnarRyan, Daniel grain structure observed in the additively manufactured Inconel 738. Ultimately, the EBM material was observed to perform comparably to the conventional cast Inconel 738.