Proceedings of the 10th International Symposium on Superalloy 718 and Derivatives

A revised mechanism for the formation of downward frecklesFreckle is proposed. The mechanism accounts for the observations of freckleFreckle formation in industrial ingot production. In particular, the association of freckling with process instability is emphasized and is part of the proposed mechanism. Additionally, we suggest that the shrinkage which takes place as the final eutectic solidifies can explain the formation of freckling when the basic gravitational force produced by negative buoyancy in the solidifying liquid appears to be insufficient for the initiation of interdendritic fluid flow. We conclude that the key factor in preventing freckleFreckle formation in industrial ingot remeltingRemelting is the maintenance of process stability.

The U.S. Department of Energy, through its Offices of Fossil Energy and Energy Efficiency and Renewable Energy, has funded programs to develop materials technology for high-efficiency energy systems that utilize supercritical steam or carbon dioxide as working fluids. These plants would operate in the 700–800 °C temperature range and require the use of nickel-base alloys to meet design creepCreep-rupture life requirements for welded structures. Specific components include large-diameter pipes, induction bends, and forged fittings. INCONEL® alloy 740H® (UNS N07740) is a γʹ-strengthened nickel-base superalloySuperalloys that was developed for this application and down-selected for a manufacturing demonstration program designated AUSC ComTest. Work was recently completed on the Phase 2 program under DOE contract DE-FE0025064 that had the goal of demonstrating the ability of US industrial supply chain to manufacture full-scale components. In another DOE contract, DE-EE0008367, that concluded in the Fall of 2022, the production of large-diameter longitudinally seam-welded pipe was demonstrated. Alloy 740H contains 16–20% γʹ and is sensitive to thermal stress cracking and auto-agingAging with implications for each stage of the manufacturing operation. Previous publications have described the production of ingot, pipe, and welded pipe made for these demonstration projects. This paper describes induction bendingInduction bending and subsequent heat treatmentHeat treatment of these pipes. The results demonstrate the capability of US manufacturers to make these bends with satisfactory dimensional control and properties. The investigation and mitigation of process cracking encountered in this work will be discussed. The test articles are now stored at Oak Ridge National Laboratory awaiting future programs for more detailed material characterization.

Nickel-base alloys have been developed, which offer higher temperature capability from ingot metallurgy than Alloy 718Alloy 718. However, these alloys have higher volume fractions of gamma primeGamma prime (γʹ) precipitates, require further processing steps and consequently, have higher material processing costs, show a greater propensity for freckleFreckle and are not readily electron beam (EB) welded. There is an appetite for an alloy that shows improved forgeability, that is EB weldable and can be used at temperatures of up to 700 °C. This study examines VDM Alloy 780VDM Alloy 780, first using laboratory compression tests and heat treatmentHeat treatment experiments to determine suitable thermo-mechanical processing (TMP) conditions. Subsequently, pancakes that were 21–22 mm in height and 130–133 mm in diameter were forged from 70 mm in diameter × 76 mm high bars, which were extracted from mid-radius locations of 8-inch diameter billet. From these, test piece blanks were extracted, and heat treated for tensile testsTensile tests at temperatures of 20, 650 and 750 °C. Results from these tests were compared with microstructureMicrostructures and data from 30 mm thick pancake forgingsForging that received different TMP conditions from work at VDM Metals International. The combined experiences provide an insight into the effect of forgingForging and heat treatmentHeat treatment conditions on the microstructureMicrostructures and tensile testTensile tests properties of VDM Alloy 780VDM Alloy 780. They show that strength levels for fine grain Alloy 720Li can be achieved if specific sizes of γ grains and γʹ precipitates can be produced.

The characterization of local and global fracture mechanical propertiesMechanical properties is carried out with destructive testing methods and is increasingly required in the specifications of forgingsForging. Especially in the case of alloy 718Alloy 718 aircraft parts, the numerical estimation of local material properties is essential for lightweight design, geometry optimization, and a significant reduction of development and experimental characterization costs. This leads to a demand for numerical models to capture initial microstructural inhomogeneities, describe the forming history, and reflect the local microstructureMicrostructures and properties of the final product. Therefore a digital twin for the complex forgingForging process was developed in order to reproduce and evaluate the resulting local microstructureMicrostructures across the complete process chain. Since the microstructureMicrostructures determines the mechanical propertiesMechanical properties like yield stress and fracture toughnessFracture toughness, a dedicated model was implemented to describe the local evolution of the relevant microstructural features.

Ni-superalloysSuperalloys are well-established for use in high temperature applications in aerospace, power generation, and automotive sectors, yet, are seldom considered as materials for hot toolingHot tooling. The operational conditions of hot forming dies potentially exceed those experienced by aircraft turbine discsTurbine discs. Fortunately, new disc alloys have pronounced elevated temperature capabilities and the current study focuses on implementing two advanced alloys, VDM 780VDM 780 and Haynes 282Haynes 282 (H282H282) as hot tool materials. There is, however, inadequate evidence of their life-limiting properties and mechanisms in the in-service temperature regime of 700–900 ºC. Thus, realistic operating conditions were replicated by combining interrupted short and long-term thermal-mechanical tests. Initially, isothermal ageing in the furnace was used to compare the extent of γ′ coarsening between the alloys, and subsequent in-situ ageing and compression testing measured the accompanying loss in strength. Compression creep testingCreep testing at stresses near the yield points (250–750 MPa) revealed accelerated creepCreep rates at high temperatures. The results indicated that even as exposure duration, temperature, and applied stress all influence microstructural evolution, the exposure temperature was pivotal in determining the effective life of these γ′ strengthened alloys. Dissolution kinetics of γ′ around near-solvus temperatures was crucial and was governed by elemental additions. As a result, the research paves the way for a better understanding and design of superalloysSuperalloys with improved thermal integrity for hot toolingHot tooling.

In the aerospace industry the microstructure evolutionMicrostructure evolution of alloy 718Alloy 718 during forgingForging and heat treatmentHeat treatment and the resulting mechanical propertiesMechanical properties are decisive in view of the high-quality requirements of aircraft components. During thermomechanical processing the temperature control and adiabatic heating lead to grain growth and, if δ-solvus temperature is exceeded, to the dissolution of the δ-phase, which further results in accelerated grain growth. To describe the history of the microstructureMicrostructures in terms of grain sizeGrain size during and after forgingForging or heat treatmentHeat treatment an existing multi-class microstructureMicrostructures model is optimized with a focus on grain growth kineticsGrain growth kinetics and parameterized by experimental results. The multi-class model describes the microstructureMicrostructures and the coarsening during processing more precisely in terms of the grain sizeGrain size distribution than previously used single-class models. A topic for other but related work is the prediction of mechanical propertiesMechanical properties such as tensile strength, fracture toughnessFracture toughness and creepCreep resistance, which requires a precise prediction of grain sizeGrain size characteristics in terms of sizes and size distributions as provided by the model presented in this work. The experimental data basis stems from in-situ high-temperature electron backscatter diffraction (HT-EBSD) investigations and supporting experiments. The experimental setup and the results are discussed in detail.

The presence 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 superalloysSuperalloys. These overgrown grainsOvergrown grains typically feature a high density of annealing twins and a precipitate distributionPrecipitate distributions similar to the one in the adjacent fine-grained matrix. Predicting the propensity of a given thermo-mechanical processing path to trigger said abnormal grain growth (AGG) has proven difficult since key aspects such as the distribution of stored energyStored energy in the as-hot-worked structure are generally not known a priori. Instead, alloy-specific AGG mapsAbnormal grain growth maps are proposed that provide a potential risk assessment for AGG to occur. Such maps were generated exemplarily for two very different Ni-base superalloysSuperalloys, HAYNES® 244® alloy and HAYNES® 233™ alloy, built on a sizeable number of different hot-worked and sub-solvus annealed product forms. The usefulness and limitations of such maps are discussed. In addition, some insight into possible mechanisms of AGG upon sub-solvus annealing of the studied alloys is provided.

The need to develop new high-temperature materials has increased significantly in the last decade owing to the demand of higher engine operating temperature. This demand has motivated the development of a new Ni-Co-based superalloyNi-Co based superalloys GH4251 with service temperature up to 700–800 °C. Based on disk alloy U720Li, the GH4251 alloy is designed by adjusting the content of Co, Cr, Ti, Nb, and other elements. On the one hand, by increasing the Co content to 25 wt. %, the stacking fault energy is effectively reduced, which makes it easy to form nano-twins and other substructures that strengthening the alloy together with γ′ precipitates. The yield strengthsYield strength of the newly designed alloy can achieve 1100 MPa at 750 ℃ and the creepCreep-rupture life is more than 500 h at 750 ℃ under 530 MPa with fine grain sizeGrain size (ASTM 8), which is superior to U720Li. On the other hand, a certain Nb element is added to replace Ti element, which can reduce the γ′solvus temperature and its precipitating dynamics, along with a changed thermal deformation behavior caused by lower stacking fault energy, leading to a significant better hot work ability and weld ability compared with U720Li. Besides being used as disk or ring forgingsForging, this novel GH4251 alloy can also be well processed by additive manufacturingAdditive manufacturing due to its low cracking tendency.

Early strain localization, parallel and adjacent to annealing twin boundaries (ATBsAnnealing twin boundaries (ATBs)), has been reported in several superalloysSuperalloys. While strain localization is generally attributed to local shear stresses developed near ATBsAnnealing twin boundaries (ATBs) due to elastic anisotropyAnisotropy, the role of local microstructural features near ATBsAnnealing twin boundaries (ATBs) is unclear. Precipitate free zones (PFZ) parallel to ATBsAnnealing twin boundaries (ATBs) in a γʺ–strengthened alloy 945X have been reported and were found to greatly influence strain localization in the alloy. However, it is unclear if such PFZs near ATBsAnnealing twin boundaries (ATBs) occur in other superalloysSuperalloys, potentially influencing strain localization. The present work investigates local microstructuresMicrostructures near ATBsAnnealing twin boundaries (ATBs) in a Ni-based superalloyNi-based superalloy—718, which is strengthened by both γʹ and γʺ phases. Based on characterization experiments, this paper reports that Alloy 718Alloy 718 shows a high density of herringbone-like γʹ−γʺprecipitates at ATBsAnnealing twin boundaries (ATBs). However, the ATBsAnnealing twin boundaries (ATBs) exhibit a much higher fraction of $$\gamma^\prime$$ γ ′ (25.3 ± 2.8%) than $$\gamma^{\prime \prime}$$ γ ″ (18.4 ± 2.4%), ultimately causing the coprecipitate fraction at ATBsAnnealing twin boundaries (ATBs) to be as high as 43%. A local HCP phase at the ATBsAnnealing twin boundaries (ATBs) within $$\gamma^\prime$$ γ ′ precipitates exhibiting Nb segregation and Al depletion is also reported. On the contrary, no appreciable change in the ATBAnnealing twin boundaries (ATBs) composition was observed within $$\gamma^{\prime \prime}$$ γ ″ . Finally, a novel mode of heterogeneous precipitationPrecipitation at ATBsAnnealing twin boundaries (ATBs) is proposed.

Next-generation energy systems require superior resistance to creepCreep deformation due to the considerably prolonged exposure times at operating stress and temperature. To improve the elevated temperature properties of INCONEL® 725 (IN725), a corrosion-resistant alloy, several variants with different Ti/Al ratios and judicious amounts of Nb and Ta were made. Furthermore, a high temperature agingAging (HTA) heat treatmentHeat treatment, designed to promote favorable precipitate phase formation, was explored. These adjustments allowed to tailor the amount and type of precipitate strengthening which led to significant increases in time to failure. The Ti/Al ratio was used to favor the formation of γ′ or γ″ precipitates. Compact morphology precipitates, consisting of γ′ precipitates surrounded by a γ″ shell, were formed in alloys with a low Ti/Al ratio. The HTA increased the creepCreep life of various alloy formulations up to a maximum improvement of 371% as compared to the standard agingAging heat treatmentHeat treatment. The Nb and Ta additions had a similar effect on increasing creepCreep life by promoting and stabilizing γ″ precipitationPrecipitation. The positive effect of the additions was even more pronounced when coupled with the HTA. A phase stabilityPhase stability study with up to 10,000 h exposure at 700 °C revealed that the compact morphology helped in slightly reducing coarsening of the γ′ precipitates, although the effect on creepCreep was not significant. The findings of this study enable design of dual superlatticeDual superlattice alloys through microstructural engineering that yields superior performance and can be applied to a wide range of alloys in the IN718IN718 and derivatives family.

Improving the efficiency of a land-based industrial gas turbine ultimately relies on novel alloy development for the turbine wheel. However, this alloy development task is challenging because it necessitates higher temperature capabilities along with phase stabilityPhase stability during low cooling rate processing of a full-scale wheel. These challenges make 718-based variant alloys an attractive choice because of their superior thermal stabilityThermal stability. However, to develop these novel alloys, their deformation behavior must also be accounted for. Thus, in the present work, we investigate the microstructureMicrostructures and creep deformation behaviorCreep deformation behavior of a novel 718-variant alloy718-variant superalloy. Detailed microstructural characterizationMicrostructural characterization reveals that the phase fraction of the $$\gamma^{\prime \prime}$$ γ ″ phase in the variant alloy is much lower than 718. In addition, the presence of Mo causes detrimental grain boundary precipitationPrecipitation which leads to final failure during tensile creepCreep deformation. The variant alloy accumulates creepCreep strain faster than 718, ultimately fracturing at 0.6% strain. Finally, a detailed characterization of the deformed variant alloy reveals extensive microtwinning.

For Ni-Co-based superalloysNi-Co based superalloys, the higher level of Co content, the lower the stacking fault energy, which promotes the formation of deformation twins and affects the mechanical propertiesMechanical properties of the materials. Mechanical tests of a novel Ni-Co-based superalloyNi-Co based superalloys at temperatures between 25 and 760 ℃ revealed that its strength maintained a sufficiently high level when the testing temperature was up to 650 ℃, above which the tensile strength decreased with test temperature, and there was a sharp drop at 760 ℃. Pre-strain tests at room temperature indicated that the density of dislocation and stacking faults increased with strains and they became obstacles for dislocation motion at elevated temperatures, resulting in an increase in yield strengthYield strength. In addition, the grain boundary of the pre-strained specimens has a higher dislocation density than the grain interior, and dislocation recovery occurred during the stress-ruptureStress-rupture process, resulting in a lower stress-ruptureStress-rupture life than the sample without a pre-strain.

The evolution of microstructureMicrostructures and ductility has been investigated for WaspaloyWaspaloy after isothermal exposure between 5 and 1800s at 750–950 °C. Gamma primeGamma prime (γ’) with 1.7 nm diameter is found in the mill-annealed condition, while precipitate-growth following a t1/3 relationship is observed for isothermal exposure. Grain boundary carbideCarbides networks are formed during isothermal exposure together with a rapid hardness increase. A drop in ductility is observed with the lowest values at 750 and 800 °C. Further ductility reduction during isothermal exposure correlates with the rapid hardness increase of WaspaloyWaspaloy. While grain boundary strengthening can compensate for the moderate age hardening observed for Haynes® 282®, the more rapid hardness increase due to γ' precipitationΓ' precipitation appears to be the dominating effect on ductility in WaspaloyWaspaloy. CarbideCarbides precipitationPrecipitation and growth kinetics are slower than those of Haynes® 282®, which further increases the relative effect of age hardening reactions on the ductility of WaspaloyWaspaloy.

In the present study, quantitative evaluation of the size and volume fraction of γ′ particles have been studied by Small-Angle Neutron ScatteringSmall-Angle Neutron Scattering (SANS) and FE-SEM in additively manufactured IN738LCIN738LC SuperalloySuperalloys employing isothermal heat treatmentHeat treatment conditions at 850 ℃. The results show that there is no observable γ′ precipitationPrecipitation in the as-deposited alloy, while a large number of small cellular and striated sub-structures can be characterized with widths ranging from 0.5 to 1.5 μm. During the 850 ℃ isothermal heat treatmentHeat treatment, the γ′ phase rapidly precipitates in large quantities and shows a unimodal irregular sphere-like shape. The average size of the γ′ particles increases with longer agingAging times. After 10 min of isothermal heat treatmentHeat treatment the average γ′ particle size is 72.5 nm. This average size increase to 137.3 nm after 120 min of ageing. However, the volume fraction (VF) of γ′ precipitates does not change after 10 min of isothermal heat treatmentHeat treatment time, at which point the VF % reaches about 40%. This result varies significantly from IN738LCIN738LC that was prepared by traditional casting processes.

Ni-based superalloysNi-based superalloy with applications to aerospace, nuclear, electrical, and automotive industries contain various alloying elements for enhanced thermal and corrosion resistance as well as improved mechanical strength. The underlying property and long-term stability of the superalloysSuperalloys is directly correlated to the processing conditions, materials structure, and alloying wt %. Hence, quantitative materials analysis of the alloying elements is essential for current and future superalloySuperalloys development and failure investigations. Alloying elements introduced in superalloysSuperalloys can segregate to grain boundaries or form nanoscale clusters either as a result of processing conditions, secondary phaseSecondary phase formation or failure from long-term use. Thus, suitable techniques at the nanoscale are necessary for a quantitative analysis and to understand the effect of clusters and segregation on material properties. APT (atom probe tomographyAtom probe tomography) is the only analysis technique that can provide 3D elemental distribution with nm spatial resolution and up to 10 ppm chemical sensitivity to investigate both grain boundary segregation and clusters. Moreover, new processing techniques including additive manufacturingAdditive manufacturing and nanoparticle-based alloys, further require suitable analytical techniques at the nanoscale, like APT, to identify chemical compositions. Herein, we use APT to better understand the influence of nanoscale effects in standard samples of a widely used Ni-based superalloyNi-based superalloy. Our analysis reveals differences in elemental distributions at low wt % that are responsible for material properties such as mechanical strength and oxidation resistanceOxidation resistance. We also compare bulk analysis using GDMS (glow discharge mass spectrometryGlow discharge mass spectrometry), ICP-MS (inductively coupled plasma mass spectrometry) with APT to show localized fluctuations in the compositions.

Supercritical CO2 (sCO2) power cycles, particularly direct-fired cycles, have the possibility of revolutionizing clean fossil energy with peak temperatures above 700 °C and wrought precipitationPrecipitation strengthened alloys like Haynes 282™ for structural components. At temperatures <650 °C, it would be desirable to use less expensive alloys, however, steels are known to be susceptible to carburization. Laboratory 300 bar sCO2 autoclave results were collected on a range of alloys including less expensive Ni-based alloys like 825 compared to advanced austenitic steels like alloy 709 at 600 °C. Both alloys 825 and 709 formed thin, protective Cr-rich oxides after 1,000 h. Alloy 825 also was exposed for 1,000 h in sCO2 at 800 °C and compared to a range of Ni-based alloys. Comparing alloys 625, 825, and 282, the mass gain increased with increasing alloy Ti content under these conditions. High Al superalloysSuperalloys did not perform significantly better under these conditions at 800 °C.

The effect of isothermal oxidationIsothermal oxidation on the fatigue performanceFatigue performance of differently processed Alloy 625Alloy 625 was studied (wrought 625, laser powder bed fusionLaser powder bed fusion, direct energy deposition). Uniaxial fatigue tests at room temperature were conducted after prior exposures at 800 °C for 24 h, 300 h, and 1000 h in either air or argon. Exposures in air resulted in chromia-scale formation, internal attack, and the formation of subsurface precipitates (i.e., δ-phase and σ-phase). Fatigue results indicated a consistent life reduction of up to 96% for the oxidized additive-manufactured test bars compared to their counterpart aged in back-filled argon. The fatigue life decreases as the oxidation exposure time increases. By contrast, any of the prior high-temperature exposures were not detrimental to the performance of the wrought Alloy 625Alloy 625. Microstructural analysis of the after-testing oxidized AM-processed bars indicated that the failure mode was attributed to the exacerbation of interfacial and subsurface defects from the oxidation exposure (i.e., internal attack, decohesion of the scale, and subsurface precipitates). These defects acted as preferential crack-initiation sites, leading to a reduction in fatigue life. On the other hand, the failure mode for the thermal-aged bars, without superficial degradation, involved fragmentation of (δ + σ) precipitate clusters favored by the Nb and Mo segregation from the as-built microstructureMicrostructures of both AM processes (LPBF and DED). A large precipitate fraction depletes the matrix, facilitating crack formation.

HAYNES 233 alloyHAYNES 233 alloy is an alumina-formingAlumina-former alloy developed for use in a variety of high temperature applications. It is unique in the marketplace for having a combination of high creepCreep strength and excellent oxidation and high temperature corrosion resistance to temperatures of 2000°F (1093 °C) and above while still being readily fabricable (formable, weldable, etc.). The alloy may be used in the solution annealed conditions at these high temperatures, or age-hardened to provide high strength at intermediate temperatures while maintaining its oxidation/corrosion resistance. This study will explore the long-term performance of 233 alloy in terms of microstructural stabilityMicrostructural stability, tensile propertiesTensile properties, and oxidation resistanceOxidation resistance. Thermal exposures of up to 8000 h were explored at temperatures ranging from 1200°F (649 °C) to 1800°F (982 °C). The effect of the initial material condition (solution annealed vs. age-hardened) was considered as well. It was found that prior age-hardening can provide the alloy with significantly improved ductility at intermediate temperatures before and after thermal exposure. A comparative year-long oxidation test at 2000°F (1093 °C) confirmed the excellent oxidation resistanceOxidation resistance of the alumina-formingAlumina-former 233 alloy compared to competing alloys (typically chromia formers).

UNS N07718 is widely used in marine service applications under a variety of conditions: alternate immersionAlternate immersion, different levels of cathodic protection, and freely corroding galvanic couples. Environmentally assisted cracking can significantly affect the performance of this alloy and constrains design as it needs to account for subcritical crack growth in service. We measured subcritical crack growth rates and thresholds in different environmental conditions for two different heat treatmentsHeat treatment of UNS N07718. The first heat treatmentHeat treatment, following AMS 5664 is widely used in the aircraft industry and for marine fasteners, and the second, following API 6A, is principally used in the marine and oil and gas industries. The material environmentally assisted cracking was studied under alternate immersionAlternate immersion to natural seawaterNatural seawater and under cathodic protection in natural seawaterNatural seawater. Microstructural modeling is presented to understand and predict how precipitates, their volume fraction, morphology, and properties, affect the evolution and accumulation of dislocation densities within the microstructureMicrostructures, influencing the fracture process at different physical scales.

The nickel-based superalloyNickel-based superalloys disk components in the turbine sections were subjected to Type II hot corrosionHot corrosion damage in the sulfur-containing salt contaminants at 650 °C–750 °C. The alloy GH4720Li with different grain sizesGrain size was corroded in a mixture of sulfates (25% NaCl + 75% Na2SO4) at 700 °C for 200 h, and the microstructure evolutionMicrostructure evolution was investigated. The experimental results showed that when the grain sizeGrain size increased from 15.9 to 127 μm, the mass loss decreased by 96%, and the corrosion layer thickness decreased by 44%. The hot corrosionHot corrosion resistance increased with the increasing grain sizeGrain size, and the corrosion failure mechanisms changed from pitting corrosion to uniform corrosion. The corrosion layer comprised NiCr2O4, Al2O3, CoO, TiO, Ni3S2, and CoS2. The oxide layer, Ni/Co-rich layer and S-rich layer were stratified and sequentially located on the alloy GH4720Li surface. The corrosion behavior was accelerated by the triangular grain boundaries (GBs) and γ′ phase, the segregation behavior of Cr elements in the GBs, as well as the γ′ phase formation promoted the tendencies for pit nucleation in the fine-grained structure. In contrast, the Ni/Co-rich layer provided better resistance to hot corrosionHot corrosion and was easier to form on the surface of the coarse-grained structure. The sulfide-oxidation cycle mechanism could well describe the hot corrosionHot corrosion behavior of the disk superalloyDisk superalloy.

Computational materials and process modeling capabilities have evolved over the past several decades. More recently, the Materials Genome Initiative (MGI) has provided focus on this technology and its application for rapid and lower cost materials and process development and implementation. Integrated Computational Materials EngineeringIntegrated computational materials engineering (ICMEICME) is now part of many organizations’ engineering and design approaches and associated infrastructures. Nearly all current new and future materials and process technology developments do or will involve application of modeling and simulationSimulation. The evolution of materials modeling and application to mainstream and emerging supply-chain processes will be reviewed with some perspectives on what the future might hold. Special emphasis will be made to review application of ICMEICME Alloy 718Alloy 718 and derivative materials and components. The use of computational modelingComputational modeling and simulationSimulation to material and process development is being extended to component qualification and certification. There are significant opportunities and prospects for materials and process modeling to enable further advancements in alloy designAlloy design and definition, materials processing methods development, and enablement of enhanced utilization of material capabilities to new product application spaces.

This comparative computational study of Applied CalphadCALPHAD aims at the simulative analysis of mechanical propertiesMechanical properties and microstructuresMicrostructures of recent C&W IN718IN718 successors with a similar application at elevated temperatures.Predictions are presented for candidates AD-730, and M647. Moreover, Rene88DT and derivatives GH4096, as well as Co-free EXP-G27, which, due to a new C&W technology involving electro-slag remeltingRemelting continuous directional solidification [1], enter the list of promising C&W candidates, are researched. The different alloying concepts are presented, and their influence on microstructuresMicrostructures during thermal processing is shown. Using the assessed MatCalc [2] multi-component CalphadCALPHAD thermodynamics and diffusion mobilities databases “mc_ni” (thermodynamic databases for fcc γ and γ′, as well as the diffusion mobilities database used here are added at the end of this paper) for Ni-base superalloysSuperalloys within the system Ni–Fe–Al–Ti–Co–Cr–Mo–Nb–W, phase stabilitiesPhase stability of Ni3Al-based strengthening precipitates, affected by different alloying contributions, are presented. The evaluated solubility behaviour of elements, and the thermodynamic dissolution behaviour of second phases in the Ni-fcc matrix are discussed. By Scheil-Gulliver simulationsSimulation of as-cast microstructuresMicrostructures, segregation trends during casting are described. The kinetic phase evolutions during supersolvus heat treatmentHeat treatment conditions are evaluated with nucleation and growth modeling by using MatCalc (version 6.04 rel0.127), predicting particle distributions, sizes, and densities.We show that fully predictive, computational strengthening trends, employing simple Applied CalphadCALPHAD and computational thermokinetics of precipitationPrecipitation, combined with strengthening models for solid solution strengthening and order strengthening for γ′, are a valuable tool for decisions on appropriate thermal operation conditions of IN718IN718 successors.

High throughput measurements of structure from simulationsSimulation and microstructural analysis were developed and combined into an adaptable data-analytics processing–structure–property modeling framework to gain insight into the design envelope for forgingsForging of Inconel 706Inconel 706. We highlight how thermal profiles from finite element simulationsSimulation (DEFORM) can establish time–temperature boundary conditions for CALPHADCALPHAD predictions of the combined γ′ γ′′ precipitate distributionPrecipitate distributions in Inconel 706Inconel 706 (structure predictors). Experimental observations of these precipitate distributionsPrecipitate distributions allow for the tailoring of the CALPHADCALPHAD interfacial energy. In this manner, a 25 × reduction in the number of physical observations of γ′ and γ′′ distribution (78 to 4) results in site-specific processing–structure, and processing–property models of forged parts with 80% predictive power. Additionally, the gradient boost modeling provides an opportunity to interpret feature importance in structure–property models to provide engineers with design insights for future development efforts. The adaptable framework would enable future DEFORM and CALPHADCALPHAD simulationsSimulation to be added to the dataset so that engineers can interpolate within the existing experimental dataset.

Requirements of stress-ruptureStress-rupture life and elongationElongation of nickel alloy 718Alloy 718 are often prescribed by specification AMS5596™ or AMS5662™, which broadly state that the stress-ruptureStress-rupture life and elongationElongation must exceed 23 h and 4% at 649 ºC (1200 ºF), respectively. Variability in product stress-ruptureStress-rupture life can range from less than 2 h to more than 1000 h depending on test load, which can cause significant delays for testing, shipping, and delivery of a product. In this work, we predict the stress-ruptureStress-rupture life and elongationElongation of HAYNES® 718 sheet product utilizing machine learningMachine learning models. The models were trained on data from 448 lots of material and inputs including composition, room temperature mechanical propertyMechanical properties data, processing data such as finish gauge, total reduction, final reduction, rule of mixture averaged properties, and environmental factors. Different sets of input features were chosen from the highest absolute Pearson correlation values, one-way ANOVA analysis, random forest (RF) model analysis methods, and generated compound features, and two separate RF models were trained using an 80–20% split between training and testing data. The resulting mean squared errors of best performing models of stress-ruptureStress-rupture life and elongationsElongation were 102 h and 7.2%, respectively. Input features of the highest importance were observed to be room temperature tensile propertiesTensile properties, finish gauge, and tramp elements such as Co, P, and Si. These models can be utilized to accelerate acceptance testing of 718 products by selecting the highest testing load that will still guarantee passing test life and elongationElongation results.

Superalloy 718Superalloy 718 and its derivatives are ubiquitous to aeroengine applications owing to their excellent formability, ultra-high strength, good thermal stabilityThermal stability, adequate weldability, and so forth. However, currently, the relatively lighter Ti-comprising refractory high entropy alloysTi-comprising refractory high entropy alloys (Ti-comprising RHEAs), having a unique combination of ambient and elevated temperature mechanical propertiesMechanical properties and corrosion resistance, are projected as potential competitors to superalloy 718Superalloy 718 and their derivatives. We analyzed the current literature data of, relatively lighter, Ti-comprising RHEAsTi-comprising refractory high entropy alloys by a novel combination of multiple attribute decision makingMultiple attribute decision making (MADM), hierarchical clustering (HC), and principal component analysis (PCA)—to identify the probable competitors to superalloy 718Superalloy 718 for aeroengine applications. The ranks assigned by six MADMs, chosen for the investigation, including ARAS (additive ratio assessment), MEW (multiplicative exponent weighing), OCRA (operational competitiveness ratio), ROVM (range of value method), SAW (simple additive method), and WEDBA (weighted Euclidean distance-based approach), were concordant. PCA consolidated the MADM ranks of the alloys, while HC identified similar top-ranked alloys. The analyses identify three Ti-comprising RHEAsTi-comprising refractory high entropy alloys, viz., ONS-BCC–Ti17.8 (Al20.4–Mo10.5–Nb22.4–Ta10.1–Ti17.8–Zr18.8), EF-BCC-Cr20–Ti20 (Ti20–Zr20–Hf20–Nb20–Cr20), and ONS-BCC–Ti27.9 (Al11.3–Nb22.3–Ta13.1–Ti27.9–V4.5–Zr20.9), having properties comparable or superior to superalloy 718Superalloy 718 and reveal their potential to substitute critical parts in aeroengines.

The superalloySuperalloys INCONEL 718Inconel 718 stands out for its excellent manufacturability and strength at ambient temperature. In most recent studies people tried to improve the mechanical propertiesMechanical properties of the alloy through adjusting different processing conditions such as solution annealing temperature, agingAging temperature and holding time, and the amount of intermediate cold work. Such studies could be expensive and time consuming. This study aims to build a CALPHADCALPHAD-based ICMEICME framework to investigate the microstructural stabilityMicrostructural stability and mechanical propertiesMechanical properties using the Thermo-Calc softwareThermo-Calc software. The evolution of precipitates is characterized using the TC-PRISMA precipitationPrecipitation module paired with the TCNI12 and MOBNI6 databases. The microstructureMicrostructures is simulated in terms of the nucleation and growth of the precipitates. The precipitationPrecipitation of the secondary phasesSecondary phase $${\gamma }{\mathrm{{\prime}}}$$ γ ′ , $$\gamma \mathrm{{\prime}}\mathrm{{\prime}}$$ γ ′ ′ and $$\delta $$ δ is simulated under different agingAging temperatures and their contributions to yield strengthYield strength are quantified. A simplified yield strengthYield strength model is applied to predict the precipitate, grain boundary and solid solution strengthening. The quantified results show good agreement with the experiment.

Inconel® alloy 740H® (UNS N07740) was the first age-hardenable nickel-based alloy approved by the ASME Boiler and Pressure Vessel Code for use in pressure-boundary applications. In recent years, advanced energy systems such as supercritical CO2 power cyclesSupercritical CO2 power cycles have utilized alloy 740H in large demonstration projects driven by the requirement for higher fluid temperatures and pressures. Stress relaxation crackingRelaxation cracking (SRxC) following post weld heat treatment (PWHT), also known as strain age crackingStrain age cracking (SAC), has been identified in a limited number of weldments during these industrial builds resulting in focused research to further clarify factors influencing this cracking tendency. This paper will summarize some of the findings from shop and field fabrication leading to successful welds and characteristics of observed SRxC. Laboratory experiments supported by microstructural characterizationMicrostructural characterization will be presented to highlight the importance of variables such as strain, material starting condition, and PWHT temperatures. Finally, the results will be summarized within the context of practical guidance for industry to successfully weld the material in boiler, heat exchanger, and piping applications.

Abradable honeycombHoneycomb sealing systems are widely used in turbines to improve efficiency and thus reduce carbon dioxide emissions. The honeycombHoneycomb sealing systems are produced by brazingBrazing together nickel-based superalloyNickel-based superalloys sheets with a nickel-based brazingBrazing metal. This investigation aimed to generate data on the mechanical performance of Haynes 214Haynes 214 metal sheets brazed with the nickel–chromium-silicon filler metal BNi-5 (71 wt.% Ni, 19 wt.% Cr, 10 wt.% Si). Tensile propertiesTensile properties of brazed metal sheet composites are tested. Interdiffusion zones and hard particles with high chromium contents are observed along the brazed joint. Even a very thin brazingBrazing layer reduces the ductility considerably.

The microstructureMicrostructures, mechanical propertiesMechanical properties, fracture behavior, and deformation mechanismsDeformation mechanism of a novel cast nickel-based superalloySuperalloys subjected to various agingAging treatments were investigated. The microstructureMicrostructures of this new alloy K4800 consists of γ, MC, M23C6, and the γ′ in two sizes after conducting a heat treatmentHeat treatment of solution annealing and double-stage agingAging. It is found that an initial higher temperature agingAging process is beneficial and somewhat necessary to optimize the tensile and creepCreep properties of the alloy. The strength of alloy K4800 rises with the volume fraction of small-sized γ′ phase; meanwhile, the precipitationPrecipitation of large-sized γ′ phase has a desirable impact on the elevated temperature ductility of the alloy. However, the existence of large-sized γ′ phase may accelerate the ripening process of γ′ phase and decrease of creepCreep life of alloy K4800. Therefore, accurate control of the content and proportion of the γ′ phases in two sizes is the key to obtaining an optimal mechanical propertyMechanical properties of K4800. TEM microstructureMicrostructures investigations show that the main strengthening mechanismsStrengthening mechanisms of the alloy are APB cutting at room temperature and Orowan bypassing at high temperature. Additionally, based on the results of a long-term agingAging experiment at 800 °C up to 2000 h, the material is not expected to precipitate any undesirable phase like σ or η, exhibiting an outstanding microstructural stabilityMicrostructural stability.

Development of Fe-based superalloysSuperalloys is highly demanded for both scientific interests and broad applications. Generally, the mechanical propertiesMechanical properties of traditional alloys strengthened by carbidesCarbides and Laves phaseLaves phases, etc., are degraded after long-term operation at HTs (>923 K) due to microstructural instability. Considering that the superiority of Ni-based superalloysNi-based superalloy benefits from their unique coherent microstructureMicrostructures, the coherent precipitationCoherent precipitation of cuboidal B2 nanoparticles will certainly improve the HT microstructureMicrostructures and mechanical propertiesMechanical properties of Fe-based superalloysSuperalloys. In the present work, we developed a new Fe-based superalloySuperalloys with cuboidal B2-NiAl nanoparticles coherently-precipitated into BCC matrix, which has not been reported. This alloy exhibits an excellent microstructural stabilityMicrostructural stability at 973 K with a slow particle coarsening rate, which is ascribed to the moderate lattice misfitLattice misfit (ε = 0.24 ~ 0.67%) between BCC/B2. Also, it is the cuboidal B2 nanoprecipitation that renders the current alloy with yield strengthYield strength of σYS = 238 ~ 258 MPa at 973 K.

Laser Powder Bed FusionLaser powder bed fusion (LPBF) of metallic components is unlocking new design options for high-efficiency gas turbine component designs not possible by conventional manufacturing technologies. Surface roughnessSurface roughness is a key characteristic of LPBF components that impacts heat transfer correlations and crack initiation from co-located surface defects—both are critical for gas turbine component durability and performance. However, even for a single material, there is an increasing diversity in laser machines (single vs multi-laser), layer thicknesses (~20–80μ) and orientations to the build plate (upskin, vertical, and downskin) that result in significant variability in surface roughnessSurface roughness. Build direction effects are particularly important when considering three-dimensional gas turbine components each having unique cooling features. This study systematically compares the external and internal surface roughnessSurface roughness of two gas turbine superalloysSuperalloys—Inconel 718Inconel 718 and Haynes 282Haynes 282—from multi-laser and multi-size machines. This presented data will be discussed in detail, to show potential applicability of a 3D process signature surface across machines and substrate orientations for additively manufactured superalloysSuperalloys.

There is increasing interest in the use of additive manufacturingAdditive manufacturing (AM) for superalloysSuperalloys due to their broad applications in the aerospace industry. As the raw material, high-quality metal powder is very important for successful powder bed fusion in AM. In this work, Haynes 230 powders manufactured by Vacuum Induction Melting Inert Gas Atomization (VIGA) and Plasma Rotating Electrode Process (PREP) were characterized and compared. Results demonstrated P-230 powder is superior to V-230 powders. P-230 powder exhibits a lower Hausner coefficient and better flowability. Meanwhile, attributed to superior sphericity and fewer satellite particles, lower dynamic angle of repose and cohesive index were achieved by P-230 powder, which means better dynamic flow and spreading of the powder during the recoating process of selective laser meltingSelective laser melting (SLM). In terms of their performance on SLM, the powder bed density of the P-230 powder is higher, and samples prepared with P-230 powder exhibited higher relative density. Although both V-230 and P-230 samples were all HIPed at 1200 ℃ for 4 h, P-230 samples revealed higher yield strengthYield strength at room temperature.

With the need to increase electric power generation efficiencies in order to reduce carbon emissions, operational temperatures of turbine components must increase. Oxide-dispersion strengtheningOxide-dispersed strengthened (ODS) has been proven to be useful in increasing the operational temperature of steels. In this study, yttria oxides (Y2O3) particles were mechano-chemical bonded (MCB) to IN718IN718 powder to investigate the ability of IN718IN718 to be strengthened by ODS particles at temperatures above 1000 °C. The Y2O3 infused IN718IN718 powder was direct energy deposited using two parameter sets for testing. Plates of standard IN718IN718 power were built using the same deposition parameters for reference. Specimen blanks were cut from plates and tested as built, with 1080 and 1200 °C homogenization heat treatmentsHeat treatment. The initial microstructureMicrostructures for all four conditions was dendric with significant Mo, Nb, and Ti segregation. Homogenization was observed to break up the initial large Mo and Nb precipitates and form Mo–, Nb–, and Ti–rich spherical precipitates. The 1080 °C homogenization heat treatmentHeat treatment retained the dendritic structure, while the 1200 °C was observed to partially recrystallize the grain structure. Minor changes in the tensile propertiesTensile properties were observed at 1050 °C by additions of Y2O3. This suggests that with further optimization, IN718IN718 may be utilized at temperatures above 1000 °C with ODS methods.

The as-printed microstructureMicrostructures of additively manufactured parts is a function of many variables that span from scan strategy to part geometry. This is particularly relevant in precipitationPrecipitation strengthened alloys (e.g. IN718IN718), as thermal history—which itself varies across a build—dictates the distribution of precipitates in the microstructureMicrostructures. Elucidation of the complex relationship between geometry, scan strategy, and resultant microstructureMicrostructures is necessary to optimize future scan strategies. In this study, a series of IN718IN718 dogbone samples were printed via selective laser meltingSelective laser melting at 0.8 mm in thickness using a contour + hatching scan strategy with both rotating (67°) and static beam directions. To quantify the effect of subsurface porosity on fatigue crack initiation, the as-built surface was left intact on the thinwall samples. As a reference, a 10 mm thick “bulk” sample was also printed and had 0.8 mm thick dogbones removed from it. To investigate the influence of scan strategy on the low-cycle fatigue (LCF) resistance of AM IN718IN718, these samples were tested to failure using a custom micromechanical test setup equipped with a linear actuator and digital image correlationDigital image correlation (DIC) to identify the onset of plasticity and the initiation and propagation of fatigue cracks across the sample surfaces. The driving hypothesis for this study is that the rotated scan strategy increases the fatigue resistance due to a more uniform microstructureMicrostructures with smaller grains. As-printed defects and fracture surfaces were characterized utilizing SEM imaging and DIC analysis and compared to observations from mechanical testing.

Nickel-based superalloyNickel-based superalloys Haynes 282Haynes 282 specimens were manufactured using the Laser Powder Bed FusionLaser powder bed fusion process with a powder layer thickness of 60 and 90 microns to study the effect of laser power, laser scan speed, and hatch distance on the melt pool dimensionsMelt pool dimensions and porosity. The melt pool dimensionsMelt pool dimensions and porosity were measured at the center of the cubes parallel to the build direction. Variation of melt pool depth and overlap exist within the same sample signifying the scatter present in the process. Laser scan speed was found to be the most significant parameter for porosity and hatch distance was found to be the most significant parameter affecting the average melt pool overlap depth in the cubes built with 60 microns layer thickness. Interaction of speed and hatch distance was found to be the most significant parameter for porosity and Laser scan speed was the most significant parameter for average melt pool overlap depth in cubes built with 90 microns layer thickness. Comparison of measured responses with individual parameters provides partial trends of melt pool dimensionsMelt pool dimensions and porosity. As the heat input is captured better in line energy and area energy density, a better trend of the melt pool dimensionsMelt pool dimensions data and marginal trend of porosity in comparison with energy densities is discussed. The ratio of maximum length to minimum length of defects such as porosity and lack of fusion is measured to determine the shape of the defects and averaged to provide insight into the dominant shape of defect for a given set of parameters.

Using laser powder bed fusionLaser powder bed fusion additive manufacturingAdditive manufacturing (L-PBF AM), a series of 10 sample walls were made, sectioned into fatigue specimens, and tested using high cycle fatigueHigh-cycle fatigue (HCF) testing with a stress ratio of 0.1 at 538 °C. Each wall was built with a different process pedigree, or set of process parameters, both to explore the effects of different process regimes (conduction, transition, keyhole) on the porosity and microstructureMicrostructures of AM IN718IN718 and to serve as a database of AM process conditions and their resultant defects. The internal porosity in the full gage regions of all fatigue specimens was characterized prior to HCF testing using X-ray computed tomography. Following fatigue testing, SEM fractography was used to identify the locations of fatigue critical flaws that led to failure. Using the data acquired in this work, the relationships between fatigue performanceFatigue performance of IN718IN718 at high temperature and the process conditions and associated porosity will be interpreted. These relationships could be adapted and used in a quality assurance model for L-PBF AM IN718IN718.

Additive manufacturingAdditive manufacturing processes such as laser powder bed fusionLaser powder bed fusion produce material by localized melting of a powder feedstock layer by layer. The small melt pools and high energy density generate very different microstructuresMicrostructures in nickel superalloysSuperalloys when compared to more traditional cast or wrought processing, including features such as cellular structures and epitaxial grain growth. The features of these microstructuresMicrostructures vary depending on local thermal history, alloy chemistry, and processing parameters. There is a need to develop a systematic understanding of the influence the local thermal conditions during solidification have on the resulting microstructureMicrostructures. Such understanding will be useful in predicting and ultimately avoiding microstructural defects such as undesirable phases or non-optimal grain structures. In this work, in-situ Longwave Infrared imaging of a laser powder bed fusionLaser powder bed fusion process is used to characterize the local thermal conditions throughout additively manufactured builds for alloy IN718Alloy IN718 processed using systematically varied process parameters. This information is then correlated to observations of the microstructural features of these alloys in the as-built condition. This correlation analysis shows clear influence of the local thermal conditions during solidification on the dimensions of the dendritic microstructuresMicrostructures formed during the build process for IN718IN718. These dendritic structures arise due to segregation of elements such as niobium during solidification, an observation which can be predicted using a Scheil modeling approach.

Ni-based alloys are used for high temperature structural components that span from small, highly complex, with fine feature resolution to large, simple shapes with low dimensional tolerances, necessitating the use of processes spanning from laser powder bed fusionLaser powder bed fusion (LPBF) to wire arc additive manufacturingWire-arc additive manufacturing (WAAM). However, there is very little understanding about how annealing behavior during post-build heat treatmentsPost-build heat treatment varies between additive manufacturingAdditive manufacturing processes. In this work, we explore the annealing behavior of IN625 and Haynes 282Haynes 282, manufactured with WAAM and LPBF, under the same annealing conditions. The results of hardness measurements after annealing indicate that for both IN625 and Haynes 282Haynes 282, the LPBF samples show larger decreases in hardness between the as-built condition and after annealing at 1200 °C for 1 h compared to the WAAM samples. LPBF IN625 and Haynes 282Haynes 282 samples annealed at 1200 °C for 1 h, all show complete and partial recrystallization, respectively, whereas none of the WAAM samples annealed at this temperature show recrystallization. For a given alloy, both LPBF and WAAM samples annealed at 1200 °C show particles with compositions consistent with MC carbidesCarbides that are predicted from thermodynamic simulationsSimulation. The MC particles present are of similar size and distribution in both LPBF and WAAM samples indicating a similar capacity for these particles to pin moving boundaries during recrystallization. In concert, these results suggest that LPBF samples have more stored energyStored energy in the as-built condition compared to their WAAM counterparts, and therefore have a higher driving force for recovery and recrystallization.

The properties of alloy 718Alloy 718 made by laser powder-bed fusion (LPBF) have been widely reported, and while their room-temperature static properties are often similar to wrought material, elevated temperature properties have proven inferior, especially at slow strain rates. Stress ruptureStress-rupture tests performed on LBPF 718 material after Hot isostatic Pressing (HIP) and heat treating in conformance with AMS 5662 have persistently led to brittle notch failures with limited elongationElongation. Creep testingCreep testing at 650 °C and 690 MPa shows a similar tendency, with samples showing capability of sustaining a load but with limited elongationElongation and abrupt, intergranular failure. Alternative heat treatmentsHeat treatment performed to enhance high-temperature ductility have shown success in adjusting delta phaseDelta phase population but without benefit for stress ruptureStress-rupture. Alternative HIP schedules have also been performed that led to changes in room-temperature and elevated temperature static strength but no benefit in stress ruptureStress-rupture behavior. The root cause of this behavior is attributed to the dispersion of NbC that is a consequence of the LPBF process that results in fine-scale segregation of Nb and C during rapid solidification. Deliberately lowering the carbon content of the powder feedstock led to a greater number of smaller Y” particles and smaller size NbC particles which resulted in an increase in static strength at room and elevated temperatures but no improvement in stress ruptureStress-rupture. The presence of a large number of NbC particles leads to environmental sensitivity of LPBF 718 that is most apparent at elevated temperatures and slow strain rates.

In this study, the microstructureMicrostructures and deformation mechanisms of selective laser melted GTD222 and TiC/GTD222 composite were studied. The results show that the TiC/GTD222 composite has finer grains and more precipitationPrecipitation phases. Meanwhile, TiC/GTD222 composite has higher yield strengthYield strength both at room and high temperatures, which can be mainly attributed to the synergistic effect of the TiC strengthening and γ′ strengthening. The deformation mechanisms of TiC/GTD222 composite at 800 °C were identified as isolated stacking faults shearing the γ′ phase, continuous stacking faults shearing the γ and γ′ phases, dislocations cutting the γ′ phase, and dislocation slip within the γ matrix. This study provides insights for understanding the influence of TiC particles on the deformation mechanisms of additive manufactured nickel-based alloys.

SuperalloysSuperalloys are commonly used in structural components of aero-engines. SuperalloysSuperalloys in general, Ni- and Ni–Fe-based superalloysSuperalloys, belong to an important group of materials used in aerospace applications. Fabrication and associated weldability aspects of structural components for the hot section of aero-engine gas turbines continue to be of high importance to the manufacturing industry within this discipline. Cracking and specifically hot crackingHot cracking as well as strain age crackingStrain age cracking is a serious concern during the weldingWelding and additive manufacturingAdditive manufacturing (AM) of these structural components. The cracking phenomena can occur during weldingWelding, AM or subsequent heat treatmentHeat treatment of precipitationPrecipitation-hardening superalloysSuperalloys. The cracking behaviour can be influenced by several factors, i.e., chemical composition in terms of hardening elements and impurities, the microstructureMicrostructures of base material, and weld zone, together with corresponding weldingWelding, AM and post-treatment process parameters. This paper provides a review of Ni- and Ni–Fe-based superalloysSuperalloys concerning fabrication and weldability aspects within the context of structural components of aero-engines. Also, the paper offers insight and analyses to research publication data of weldingWelding and AM of superalloysSuperalloys in the context of annual publication developed over the years as well as specific contributions from countries, affiliations, and specific researchers.

The anisotropyAnisotropy in tensile propertiesTensile properties of Wire Laser Metal Deposited Inconel 718Inconel 718 (LMD-w)LMD-w has been investigated from room temperature up to 750 °C at a strain rate of 5.0 10–4 s−1. These properties have been investigated along, at 45° and perpendicular to the building direction. Moreover, different heat treatmentsHeat treatment have been used: as-built, solution heat treated to dissolve Laves phasesLaves phases, solution treated + aged to trigger γ′/γ″ precipitationPrecipitation and direct-aged. According to this extensive characterization of tensile propertiesTensile properties, complemented by SEM and EBSD characterizations, it is shown that, whatever the temperature, Yield stress and tensile resistance have a very weak anisotropyAnisotropy and that tensile propertiesTensile properties are mostly dependent to the prior heat treatmentHeat treatment state. The anisotropyAnisotropy is mostly observed on elastic properties, due to a pronounced crystallographic texture inherited from the directional thermal gradient during the building process. Moreover, Laves phasesLaves phases do not seem to have a strong impact on tensile propertiesTensile properties for this coarse grain material. Tensile strength in such an LMD-processed Inconel 718Inconel 718 is mostly controlled by the γ′/γ″ precipitationPrecipitation and stored “processing” dislocations. A loss of tensile ductility has been evidenced at 750 °C, due to grain boundary oxidation.

The microstructureMicrostructures and tensile propertiesTensile properties of direct-aged WaspaloyWaspaloy manufactured using wire arc-based Cold Metal Transfer (CMT) have been investigated. Samples were exposed to temperatures ranging from 700 to 900 °C, for up to 96 h. In the as-deposited condition, pronounced chemical segregation is inherited from the process, leading to heterogeneous γ′ precipitationΓ' precipitation between dendrite cores and interdendritic spacings. γ′ size and distribution were measured in both areas for each heat treatmentHeat treatment, and a diffusion-controlled coarsening behavior following the Lifshitz–Slyozov–Wagner theory was observed for temperatures above 760 °C. Activation energies were calculated. Tensile testsTensile tests at room temperature were carried out not only on the additively processed alloy before and after agingAging but also on wrought sub-solvus and super-solvus treated material for reference. Results showed that heat treatmentHeat treatment significantly increased the yield strengthYield strength and ultimate tensile strength of the CMT samples, of up to +340 MPa compared to the as-built conditions. ElongationElongation, however, decreased from 40–45% to 16–28%. Direct-aged CMT WaspaloyWaspaloy exhibited similar behavior to that of wrought super-solvus WaspaloyWaspaloy, due to their large grains (~200–250 µm). AnisotropyAnisotropy in tensile propertiesTensile properties was estimated by calculating the ratio of properties for horizontal and vertical specimens. Finally, the formation of intermetallic phases was assessed. Thermodynamic calculations predicted the formation of M23C6, η, and σ phases in interdendritic spacings at thermodynamic equilibrium in the range 700–900 °C. Using electron diffraction patterns and energy-dispersive X-ray spectrometry, intergranular (Cr, Mo)23C6 secondary carbidesCarbides decorating grain boundaries and located near (Ti, Mo)C primary carbidesCarbides in the interdendritic spacings were observed to nucleate and grow.

Cold gas spray is an established process for coating substrates with similar or dissimilar materials. By use of a high-pressure process gas stream, solid particles are accelerated onto a substrate at supersonic velocities. The method is particularly suited for repair applications since neither structural changes nor oxidation occur during the process. To investigate the suitability of the cold gas spray process for the repair of major defects with up to 4 mm depth in Inconel 718Inconel 718 components, sample geometries were manufactured, containing tapered cavities. The specimen cavities were filled with Inconel 718Inconel 718 particles by a cold gas spray process. Non-destructive high-resolution neutron diffractionNeutron diffraction experiments were performed by use of the SALSA instrument at the Institut Laue-Langevin (ILL) to evaluate the local residual stressResidual stress analysis state in the as-sprayed condition. 2D maps of the residual stress distribution over the cross-sectional area of the filled cavities were determined. The results indicate compressive residual stresses within the filled process zone. Metallographic examinations show a good bonding between the repair filling and the substrate as well as strongly deformed particles within the repaired region. The latter indicates significant plastic deformation during cold gas spray, which is also in good agreement with increased diffraction line width from the neutron diffractionNeutron diffraction analyses in the filled process zone compared to the surrounding substrate.

In this work, two graded transition interlayers were designed using a CALPHADCALPHAD-based ICMEICME framework (CALPHADCALPHAD: Calculation of Phase Diagrams; ICMEICME: Integrated Computational Materials EngineeringIntegrated computational materials engineering) for joining Inconel 740H superalloySuperalloys with P91 steel. Successful builds with the designed interlayers (content of P91 steel are 60 and 85 wt.%) sandwiched between the constituent materials were fabricated using wire-arc additive manufacturingWire-arc additive manufacturing. 60% P91 interlayer exhibited an FCC structure with low hardness, while the 85% P91 interlayer had a martensitic structure with high hardness. A two-step post-heat treatmentHeat treatment consisting of homogenization at 1150°C and agingAging at 760°C was designed. 60% P91 interlayer showed no improvement in hardness after agingAging. It agrees with the CALPHADCALPHAD modeling that predicts a lack of effective strengthening precipitates at 760°C, whereas the hardness of 85% P91 increased significantly after agingAging for 8 h due to the precipitationPrecipitation of the M23C6 phase. Mechanical tests equipped with digital image correlationDigital image correlation were performed to determine the location of the failure and tensile propertiesTensile properties. As-built and heat treated 60% P91 build failed in the graded alloy block, whereas the as-built 85% P91 alloy failed at the 85% P91/740H interface, and the aged alloy failed in the pure P91 region. This proves that post-heat treated 85% P91 is much stronger than pure P91, and the alloy designAlloy design strategy used in this work proves successful for developing interlayers for dissimilar joining.

Post-heat treatmentHeat treatment optimization is imperative to improve the mechanical propertiesMechanical properties of superalloysSuperalloys prepared by additive manufacturingAdditive manufacturing. In this work, the effect of solution treatment on the microstructural heterogeneity and γ′ precipitationΓ' precipitation of Haynes 282Haynes 282 fabricated by wire-arc additive manufacturingWire-arc additive manufacturing (WAAM) has been investigated. The results suggest that the standard solution treatment carried out at 1150 °C for 2 h is insufficient to remove the grain texture developed during WAAM. Instead, a heat treatmentHeat treatment at 1250 °C for 2 h facilitates homogenization and recrystallization without causing excessive coarsening. Furthermore, solution treatment temperature affects the kinetics of γ′ precipitate growth. By increasing the solution heat treatmentHeat treatment temperature from 1150 to 1250 °C, strengthening γ′ precipitates grow faster, achieving the peak hardness earlier. Moreover, the increase in solution temperature favors the development of a bimodal distribution of γ′ precipitates during agingAging. This work demonstrates the need for an effective post-heat treatmentHeat treatment to eliminate the heterogeneities that are formed during the WAAM process and alter the γ′ size distribution to improve the mechanical performance of Haynes 282Haynes 282 alloy.

The usage of additive manufacturingAdditive manufacturing as a process for component production is becoming increasingly important, as it offers enormous potential for material savings and therefore cost reduction. In particular, Wire Arc Additive ManufacturingWire-arc additive manufacturing (WAAM) processes are arousing a great deal of interest in several industries by its high deposition rates at low equipment acquisition costs and the low buy-to-fly ratio. This process is being specifically investigated for aerospace and space applications, as it allows the production of large structural complex near-net-shape components in small batches. However, a major drawback of this technology is the high anisotropic behaviour of the manufactured structures in the as-welded state. Since the Ni–Fe alloy Inconel 718Inconel 718 is an anisotropic material, in which introduced textures strongly influence the mechanical propertiesMechanical properties, the impact of the WAAM processing route on the mechanical propertiesMechanical properties as well as the underlying microstructureMicrostructures is specifically focused on in this study. Using a plasma arc as heat source and Inconel 718Inconel 718 wire as feedstock material, test walls are produced in order to characterize the created material. In addition to the identification of factors influencing the process, temperature cycles are measured at different positions during the build-up. The resulting microstructureMicrostructures is subsequently evaluated macroscopically as well as microscopically and examined regarding pores and precipitates. SEM/EDS analysis is carried out to investigate the underlying microstructureMicrostructures of the additively manufactured parts. Furthermore, mechanical propertiesMechanical properties are evaluated in the build-up direction as well as transversal to this direction in order to characterize the anisotropyAnisotropy of the material.

The influence of keyhole TIG (K-TIG) weldingWelding parameters on the weld geometryWeld geometry and defects of a new Co-lean nickel-based superalloyNickel-based superalloys G27 was studied, and the microstructuresMicrostructures of the heat-affected zone (HAZ) and fusion zone (FZ) of the K-TIG-welded Alloy G27 were characterized. No cracks are found in the FZ and HAZ. Minimum weld width and face underfill statistically were significantly influenced by travel speed and interaction current*travel speed. Root excess weld metal was only significantly influenced by travel speed. Face excess weld metal was significantly influenced by all the factors, including their interaction. On the other hand, all the factors, including their interaction, did not significantly influence the average pore diameter. In the FZ microstructureMicrostructures, interdendritic microconstituents are identified as (Nb, Ti)C particles and γ/Laves eutectic constituents. In addition, a plate-like phase is observed surrounding the Laves phaseLaves phases, and γʹ precipitates are found to be inhomogeneously precipitated in the FZ. In the partially melted zone (PMZ), (Nb, Ti)C, Laves phaseLaves phases, γʹ precipitates, and plate-like particles are found in the liquated and resolidified regions, suggesting the solidification behavior in PMZ is likely to follow a similar pattern to the one observed in FZ.