Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives: Energy, Aerospace, and Industrial Applications

Advanced Ultrasupercritical (A-USC) steam Rankine cycles and Supercritical Carbon Dioxide (sCO₂) Brayton cycles are under intensive development to enable low carbon generation of electricity. These high-efficiency power cycles, aimed at fossil and in some cases renewable energy, require higher temperatures and pressures compared to traditional steam cycles for pressuring retaining components such as tubing, piping, heat exchangers, and turbine casings. Extensive research and development to produce and characterize age-hardenable nickel-based alloys containing Al, Ti, and Nb in judicious amounts have allowed designers to now consider supercritical fluid temperatures up to ~760 °C which is much greater than today’s supercritical steam technology based on steel metallurgy up to ~610 °C. This paper will focus on the alloys developed around the world to enable these advanced power cycles, and a discussion on their key properties: long-term creep strength (100,000 h+), fabricability, and weldability/weld performance. Most of these alloys contain less than 25% gamma prime, such as alloy 740H, 263, and 282, due to the need for heavy section weldability, unique to these applications. While welding processes have now been developed for many of these alloys using a variety of filler metals and processes, key research questions remain on the applicability of processes to field power plant erection, the potential for cracking to occur during service, and the long-term weld creep and creep-fatigue performance.

Alloy 718Alloy 718 (UNS N07718UNS N07718) was the culmination of a research project started in the mid-1950s to develop a stronger pipe alloy for coal-fired power plants. It was never used for that application, but it was quickly adopted for aircraft turbine engines because of its very high strengthHigh strength, thermal stability, formability and weldabilityWeldability compared to the γʹ-strengthened alloys available in the 1960s. Alloy 718Alloy 718 derives its unique combination of strengthStrength and fabricability from a coherent ordered tetragonal phase γʺ and the slow diffusion rate of its main constituent, niobiumNiobium. Very early in its commercial life, alloy 718Alloy 718 was recognized as having attributes for both ambient and cryogenic temperatureTemperature uses as well. Impact toughness, aqueous corrosionAqueous Corrosion resistance, and non-ferromagnetic properties were important attributes. Alloy 718Alloy 718 replaced established age-hardened iron/nickel-base alloys such as A-286, K-500 and X-750 as well as martensitic steels in a wide range of components in the space launch, oil and gasOil and Gas, marine, nuclear and superconducting magnet industries. As the applicationsApplications became more specialized, so did the heat treatments and microstructuresMicrostructures to accentuate specific properties. A common thread through these low temperatureTemperatureapplicationsApplications has been hydrogen embrittlementHydrogen embrittlement and a substantial body of literature documents our increasing awareness of its role in service performance. In recent decades, new alloys based on alloy 718Alloy 718 and the γʺ strengthening phase have been introduced, especially for oil and gasOil and Gas production equipment. This paper describes the early development of alloy 718Alloy 718 for these important applicationsApplications, along with alloy, microstructureMicrostructure and heat-treatment evolution and current status.

Alloy 718Alloy 718 has been utilized successfully in both static and rotating turbo machinery applicationsApplications for four decades. The combination of high strengthHigh strength, fatigueFatigue capability, rupture strengthStrength, corrosion and creepCreep resistance at temperatures through 650 °C are key attributes of this alloy. Conventional manufacturing routes include cast, wrought, sheet, joining and fabrication by weldingWelding and brazing, powderPowder metallurgical processing and metal injection molding. Recent investigation of aerospace materials like Alloy 718Alloy 718 produced by additive manufacturingAdditive manufacturing technology has provided an opportunity for disruptive component manufacturing methods, geometries, and component capabilities which expand designDesign space for complex applicationsApplications. At GE Aviation (GEA), development of laser powderPowder bed direct metal laser meltingMelting (subsequently referred to as DMLM) Alloy 718Alloy 718 was a natural choice following the successful commercial application of DMLM CoCrMo in GE Aviation and Safran’s LEAP platform fuel tip component and the GE90 T25 sensor part. GEA’s DMLM 718718 development started with demonstrator military applicationsApplications, and now has expanded to include multiple commercial engine applicationsApplications across the size range of the GEA product line. The additive (laser) process development for Alloy 718Alloy 718 involved a combination of laser processing parameter investigation and heat treatmentHeat treatment development to produce both acceptable build geometry and metallurgical microstructuresMicrostructures. Initial developments started with 20 μm build layer thickness, and expanded to 50 μm thicknesses for improved build productivity. From the onset, the materials engineering focus was on isotropic, fine grain, pore free and fully developed microstructuresMicrostructures for Alloy 718Alloy 718 by heat treat designDesign. Mechanical characterizationCharacterization included consideration of build direction, machine type, machine to machine variation and processing gas effects. This paper will discuss various metallurgical challenges and related, microstructureMicrostructure & mechanical characterizationCharacterization of DMLM Alloy 718Alloy 718 .

This paper reviews currently available information on the processing and understanding of Alloy 718718 fabricated through powderPowder bed additive manufacturingAdditive manufacturing processes, specifically selective laser meltingSelective laser melting, electron beam meltingElectron beam melting, and binder jet additive manufacturing. In each instance, the microstructuresMicrostructure formed exhibit attributes unique to the process used. Through post-processingPost-processing, these materialsMaterial are capable of achieving property behaviors similar to that of the long utilized wrought material. While AM processes are complex, computational modeling has been successfully applied to capture the heat and mass transfer, microstructure evolutionMicrostructure evolution, and constitutive response of the material.

PowderPowder-bed additive manufacturingAdditive manufacturing processes use fine powders to build parts layer-by-layer. Alloy 718Alloy 718powderPowder feedstocks for selective laser meltingSelective laser melting (SLMSLM) additive manufacturingAdditive manufacturing are produced commercially by both gas and rotary atomization and are available typically in the 10–45 or 15–45 µm size ranges. A comprehensive investigation was conducted to understand the impact of powderPowder variability on the microstructureMicrostructure and mechanical behavior of SLMSLM718718 heat treated to Aerospace MaterialMaterial Specification (AMS) 5664. This study included sixteen virgin powders and three once-recycled powders within the 10–45 and 15–45 µm size ranges that were obtained from seven direct source suppliers and one reseller. Although alike as highly regular spheroids, these powders showed distinct differences in compositionComposition (especially Al, C and N contents), particle size distributions, and powderPowder features such as degree of agglomeration, fusion and surface roughness. Compositional differences expectedly had the strongest impact on microstructureMicrostructure. High N and C contents formed TiN-nitrides and/or (Ti, NbTi/Al ratio, Mo)-C carbides on the grain boundariesGrain boundaries, prevented recrystallizationRecrystallization during heat treatmentHeat treatment, and resulted in retained (001)-scalloped shaped grains that ranged from 19 to 41 µm in average size. In the absence of this particle pinning, the average grain sizeGrain size of the heat treated SLMSLM718718 ranged from 51 to 90 µm. Room temperatureTemperature tensile and high cycle fatigueHigh cycle fatigue (HCF) testing compared as-fabricated (AF) and low stress ground (LSG) surface conditions. Tensile testing revealed consistent behavior between the two surface conditions and amongst the powderPowder lots. The finer grained SLMSLM718718 builds displayed the lowest tensile propertiesTensile properties. A SLMSLM718718 build fabricated from a powderPowder with eight times lower C content showed statistically better tensile propertiesTensile properties presumably due to enhanced coarsening of δ-Ni₃Nb precipitates. The specimens from once-recycled powders had slightly higher tensile strengths and slightly higher ductility compared to their virgin equivalents; once-recycling also did not substantially degrade the mean HCF life. The LSG fatigueFatigue lives agreed with conventionally manufactured 718718 data, while AF lives exhibited a knock-down due to surface roughness. The fatigueFatigue lives of AF specimens were statistically equivalent across powderPowder lots except for one and failures typically initiated at stress concentrators associated with SLMSLM surface asperities. Fatigue testingFatigue testing of low stress ground specimens result in both transgranular and within facet crack initiations. More than half of the cracks initiated from these facets for the machined condition; however, these facets appeared to be within grains that were larger-than-average in size. A nitrogen-atomized powderPowder with fine prior particles of TiN-nitrides and M(TiTi/Al ratio, Nb, Mo)C carbides from atomization on powderPowder surfaces resulted in the best fatigue performanceFatigue performance with segregation of these particles to the SLMSLM718718grain boundariesGrain boundaries leading to higher resistance to early-stage crack propagation. Typically the fine-grained builds with minor phases along the grain boundariesGrain boundaries did not perform well in fatigueFatigue, whereas a larger-grain build with lower carbon content and coarser δ-Ni₃Nb precipitates showed the next best HCF response. Further details of the build microstructureMicrostructure and its impact on tensile and fatigueFatigue behavior was considered.

Additive manufacturingAdditive manufacturing (AM) of Ni-based superalloysNi-based superalloys such as Alloy 718Alloy 718 may obviate the need for difficult machiningMachining and weldingWelding operations associated with geometrically intricate parts, thus potentially expanding designDesign possibilities and facilitating cost-effective manufacture of complex components. However, processing AM builds completely free from defectsDefects, which may impair mechanical propertiesMechanical properties such as fatigueFatigue and ductility, is challenging. Anisotropic properties, microstructural heterogeneities and local formation of undesired phases are additional concerns that have motivated post-treatmentPost-treatment of AM builds. This work investigates the microstructural changes associated with post-treatmentPost-treatment of Alloy 718Alloy 718 specimens produced by Electron Beam MeltingElectron beam melting (EBMElectron beam melting (EBM) process) for as-built microstructuresMicrostructures at 3 build heights: near base plate, in the middle of build and near the top of the build. Two different post-treatmentPost-treatment conditions, hot isostatic pressing (HIPHIP) alone and a combined HIPHIP with solutionising and two-step agingAging were examined and compared to the results for the as-built condition. The influence of various post-treatments on minor phase distributions (δδ, γ″, carbides), overall porosity, longitudinal grain widths and Vickers microhardness was considered. The HIPHIP treatment led to significant reduction in overall porosity and dissolution of δδ phase, which led to appreciable grain growth for both post-treatmentPost-treatment conditions. The variation in hardnessHardness noted as a function of build height for the as-built specimens was eliminated after post-treatmentPost-treatment. Overall, the hardnessHardness was found to decrease after HIPHIP and increase after the full HIPHIP, solutionising and agingAging treatment, which was attributed to dissolution of γ″ during HIPHIP and its re-precipitationPrecipitation in subsequent heat treatmentHeat treatment steps.

Metal additive manufacturingAdditive manufacturing (AM) is an innovative and enabling manufacturing technology that is also pervasive/cross cuttingCutting in terms of system applicationsApplications, dual-use potential and interest from multiple agencies. AM technologies build near-net/net shape components, one layer at a time, using digital data from 3D CAD models. In addition, AM has the potential to enable novel product designs that could not be fabricated using conventional subtractive processes. The goal of this Metals Affordability Initiative (MAI) project (HON-9 Agreement Order Number FA8650-14-2-5204) is to create a cross-functional team focused on developing the necessary Integrated Computational Materials EngineeringIntegrated computational materials engineering (ICME) based framework, knowledge and supporting models to enable powderPowder bed AM production of nickel-based superalloyNickel-based superalloy aerospace and space components. An Activity Integrated Project Team (AIPT) comprising of Honeywell Aerospace (Lead), Aerojet Rocketdyne, ATI PowderPowder Metals, Carpenter PowderPowder Products, Lockheed Martin, Northrop Grumman, Rolls-Royce Corporation, Arconic Inc. along with Applied Optimization and QuesTek as major subcontractors was formed. The AIPT successfully completed the concept feasibility demonstration for additively manufactured Alloy 230 components. A focused series of designDesignof experiments (DOEDesign of Experiments (DOE) to establish AM build parameters) related to machine parameters and post processing operations were designed and implemented within Concept Laser Cusing M2 machine. The collected empirical data was used to optimize process parameters, calibrate ICME models, and improve tool maturity level (TML) of the ICME framework for AM of Ni superalloySuperalloy components. A preliminary business case was developed for parts from Honeywell Aerospace, Aerojet Rocketdyne, Rolls-Royce Corporation, Northrup Grumman and Lockheed Martin.

In this paper, we will study the co-precipitationPrecipitation kinetics of phases in Superalloy 718Superalloy 718 using the simulationSimulation tool we have developed using the CALPHAD approachCALPHAD approach. This tool considers concurrent nucleation, growth and coarsening of these precipitates. Furthermore, it is directly integrated with thermodynamic calculationThermodynamic calculation engine to obtain instant update of phase information, such as the compositionComposition of the matrix and the nucleation driving force for each precipitate. In addition to the average particle size, the more advanced KWN (Kampmann and Wagner Numerical) model was implemented to allow for predication of the full evolution of the particle size distribution (PSD). In this paper, we will perform virtual experiments using this tool to simulate the co-precipitationPrecipitation of the γ′ and γ″ phases under different heat treatmentHeat treatment conditions. SimulationSimulation results, such as temporal evolution of volume fraction, number density, and mean size of the precipitates, as well as the final particle size distribution will be presented and discussed. The impact of δδ precipitate and the initial microstructureMicrostructure will also be briefly discussed. These virtual experimental results can be used to understand the microstructural features of Superalloy 718Superalloy 718 and serve as guidance for further optimization of heat treatmentHeat treatment schedule.

As power generation systems move towards higher efficiency operation above 700 ℃, wrought superalloys are the leading structural alloy candidates, including precipitationPrecipitation strengthened (PS) alloys 740 and 282 for the highest temperatures. To evaluate the performance of these alloys for these applicationsApplications, a range of 500–5000 h evaluations have been conducted in environments including steamSteam, supercritical CO_{2Supercritical CO2} (sCO₂) and simulated combustion exhaust with H₂O and/or SO₂ at 700–800 ℃ and compared to baseline exposures in laboratory air and 1 bar CO₂. These alloys primarily rely on the formation of an external Cr-rich oxide layer or scale for environmental protection and the reaction rates in all of these conditions are similar and relatively low. However, compared to a conventional solid solution strengthened alloy, like 625, the mass gains are higher for PS alloys due to the internal oxidationOxidation of the γ′ forming additions, Al and TiTi/Al ratio. Post-exposure characterizationCharacterization has quantified the reaction products and the depth of internal oxidationOxidation is not a concern and does not appear to increase above the baseline behavior in laboratory air. Likewise, there is no indication of internal carburizationCarburization in the sCO₂ environment at 750 ℃/300 bar. The addition of 0.1% SO₂ in CO₂-10% H₂O at 800 ℃ actually suppressed the internal oxidationOxidation at 1 bar but SO₂ may be a concern when the total pressure is higher.

Two Ni-Cr-Mo-Nb superalloysNi-Cr-Mo-Nb superalloys (Alloy 625 and Alloy 718Alloy 718) were corroded in high temperatureTemperature supercritical-CO₂ (S-CO₂) at 700 ℃ (20 MPa) for 500 h and compared in terms of oxidationOxidation and carburizationCarburization behavior. A continuous chromia (Cr₂O₃) layer was formed on the surface of Alloy 625, whereas Ni- and Fe-rich oxide nodules were also formed with chromia on Alloy 718Alloy 718. Meanwhile, the extent of carburizationCarburization by formation of an amorphous C layer at the oxide/matrix interface was comparatively low for Alloy 625. This difference did not seem to stemSTEM from oxide type or underlying microstructureMicrostructure, and was thought to be associated with oxide properties. In terms of mechanical propertiesMechanical properties, only Alloy 625 exhibited decrease in ductility after exposure to S-CO₂. This was ascribed to the microstructural evolutionMicrostructural evolution of the alloys during the high temperatureTemperature exposure.

A precipitationPrecipitation hardened Ni-base alloyNi-base alloy has been developed to fulfil the recent stringent requirements of the oil and gasOil and Gas industry. The new alloy is commercially designated as AF955AF955 and the Unified Numbering System (UNS) assignment of N09955. The new alloy is patented and accepted for inclusion in NACE MR0175/ISO 15156. It is produced at strengthStrength levels of 827 MPa (120 ksi) 0.2% offset minimum yield strengthStrength (MYS) and 965 MPa (140 ksi) MYS with very good ductility and toughness, and with a microstructureMicrostructure characterized by fine γ′ and γ″ strengthening precipitates, uniform and equiaxed grain sizeGrain size distribution, minimized secondary phase precipitationPrecipitation and free of continuous grain boundary precipitates. It exhibits good corrosion resistance and low susceptibility to hydrogen embrittlementSusceptibility to hydrogen embrittlement. These properties make alloy AF955AF955 a very promising materialMaterial for widespread applicationsApplications from oil and gasOil and Gas industry to power generation, chemical applicationsApplications and many others. Details of the manufacturing processManufacturing process, The and properties of AF955AF955 are presented and discussed in this paper.

Nickel base alloys such as 718718 are used for many applicationsApplications in the drilling, completion and production segments in the Oil and Gas Industry. The alloy selection is based on high strengthHigh strength levels while exhibiting resistance to embrittlement and environmental crackingCracking. Hydrogen embrittlementHydrogen embrittlement can be a limiting factor to applicationsApplications and this investigation was undertaken to better understand the mechanisms and characteristics of hydrogenHydrogenin 718IN 718. Saluted hydrogenHydrogen into metal could be presented in different conditions: diffuse-active and trapped by different defectsDefects and structure elements. FatigueFatigue was used in current work as a tool for (1) the generation of structure defectsDefects and (2) hydrogenHydrogen effects on crack growthCrack growth. The following items were studied: (1) hydrogenHydrogen solubility into different versions of 718718 alloy; (2) effects of increased surface and volume defectsDefects density on hydrogenHydrogen solubility; (3) hydrogenHydrogen effects on stress-strain evolution; and (4) effects of hydrogenHydrogen at different locations within the structure on crack growthCrack growth rate. The specifics of each type of hydrogenHydrogen location within the structure on crack propagation including diffusion-active and trapped by different defectsDefects and structure elements were discovered and presented.

OxidationOxidation of Alloy 718Alloy 718 manufactured by electron beam melting (EBM) processElectron beam melting (EBM) process has been undertaken in ambient air at 650, 700, and 800 °C for up to 168 h. At 800 °C, a continuous external chromia oxide enriched in (Cr, TiTi/Al ratio, Mn, Ni) and an internal oxide that was branched structure of alumina formed, whereas at 650 and 700 °C, a continuous, thin and protective chromia layer was detected. The oxidationOxidationkinetics of the exposed EBMElectron beam melting (EBM) processAlloy 718Alloy 718 followed the parabolic rate law with an effective activation energy of ~248 ± 22 kJ/mol in good agreement with values in the literature for conventionally processed chromia-forming Ni-based superalloysNi-based superalloys. The oxide scale formed on the surface perpendicular to the build direction was slightly thicker, and more adherent compared to the scale formed on the surface along the build direction, attributed to the varied grain textureGrain texture in the two directions of the EBMElectron beam melting (EBM) process-manufactured specimens. The increased oxygen diffusionOxygen diffusion and high Cr depletionDepletion found on the surface along the build direction were attributed to the fine grains and formation of vacancies/voids along this grain orientation.

This paper presents a comprehensive computational model for the prediction of the transient Electroslag RemeltingElectroslag remelting (ESRESR) process for cylindrical ingots based on a two-dimensional axisymmetric analysis. The model analyzes the behavior of the slag and growing ingot during the entire ESRESR process involving a hot-slag start with an initial transient, near-steady meltingMelting, hot-topping and subsequent solidificationSolidification of the slag and ingot after meltingMelting ends. The results of model application to an industrial ESRESR process for a 1.12 m diameter nickel-iron-chromium superalloySuperalloy and its validation are presented. They demonstrate the comprehensive capabilities of the model in predicting the behavior of the ingot and slag during the entire process and properties of the final ingot produced. Such analysis provides significant benefits for the optimization of existing process schedules and designDesign of new processes for different alloys and ingot sizes.

In order to reduce the porosity and improve the performance stability of IN718C alloy castings, hot isostatic pressing (HIPHIP) process are usually conducted. Due to the different solidificationSolidification conditions, the segregation and microstructureMicrostructure present various features in different positions of castings, especially for castings showing large difference in wall thickness. The above mentioned factors can influence the effect of HIPHIP, cause incipient meltingMelting of low meltingMelting point phase, such as Laves phaseLaves phase, then induce the scrapping of castings eventually. So it is necessary to conduct homogenizationHomogenization treatment for the castings with large differences in solidificationSolidification conditions before HIPHIP process. In this article, the as-cast microstructuresMicrostructures of IN718C alloy with different solidificationSolidification cooling rates were studied, including segregation, phase constitution, etc. Then the microstructuresMicrostructures of samples after homogenizationHomogenization treatment from 1075–1125 °C were analyzed. The effect of homogenization temperatureHomogenization temperature on the elimination of segregation and dissolution behaviour of different phases were also investigated. Some relationships about homogenizationHomogenization temperatures, microstructuresMicrostructures and solidificationSolidification cooling rates for IN718C alloy have been determined. They can be used to choose homogenization temperatureHomogenization temperature for IN718C investment castings with complex shapes before HIPHIP process.

The mixed refiner of Co₃FeNb₂ and CrFeNb ternary intermetallic compounds and melt superheating treatmentMelt superheating treatment under the modified thermally-controlled solidificationSolidification (TCS) process have been designed to achieve grain refinement and further improve mechanical propertiesMechanical properties of IN718 superalloySuperalloy. It is found that the refinement effect of addition grain refinersGrain refiners under TCS process is quite superior to that by convention castingCasting (CC) process with refiner, as experiment results show that the grain sizeGrain size is refined from 3340 to 126 μm and greatly reduce the amount of porosity, during the refinement process. In addition, the applied melt treatment can refine grain sizeGrain size from 6420 to 89 μm and greatly reduce the amount of porosity. Meanwhile, after grain refinement, the stress ruptureStress rupture property under 650 °C/620 MPa is significantly improved. Grain refinement mechanisms are also discussed.

Vacuum induction meltingMelting (VIM) and electroslag remeltingElectroslag remelting (ESRESR) are techniques used to produce ingots of alloys with complex chemistries while lowering the amount of defectsDefects, inclusions or extent of elemental segregation. Those practices are widely employed in aerospace applicationsApplications and more recently in fossil-fueled power plant components due to the increasingly demanding operating conditions. Consequently, research is ongoing to improve and control the meltingMelting of commercially available alloys for optimal performance in service. In this investigation, a laboratory-scale (200 mm–200 kg) ESRESR furnace was used to remelt various alloys with a focus on the ingot quality. Several approaches were considered to study and improve the meltingMelting characteristics. Targeted additions of minor elements in master alloys were found to improve the melt range which affected the melt pool volume and subsequently increased the remelting efficiency. Furthermore, the melt parameters during ESRESR of some select alloys were modified to improve the meltingMelting characteristics. Finally, the influence of the size of the ESRESR electrode was observed and provided a better understanding of the mixing mechanisms in the slag region and their effect on the voltage swing and melt rateMelt rate. The results are presented using a combination of experimental and computational (thermodynamic and CFD-based) data.

The current state of the art of superalloys manufacturing process makes possible to obtain materials substantially clean of oxides associated with a fine grain structure. However, little progress happened in the elimination of nitrides from the Cr-Nb-TiTi/Al ratio containing superalloys. The nitrogen is mostly carried into the melt by chromium and niobiumNiobium master alloys, associate with leaks in the melt chamber. The present development obtained by CBMM’s materials and processingMaterials and processing research program provides a process for the manufacturing of materialMaterial above mentioned in which the precipitationPrecipitation of nitrides during the solidificationSolidification can be essentially eliminated. This is done by a two-step meltingMelting process. In the first one the nitrogen is taken out a bath formed with the non-reactive metals (Ni, Fe, Mo) by a controlled carbon-boil. Subsequently Cr and Nb bearing master alloys with extra-low nitrogen content (produced by a special smelting process developed by the company) are added to the bath, together with TiTi/Al ratio, Al and B. The VIM furnace utilized is designed in a way that it is virtually atmospheric leak-free. That will result in an ingot whose nitrogen content is below the limit for the TiN precipitationPrecipitation during the solidificationSolidification of alloy 718Alloy 718.

Although the relationships between processing and the resulting properties are relatively well known for alloy 718Alloy 718, a better understanding of the deformation mechanisms activated across its usable temperatureTemperature range is needed to create more mechanistically accurate property models. In this work, direct atomic-scale imaging with high angle annular dark field scanning transmission electron microscopy (STEMSTEM) has been complemented by phase field modelingPhase field modeling informed by generalized stacking fault surface calculationsGeneralized stacking fault surface calculations using density functional theory. This coupled experiment/modeling approach has shed light on the complex shearing processes occurring in alloy 718Alloy 718 following a standard commercial heat treatmentHeat treatment, which produces both monolithic γ and γ″ particles, as well as composite particles. Deformation at room temperatureTemperature occurs through complex shearing of γ″ into intrinsic stacking fault configurations that were restricted to the precipitates. Exploration of possible shearing sequences with the aide of phase-field dislocation dynamics has revealed that precipitate shearing by motion of coupled 1/2<110> dislocations of non-parallel Burgers vectors on the {111} glide plane is the dominant deformation mechanism at lower temperatureTemperature. A “fast-acting” yield strengthStrength model is discussed which takes into account microstructureMicrostructure variations and deformation mechanism transitions. Deformation at higher temperatureTemperature (427 and 649 °C) has revealed a distinct transition in deformation modes, including stacking faults extending into the matrix, as well as microtwinning. The possible origin of the temperatureTemperature and rate dependence of the stacking fault and microtwinning modes and temperatureTemperature will be discussed.

Quantitative evaluation of the size and volume fraction of γ′ particles has been studied for the first time by Small-Angle X-ray Scattering (SAXS) in the newly developed HGN300 superalloySuperalloy employing different agingAging conditions. In addition to γ′ particles of about 20 nm in diameter formed during cooling, particles with D (diameter) < 10 nm form on agingAging. The average size of the γ′ particles with D < 10 nm increases on prolonging the time at a fixed temperatureTemperature as well as by increasing temperatureTemperature for fixed lengths of time. However, their size remains in the range under 10 nm except for the highest temperatureTemperature and the longest time. The γ′ particle volume fraction also increase on increasing the agingAging time at a fixed temperatureTemperature but is roughly constant after 24 h of heating, independent of temperatureTemperature. Ultra Small-Angle Scattering (USAXS) reveals that the volume fraction of γ′ particles in the size range from 20 to 100 nm in diameter increase in the early stage of agingAging, while it decreases in the late stage. These results suggest that the disappearance of the particles in the size range of 20 < D < 100 nm stimulate the formation of γ′ particles with D < 10 nm in the single step agingAging. Comparison with Scanning Electron Microscope (SEM) images indicates that the regions with a low number density of γ′ particles in the size range from 20 to 100 nm in SEM images are the sites where the particles with D < 10 nm form. An increase in the number density of γ′ particles with 20 < D < 100 nm occurs in the following second step agingAging and cause an increase of the Vickers hardnessHardness. Consequently, statistically representative parameters obtained by SAXS in a larger sample volume than that viewed by direct observations show that the formation pathway of γ′ particles with 20 < D < 100 nm after two steps agingAging is through the dissolution of γ′ particles with 20 < D < 100 nm formed during cooling and the growth of newly formed particles with D < 10 nm.

As the optimization of highly demanding engine parts like engine disks pushes the materials used to their limits, the estimation of the materialMaterial properties becomes increasingly important. The complex interactions of strengthening mechanismsStrengthening mechanisms in direct aged (DA) Alloy 718Alloy 718 forgings demand detailed modeling of each mechanism. As some strengthening mechanismsStrengthening mechanisms are influenced during the billet processingBillet processing it is essential to consider the forgingForging stock manufacturing in the designDesign process of forged engine disks. Therefore, a finite element analysis of the billet processingBillet processing was incorporated in an existing simulation chainSimulation chain of the closed die forgingForging process of engine disks. It includes all thermo-mechanical operations after vacuum arc remelting, i.e. homogenizationHomogenization, upsetting, drawing and radial forgingForging of the billet as well as prepressing, forgingForging and heat treatmentHeat treatment of a diskDisk. In order to implement the local variations of microstructureMicrostructure caused by the billet processingBillet processing a newly developed grain class model was applied. Furthermore a duplex microstructureMicrostructure, which is often present in the surface region of the billet, was considered using the grain class model. For a sound precipitationPrecipitation modeling the local temperatureTemperature history of the whole simulation chainSimulation chain was considered in the thermo-kinetic software tool MatCalc. After parameterization of the MatCalc model, parameters for the precipitationPrecipitation, solid solution and grain boundary contribution to the total yield strengthStrength was calculated. For the determination of the DA-effectDA-effect a semi-empirical, deterministic model was developed. The established, automated simulation chainSimulation chain takes all mentioned mechanisms into account and calculates the local yield strengthStrength of the forged engine diskEngine disk.

ATI 718Plus®ATI 718Plus® is a γ′-strengthened nickel-based superalloyNickel-based superalloy developed to substitute the widely used Alloy 718Alloy 718 in aero-engine applicationsApplications. This newer superalloySuperalloy is a candidate materialMaterial for aero-engine turbine structures, with the requirement to withstand impact loading occurring at high strain rates during turbine blade out events. Furthermore, the understanding of the high strain rateStrain rate response of ATI 718Plus® is important in optimising its machinabilityMachinability during cuttingCutting operations. To predict and model the behaviour of ATI 718Plus® during these events and in other dynamic impact applicationsApplications, proper understanding of the high strain rateStrain rate behaviour of the alloy is important, but not presently available. Therefore, in this work, the influence of microstructural condition and strain rates on dynamic impact behaviour of ATI 718Plus®, using a modified version of direct impact Hopkinson bar, is investigated. It is observed that the age-hardened alloy exhibits a significantly reduced strain hardening and strain rateStrain rate hardening capabilities compared to the solution heat treated microstructureMicrostructure. Furthermore, microstructural examination of the deformed samples shows that the formation of adiabatic shear bands, which usually serve as damage nucleation site, is substantially suppressed in the solution heat treated microstructureMicrostructure, while the aged microstructureMicrostructure exhibits high propensity to form localised shear bands.

A three-dimensional (3D) stochastic model for simulating the evolution of dendritic crystals during the solidificationSolidification of alloys has been developed. The model includes time-dependent computations for temperatureTemperature distribution, solute redistribution in the liquid and solid phases, curvature, and growth anisotropy. The current 3D simulationSimulation software was written in Fortran 90 with threading (shared memory system with multiple cores) in both native Linux and Windows threads. This 3D software is highly efficient. It can run on PCs with reasonable amount of RAM and CPU time. 3D stochastic mesoscopic simulations at the dendrite tip length scale were done to simulate the evolution of the columnar and equiaxed dendritic microstructuresMicrostructures, columnar-to-equiaxed transition and segregation patterns during solidificationSolidification of alloy 718Alloy 718.

Influence of microporosityMicroporosity on the mechanical propertiesMechanical properties of superalloySuperalloy have been studied by numerical and experimental methods for long time. In this paper, the real morphology of microporosityMicroporosity in polycrystalline Alloy 718Alloy 718 specimens was inspected by synchronization radiation X-ray micron computerized tomography (μCT) and 3D model was reconstructed for the numerical simulationSimulation based on finite element method (FEMFEM). Fatigue testingFatigue testing of the heat treated samples and heat isostatic pressured samples were conducted to verify the numerical analysis. Response surface analysis based on FEMFEM was used to establish the linear function relationship between fatigueFatigue life and factors including of porosity, pre-deformation and residual stressResidual stress. The simulationSimulation results showed that the interdendritic region on the surface of microporosityMicroporosity was the place of stress concentration in metal and the crack site of fatigueFatigue failure. Experimentally, heat treated sample fractured at microporosityMicroporosity, while the HIPed samples disrupted due to the matrix ductile fractureFracture. The good agreement was obtained between the simulationSimulation and experimental results.

ATI 718PlusATI 718Plus® is a polycrystalline multi-phase strengthened Ni-base superalloyNi-based superalloy suitable for turbine discTurbine discapplicationsApplications in aero engines. A typical turbine disc experiences temperaturesTemperature in the rim up to 650–700 $$^{\circ }$$C and stresses as high as 1000 MPa in the cooler parts of the component. The manufacturing process, especially forgingForging and heat treatmentHeat treatment, plays a pivotal role in achieving the final microstructureMicrostructure and associated mechanical propertiesMechanical properties needed to withstand this harsh environment. The microstructural evolutionMicrostructural evolution during aforementioned processes is directly related to dynamic recrystallizationRecrystallization. The changes occurring during the initial stages of deformation are of particular interest as partially recrystallized areas with a broad range of grain sizeGrain size can be detrimental for the mechanical properties of the component. ATI 718Plus® has been used in this work to study grain nucleation and growth during recrystallization. To evaluate the importance of the deformation conditions on the outcome of the forging, a set of specimens were produced from a single billet of ATI 718Plus®. Hot compression tests were performed on Rastegaev-samples across a wide range of temperaturesTemperature, strains and strain ratesStrain rate. In addition, the characteristics for meta-dynamic recrystallizationMeta-dynamic recrystallization were evaluated on a samples subset. The evolution of the flow curves as well as the development of the microstructureMicrostructure will be presented. This study specifically focuses on the role of deformation parameters as well as their influence on the recrystallized fraction and grain boundary misorientationMisorientation angle.

With a combination of high strengthHigh strength, toughness and resistance to corrosion, INCONEL® alloy 725 has been widely used in marine, aerospace, and land-based power industries. Typically, the alloy presents a conventional precipitate-strengthened γ-γ′/γ″ microstructureMicrostructure when appropriate agingAging treatments are employed. Although the corrosion resistance of INCONEL® alloy 725 is significant, its use is limited to relatively low temperatures when compared to other γ′ precipitate-strengthened Ni-based superalloysNi-based superalloys. This can limit the use of the alloy as turbine engine components or in other power-generation applicationsApplications since future engine designs suggest increases in the operating temperatureTemperature. This investigation aims at modifying the compositionComposition of the alloy to assess its high-temperatureTemperaturemechanical propertiesMechanical properties. Variations to the TiTi/Al ratio/Al ratioTi/Al ratio were considered with respect to the precipitate phases stability as well as additions of Ta and Nb. Thermodynamic and kinetic predictions, such as phase fraction/stability and time–temperatureTemperature–transformation diagrams, were used to help in the designDesign process and were validated experimentally. The various compositions and relative agingAging treatments investigated produced microstructuresMicrostructures differing in grain boundary phases and γ′ precipitate sizes and fractions. Tensile and creepCreep testing were performed and the effect of the various compositions and microstructuresMicrostructures on the mechanical performance of the modified INCONEL® 725 alloys was examined.

Lattice misfitLattice misfit is important in Inconel 718Inconel 718 (IN718) as coherency strain strengthening from γ″ precipitates is the principal source of strengthening in this alloy. The value of lattice misfitLattice misfit is defined by the difference in lattice parameter between each phase. In this study, high resolution time-of-flight (TOF) neutron diffractionNeutron diffraction has been used to measure the constrained lattice parameters and lattice misfitsLattice parameters and lattice misfits of a bulk IN718 sample. Thanks to the high resolution of the diffractometer, overlapped diffraction peaks are successfully deconvoluted, and the lattice parameters of different phases and lattice misfits are determined. The measured constrained lattice parameters of γ″ phase are much less than the reported unconstrained lattice parameters measured from extracted γ″ precipitates. The lattice misfitLattice misfit between γ″/γ phases along c-axis of γ″ phase is −0.015% indicating the γ″ precipitates are largely constrained and highly coherent with γ matrix in bulk IN718.

It was recently found that the creepCreepstrengthStrength and creepCreep life of wrought alloy 718Alloy 718 samples showed an interesting dependence on the cooling rate in the standard subsolvus solution temperatureTemperature: the time to 0.2% creepCreep strain varies from 5 to 400 h depending on the cooling rate between 1 and 199 °C/min with the maximum value at an intermediate cooling rate of 51 °C/min under a creepCreep condition of 621 °C/724 MPa. In the present paper, microstructuresMicrostructures were observed for samples after creepCreep deformation to 0.2% creepCreep strain to understand the dependence of the creepCreepstrengthStrength on the cooling rate. The microstructural observations using a field emission type scanning electron microscope and transmission electron microscope showed that the size of coherent γ″/γ′ precipitates in grains decreased and the number density increased with increasing the cooling rate but they saturated around the cooling rate at which the creepCreepstrengthStrength was maximized. A careful observation on grain boundariesGrain boundaries revealed that the precipitationPrecipitation of fine γ″ phase precipitates was enhanced along grain boundariesGrain boundaries and coherent twin boundaries at intermediate cooling rates and that of δ phase platelets was pronounced along grain boundariesGrain boundaries and incoherent twin boundaries at slower cooling rates. These microstructural features suggest that the maximized creepCreepstrengthStrength is related to an enhanced precipitationPrecipitation of γ″/γ′ particles along grain/twin boundaries as well as a high number density (fine size) of fine γ″/γ′ particles in grain interiors.

Expected service lifetimes for high temperature components can be many thousands of hours, and during this time microstructural degradation including surface oxidation are major concerns for the designer. Nickel based alloy 751 in two different grain sizes was subjected to exposure at 875 ℃ for up to 4,000 h in air. Fatigue testing was performed at 840 ℃ after exposure and the microstructures and fracture surfaces were characterized. Both grain sizes had similar amounts of oxide and alloy depletion, but it was found that the fine-grained material experienced significant depletion induced grain growth in the near-surface region. While 875 ℃ is significantly below the temperatures where grain growth is active for the bulk alloy composition, the depletion of Al, Ti, and Cr in the near surface region decreased the γ′ solvus and increased grain boundary mobility. Property testing was performed on the as-oxidized surfaces as well as post-machined surfaces to study the effect of this microstructural change. The experimental results show that the coarse-grained material had longer fatigue life after 4000 h, and that surface degradation had a more significant effect on the life of the fine-grained material.

Many Ni-base superalloys are susceptible to intergranular brittle fractureFracture during holding under high stresses at relatively low temperatures, known as stress-assisted grain boundary oxidationStress-assisted grain boundary oxidation (SAGBO). These superalloys are strengthened by geometrically close-packed precipitates such as Ni₃Al (L1₂) and Ni₃Nb (D0₂₂) within grain interiors, whereas role of grain-boundary precipitates in strengthening of the alloys is limited. It is thus interesting to know how the presence of grain-boundary precipitates affects the SAGBO behavior. Recently Takeyama et al. has developed a novel austenitic heat-resistant steel Fe-20Cr-35Ni-2.5NbFe-20Cr-35Ni-2.5Nb (at.%) strengthened by intermetallic phases of topologically close-packed Fe₂Nb Laves precipitated at grain boundariesGrain boundaries, together with Ni₃Nb within grain interiors. The creepCreep rupture strengthStrength of the steel at 800 °C is comparable to Ni-base alloys, and the higher the area fraction ρ of the grain boundariesGrain boundaries covered by the Laves phaseLaves phase, the lower the creepCreep rate, thereby leading to the superior strengthStrength. This strengthening mechanism is called “Grain-boundary Precipitation StrengtheningGrain-boundary precipitation strengthening (GBPS)”. In this study, the relationship between the GBPS and SAGBO behavior has been investigated using the novel steels based on Fe-20Cr-35Ni-2.5NbFe-20Cr-35Ni-2.5Nb. Two specimens with similar strengthStrength but different ρ values were prepared. Constant-load tests were conducted at 90% of the yield stress and 600 °C in air. Rupture time and rupture elongation become larger in high ρ specimen. Ductile morphologies were observed on the fractureFracture surfaces in the high ρ specimen, whereas intergranular surfaces were more obvious in the low ρ specimen. The results clearly demonstrate that GBPS is effective in suppressing susceptibility to SAGBO under high load condition.

VDM Alloy 780 Premium is a new 718718-type superalloySuperalloy recently developed in a cooperation between Technical University Braunschweig and VDM Metals GmbH [1, 2]. It contains the γ′-phase for strengthening in addition to a second phase, in the following referred to as δ-phase (Note that determination of the crystal structure is pending. It may not have the same DO_a structure as the δ-phase in Alloy 718Alloy 718.). As in Alloy 718Alloy 718, the δ-phase may be used for grain refinement and strengthening of grain boundariesGrain boundaries at elevated temperatures. Most important differences in chemical compositionComposition compared to Alloy 718Alloy 718 are the essential replacement of Fe by about 25% Co and a higher Al-content (about 2%) in combination with a lower TiTi/Al ratio-content (about 0.2%). Firstly, these measures stabilize the strengthening phase and allow for application temperatures of up to 750 °C. Secondly, the δ-phase is preserved despite the relatively high aluminum content. To make best use of Alloy 780, the precipitationPrecipitation kinetics of both phases along with the concurrent evolution of microstructureMicrostructure and mechanical propertiesMechanical properties must be understood. Consequently, this article deals with the influence of heat treatments on the phase kinetics, microstructure evolutionMicrostructure evolution and mechanical propertiesMechanical properties. Firstly, it will be demonstrated that proper control of the heat treatmentHeat treatment parameters allows for a distribution of the δ-phase as in Alloy 718Alloy 718. Secondly, Vickers hardnessHardness testing along with creepCreep rupture experiments will be presented, revealing interdependencies between heat treatmentHeat treatment strategies and mechanical propertiesMechanical properties. At ambient temperatureTemperature, the strengthStrength of the new alloy meets or exceeds that of Alloy 718Alloy 718 despite absence of the γ″-phase, while its creepCreepstrengthStrength is far superior.

CreepCreep of additive manufactured (AM) and wrought alloy 718Alloy 718 has been examined in order to understand the difference in nature between the AM and wrought alloy. Three types of homogenizationHomogenization (solution) treatments, 982 °C (subsolvus: SUB), 1060 °C (supersolvus: SUP) and 1200 °C (ultra supersolvus: USS) for 2 h, were employed, prior to the standard agingAging treatment (760 °C/10 h + 650 °C/8 h→AC) known as supersolvus agingAging in the wrought alloy. Note that, unlike the wrought alloy, fine particles of a few tens of nm in diameter remain with a high number density even after the USS treatment. All the specimens exhibit a similar hardnessHardness value of 500 Hv after the agingAging. CreepCreep test was conducted at 650 °C under a constant stress of 630 MPa in air, and the creepCreep rate was monitored through extensometer by linear valuable differential transducer. The creepCreep rate ($$ {{\dot{\upvarepsilon}}} $$) in the transient stage becomes slower with increasing homogenization temperatureHomogenization temperature; the minimum creepCreep rate of USS and of SUP at ~0.5% creepCreep strain is one order of magnitude slower than that of SUB at 0.8% strain. However, in the accelerating stage, $$ {{\dot{\upvarepsilon}}} $$ at 2.5% strain becomes the same among the three specimens. Eventually the rupture time increases with increasing the homogenization temperatureHomogenization temperature, which is 282 h, 822 h and 1006 h for SUB, SUP and USS specimens, respectively. These values are almost the same as or a little shorter than those of the wrought alloy. The rupture elongation is, however, limited to less than 5%, which is the highest for the USS specimen. The difference in creepCreep deformation among the specimens and between the AM and wrought specimens is discussed in terms of creepCreep rate versus time(strain) curves in conjunction with the nature of the AM and wrought microstructuresMicrostructures.

Nickel-based superalloys have been developed extensively and have proven attractive for various industrial applicationsApplications over the past four decades, due to excellent mechanical propertiesMechanical properties that are maintained at high temperatureTemperature. This study investigates selective laser meltingSelective laser melting (SLMSLM) of the nickel-based superalloyNickel-based superalloyIN-718718 and documents the effects of post-processingPost-processing treatments on the resulting microstructureMicrostructure and mechanical propertiesMechanical properties. Comprehensive microstructural characterizationCharacterization was performed on both as-deposited and post-processed materials using various techniques (e.g. EBSDEBSD, OM, BSE, CT). The as-deposited alloy exhibited fine and elongated grains that contribute to the mechanical anisotropy presented in a companion paper while post-processingPost-processing produced a more equiaxed microstructureMicrostructure and reduced the mechanical anisotropy. Tensile, fracture toughnessFracture toughness, and fatigueFatiguecrack growthCrack growth tests conducted at room temperatureTemperature are reported in this work while elevated temperatureTemperature properties are reported elsewhere. This work revealed the changes of the microstructureMicrostructure (γ morphology, γ crystallography, and precipitate distributions) and hence the mechanical behavior due HIPHIP+HT which resulted in mechanical propertiesMechanical properties that approach wrought alloys after comparable heat treatmentHeat treatment. The results are discussed in this light and focus on the differences in microstructureMicrostructure resulting from the AM process.

Large and reliable components by wrought γ′ strengthened Ni and Ni-Fe based superalloySuperalloy are strongly demanded for advanced ultra-supercriticalAdvanced ultra-supercritical (A-USC) power generation, which is one of the most promising and efficient technologies with steamSteamtemperatureTemperature of above 700 °C. In this work, the effect of phosphorusPhosphorus on the creep propertiesCreep properties and microstructureMicrostructure change of wrought γ′ strengthened Ni-based superalloySuperalloy (alloy 282Alloy 282), was investigated, focused on the effects of carbides precipitationPrecipitation. In an alloy with phosphorusPhosphorus content of 8 ppm, precipitationPrecipitation of M₂₃C_6M23C6 carbides was observed in both grain boundariesGrain boundaries and grain interior prior to the creepCreep tests. Grain boundary coverage by carbideCarbide increased until the phosphorusPhosphorus content up to about 30 ppm. On the other hand, the amount of M₂₃C_6M23C6 in grain interior decreased with increasing phosphorusPhosphorus content. Results of the creepCreep tests revealed the relationship between the time to rupture and the grain boundary coverage by carbideCarbide. MicrostructureMicrostructure of the crept specimen showed the existence of misorientationMisorientation at the vicinity of grain boundary without carbides by means of an electron backscattered diffraction (EBSDEBSD) analysis. These results suggest that the improvement of the time to rupture is due to the grain boundary precipitationPrecipitation strengthening mechanism by grain boundary carbides and phosphorusPhosphorus content affects precipitation behaviorPrecipitation behavior of M₂₃C_6M23C6 carbides in grain interior and grain boundariesGrain boundaries.

Inconel 718Inconel 718 is Ni-based superalloySuperalloy commonly used for manufacturing engine components in aerospace industry. Nickel-based super-alloys are particularly well suited for service in high temperatureTemperature environment where resistance to creepCreep, corrosion, and thermal shock are of primary requirements. Alloy 718Alloy 718 forms various nano-precipitates which endow higher strengthStrength to the materialMaterial and thermal stability to the microstructureMicrostructure up to 650 °C due to grain boundary pinningBoundary Pinning of these nano-precipitates. In order to further improve the strengthStrength of the alloy at elevated temperatures, we utilized high rate severe plastic deformation (HRSPD). MachiningMachining was used to impose HRSPD on the alloy, and the chip thus formed was used to study the response of thermomechanical process. MachiningMachining led to significant microstructural transformation and resulted in “bi-modal” grain sizeGrain size distribution. Grain refinement led to significant increase in strengthStrength. Further improvement in strengthStrength was attained after extended heat-treatment at 600 °C for 10 h. This increment in strengthStrength can be attributed to the formation of nano-precipitates which results in obstructing dislocations movement and pinning of grain boundariesGrain boundaries. However, it is known that deformed microstructureMicrostructure with very fine grains saturated with dislocations, results in brittle behaviour of alloy rendering them unusable for most applicationsApplications. In order to alleviate this problem, post-process heat-treatment was designed. Three different temperatures were selected for short heat-treatment at 700, 800, and 900 °C for 15 min. This short heat-treatment resulted in varying amount of recrystallizationRecrystallization of the matrix and precipitationPrecipitation and ripening of γ″ and γ′ phase. Transformation in microstructureMicrostructure and ensuing properties after thermomechanical processing are discussed in this paper.

The changes in the stress-strain stateStress-strain state as a result of hydrogenHydrogen saturation of Alloy 718Alloy 718 produced via traditional and 3D metal printing3D metal printing routes were studied. The stress-strain stateStress-strain state of the materialMaterial after electrochemical hydrogen chargingHydrogen charging was measured by X-ray technique using diffractometers with two power levels to obtain the information from different depths from the surface. It was found that most of the hydrogenHydrogen accumulated in the thin surface layer. It was found that the microstrains and sub-microstrains changed up to 35% in a depth of ~5–30 μm as a result of the electrochemical hydrogen chargingHydrogen charging. The changes in the residual stressResidual stress (macrostress) as a result of cathodic charging were not detected in the studied materials. As a result of hydrogenHydrogen desorption at 200–800 °C, the stress-strain stateStress-strain state of the investigated alloy returned to its initial state for traditionally made materialMaterial. The bulk of hydrogenHydrogen after cathodic charging was removed by heating to 300 °C.

HAYNES® 282® alloy was developed by Haynes International as an age-hardenable, yet fabricable, wrought Ni-base superalloySuperalloy. To date, the alloy has been specified for a number of applicationsApplications, most notably in the hot sections of both aero- and industrial gas turbines. A Time-TemperatureTemperature-Transformation (TTT) diagram is a useful guide to assess an alloy’s behavior during thermo-mechanical processing and heat treating. Starting with the (commercially relevant) 1149 °C (2100 °F) mill-annealed condition, a time (0.1–100 h)—temperatureTemperature (649–1121 °C (1200–2050 °F)) exposure matrix was executed. Precipitates, which had formed as a result of these exposures, were analyzed by light microscopy, scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), and, in selected cases, by x-ray diffraction (XRDXRD). Chiefly, γ′ and two types of carbides, Cr-rich M₂₃C_6M23C6 and Mo-rich M₆C, were found. Approximate C-curves for these phases were constructed, and some implications for the alloy’s processing and properties characteristics are discussed.

The interior residual stresses, which were generated at previous stages of forgingForging and water quenching, exist as a pre-condition of γ″ precipitationPrecipitation during the subsequent thermal ageing. To clarify the issue, a refine investigation for the preferentially orientation of γ″ precipitates in Inconel 718Inconel 718 can be achieved by means of TEMTEM, FE-SEM and small angel neutron scattering. Compared with the isotropic nucleation in the early stage of ageing, the preferential orientationPreferential orientation during the coarsening process has a specific orientation-relationship, due to the volume diffusionVolume diffusion of niobiumNiobium atoms affected by elastic strain, with the load direction in a multiple crystal system. The inhomogeneity and anisotropy in microstructureMicrostructure give rise to a deterioration and anisotropy of mechanical propertiesMechanical properties.

Approximately 3600 samples have been printed to characterize the build parameters for laser powderPowder bed fusion (L-PBF) fabrication of Inconel 718Inconel 718. The tested samples connect pore formation to part orientation, part location and the use of recycled powder. These data serve as the basis for development of a Random Forest Network machine learning (ML) model capable of two-way modeling of process–property and process–structure relationships. These results show how common procedural steps in the setup and execution of L-PBF effect porosity, particularly the formation of keyhole and lack of fusion (LOF) defectsDefects, and how data collection, processing, and validation can expose even subtle connections between input features and output parameters using a general ML framework.

The use of Ni-based superalloysNi-based superalloys as structural components of advanced ultra-supercriticalAdvanced ultra-supercriticalsteamSteam and CO₂ turbines is becoming necessary due to the increasing performance requirements of future power plant designs. Although numerous Ni-based superalloysNi-based superalloys can respond to the demanding mechanical performance, very few present a combination of high-strengthStrength and creepCreep resistance while maintaining good ductility and weldabilityWeldability. Furthermore, the improved performance is often associated with wrought alloys while the fabrication of large castings is the primary target for those applicationsApplications. With the comparatively lower mechanical propertiesMechanical properties of cast alloys, new designs based on compositional changes are necessary to allow for the use of Ni-based superalloysNi-based superalloys in A-USC castings. This lack of performance is primarily associated with the large and inhomogeneous grain sizeGrain size as well as elemental segregation and other castingCasting anomalies. This investigation presents various alloys with compositions within the range of INCONEL® 740INCONEL® 740/740H designed to overcome the issues associated with the use of Ni-based superalloysNi-based superalloys in A-USC castings. Modifications to the chemistry were based on thermodynamic simulations and experimental results. Particular attention was given to reactive element additions, element partitioning, γʹΓʹ precipitate phase promoting elements and grain boundary carbides. The effects of the various compositional changes on the tensile and creep propertiesCreep properties of INCONEL® 740INCONEL® 740/740H will be discussed.

Nickel based superalloys are used in the aerospace industry as turbine rotor materialMaterial due to their high strengthHigh strength and excellent fatigueFatigue resistance at high temperatureTemperature. High strengthHigh strength is required for the high stress and high temperatureTemperature environments in which these alloys operate. This is achieved by their microstructureMicrostructure of gamma matrix/gamma primeGamma prime precipitates. The latter can cause thermomechanical processing issues since they can make the materialMaterial extremely hard. The purpose of this work is to characterize the gamma primeGamma prime precipitates which are present in Rene 65, a newly developed cast and wrought nickel-based superalloyNickel-based superalloy by General Electric and ATI. The main precipitate that was examined was the gamma primeGamma prime phase [Ni3 (Al, TiTi/Al ratio)], which largely determines the mechanical propertiesMechanical properties of the alloy. The gamma primeGamma prime in this particular alloy was found to form in three different sizes; primary, secondary and tertiary. After various heat treatments, the differences in the volume fraction and morphology of each precipitate type is described and the effect of the precipitates on hardnessHardness is determined.

Electron beam meltingElectron beam melting (EBM) process (EBMElectron beam melting (EBM) process) is a powderPowder-bed fusion process within the group of additive manufacturingAdditive manufacturing (AM) technology that is used to fabricate high performance metallic parts. Nickel-Iron base superalloys, such as Alloy 718Alloy 718, are subjected to successive heating and cooling at temperatures in excess of 800 °C during the EBMElectron beam melting (EBM) process process. CharacterizationCharacterization of the dendritic structure, carbides, Laves and δ-phase were of particular interest in this study. These successive thermal cycles influence the microstructureMicrostructure of the materialMaterial resulting in a heterogeneous structure, especially in the building direction. Hence, the aim of this study was to gain increased fundamental understanding of the relationship between the processing history and the microstructureMicrostructure formed within a single layer. Different numbers of tracks with equal heights were for this purpose produced, varying from one to ten tracks. All tracks used the same process parameters regardless of number and/or position. MicrostructureMicrostructure characteristics (sub-grain structure, grain structure and phases) were analyzed by optical microscopy, scanning electron microscopy equipped with energy disperse spectroscopy and electron backscatter diffraction. The direction of dendrites changed in the overlap zones within the tracks due to re-meltingMelting of materialMaterial in the overlap zone. The primary dendrite arm spacings slightly increased along multi-tracks owing to a slight decrease in cooling rate by addition of the next tracks. Epitaxial growth of grains were observed in all samples due to partial re-meltingMelting of grains in previous layers and surface nucleation was also found to occur in all tracks.

A comprehensive elasto-plastic polycrystal plasticity model is developed for Ni-based superalloysNi-based superalloys. To demonstrate the microstructureMicrostructure sensitive predictive characteristics, the model is applied to an Inconel 718Inconel 718 (IN718) superalloySuperalloy that was produced by additive manufacturingAdditive manufacturing (AM). The model with the same set of materialMaterial and physical parameters is compared against a suite of compression, tensionTension, and large strain cyclic mechanical test data applied in different AM build directions. The model embeds the contributions of solid solution, precipitates shearing, and grain sizeGrain size and shape effects into the initial slip resistance. The hardening law is based on the evolution of dislocation density. It is demonstrated that the model is capable of predicting the particularities of both monotonic and cyclic deformation to large strains of the alloy including decreasing hardening rate during monotonic loading, the non-linear unloading upon the load reversal, the Bauschinger effectBauschinger effect, the hardening rate change during loading in the reverse direction as well as anisotropy and concomitant microstructure evolutionMicrostructure evolution. The microstructureMicrostructure constituents and behavior of IN718 under these conditions is similar to other Ni-based superalloysNi-based superalloys, and therefore, it is anticipated that the general model developed here can be applied to other superalloys fabricated using AM and other approaches. Additionally, the materialMaterial is tested in fatigueFatigue and results are presented and discussed.

Alloy 718 superalloySuperalloy has been used for blades of air craft jet engine and gas turbineGas Turbine. The failures of these parts derive from fatigueFatigue and creepCreep, but it is difficult to estimate applied stress from observation of samples from damaged parts. OxidationOxidation and crush on fractureFracture surfaces prevent from the estimation by fractgraphy especially. Therefore, we established the novel observation of the microstructureMicrostructure on the cross section beneath cracks by using EBSDEBSD. Tensile testTensile test, fatigueFatigue test and creepCreep test were performed, and the change in dislocation density and the difference of strain contour of these specimens were specified by the new method. Finally the applied stress could be estimated and the test temperatureTemperature could be also estimated.

Recent work has identified a novel Ni-Cr-Al-Nb alloy capable of generating a γ-γ′-γ″ dual-superlattice microstructureMicrostructure with excellent microstructural stabilityMicrostructural stability and tensile strengthStrength. In this study, we report on the effect of additions of alloying elements that traditionally partition to the γ matrix. Attention is focussed on their effects upon the microstructureMicrostructure, hardnessHardness and lattice misfitLattice misfit of dual-superlattice superalloys. The results obtained from these studies indicate that judicious alloying may further enhance the properties and stability of these dual-superlattice superalloys and that they offer the potential of greatly increased temperatureTemperature capability compared to other alloys reinforced by γ″ precipitates.

Alloy 718Alloy 718 is commonly used for turbine disks in aircraft engines. The very demanding loading conditions require top notch mechanical propertiesMechanical properties of the materialMaterial. In order to maximize the mechanical propertiesMechanical properties a perfect use of the direct age (DA) effect is necessary. An optimal DA effect contributes approximately 150 MPa to the total yield strengthStrength. However, the effect is only present in a certain, narrow process window. It was observed that small deformations at specific local temperatures during the last forgingForging step lead to an ideal DA-effectDA-effect. As the ideal process window in respect to the DA-effectDA-effect can hardly be maintained in the whole forgingForging, regions with a decreased DA-effectDA-effect occur. A connection between the regions with decreased DA-effectDA-effect and abnormal respectively irregular grain growthIrregular grain growth (AGG and IGG) was analyzed based on forgingForging simulations. Therefore, the meta-dynamic recrystallizationMeta-dynamic recrystallization velocity parameter $$ v_{mrx} $$ was developed for indication of AGG and IGG. A correlation of the introduced parameter and the DA-effectDA-effect was found. In addition, $$ v_{mrx} $$ was used for the designDesign of solution annealed forgings containing coarse grain regions. Those forgings were manufactured and analyzed using laser ultrasonic testingLaser ultrasonic testing (LUS) and metallography on cut-ups. The occurrence of ALA grainsALA grains was identified but no white spots (light etch indications) were found.

Additive manufacturingAdditive manufacturing of complex parts in superalloy 718Superalloy 718 by Laser Beam MeltingMelting (LBM, also referred to Selective Laser MeltingSelective laser melting, SLMSLM) is currently under the evaluation by the aerospace industry, due to the opportunity to combine alloy 718Alloy 718 excellent properties and versatility of use, with the benefits and increasing maturity of LBM technology. In this work, the interactions between fractureFracture modes and deformation modes for LBM-manufactured 718718 were studied at 20, 450 and 650 °C. Vertical and horizontal tensile specimens were fabricated, then heat-treated with two sets of standard solution-agingAging treatments, before being tested in air over a range of strain rateStrain rate from 8 × 10–5 to 3 × 10−2 s−1. Results of these tests showed evidence of a coupling effect between oxidationOxidation and mechanical loading, resulting in oxidationOxidation-assisted crackingCracking of LBM 718718 alloy for the same temperatureTemperature and strain rateStrain rate conditions than conventionally-manufactured alloy 718Alloy 718. Also, in spite of consisting of fundamentally different microstructuresMicrostructures, relationships between fractureFracture modes and deformation modes for laser beam melted 718718 were found to be surprisingly consistent with the ones previously established for conventional 718718. These results suggests that microstructureMicrostructure parameters such as grain sizeGrain size and morphology, or phase distribution are not involved at the first order in the mechanisms controlling these interactions. To further describe these phenomena, the interactions between solute elements, mobile dislocations and interfaces must be considered.

The aim of the present work was to investigate the precipitation behaviorPrecipitation behavior and mechanism of the sigma (σ) phase in Alloy 925Alloy 925 by employing several complementary techniques of microstructural analysisMicrostructural analysis and thermodynamic calculations. The thermal exposures were performed at 650, 750 and 850 ℃ for different times, after which the thermally exposed specimens were studied and compared in terms of their microstructureMicrostructure and the micro-hardnessHardness of γ matrix. It was found that the chemical compositionComposition of σ phase in Alloy 925Alloy 925 could be represented as (Cr,Mo)₆(Ni,Fe)₅ with the lattice parameters a = 0.878 nm and c = 0.457 nm. Higher local concentrations of Cr and Mo in the γ matrix caused by the formation of γ′ phase and η phase resulted in the formation of σ phase along the grain boundariesGrain boundaries and in the vicinity of η phase. Higher contents of Cr, Mo, Al, TiTi/Al ratio and Nb in the alloy could accelerate the formation of σ phase. The increase in C content could inhibit the precipitationPrecipitation of σ phase. The formation of σ phase most likely occurred during the thermal exposureThermal exposure at 750 ℃. The amount of σ phase was remarkably increased with the thermal exposureThermal exposure time increasing, which led to the softening of γ matrix.

The lack of a reliable theoretical model of the processing-microstructureMicrostructure relationship of AM (Additive ManufacturingAdditive manufacturing) materialMaterial is preventing AM technology from being widely adopted by the manufacturing community. The goal of this work is to establish the link between the microstructureMicrostructure (textureTexture) and the process parameters of metal AM processes. A quantitative method based on the epitaxial growth of columnar grains within and across melt pools is proposed to predict the textureTexture formation during a metal AM process. The state-of-the-art CALPHAD-informed FEMFEM (finite element method) simulationSimulation has been used to predict the geometry and thermal profile of the quasi-steady melt pool. The thermal gradient distribution within the 3D melt pool determines the crystallography direction and growth direction of the columnar grains within each deposited single tracks. The single tracks with the predicted geometry are amalgamated together to represent the bulk part, and the epitaxial growth of grains across the boundary of neighboring tracks are quantitatively modeled. The proposed method is calibrated and validated by experimental studies of metal AM processed Alloy 718Alloy 718.

The 718718 alloy possesses excellent mechanical propertiesMechanical properties at high temperatures, good process ability, therefore, it has been widely used in aero engine turbine disks, compressor disks, and power turbine shafts (i.e., rotating components). The fatigueFatigue properties of the 718718 alloy are a key factor that determines the safety and reliability of the engine. In this paper, the fatigueFatigue properties of the 718718 alloy are investigated under high temperatureTemperature conditions at 455 and 600 °C. The initiation of fatigueFatigue cracks, and the relationship between fatigueFatigue life and grain sizeGrain size are discussed. The results show that the twin boundaries in large grains and carbides are acting as a crack initiation site for plate specimens, and string-type or heap-type carbides distribution promotes crack propagation and shortens fatigueFatigue life. Carbides respond as a crack initiation site for cylindrical specimens. The grain sizeGrain size is smaller, and the low cycle fatigueLow cycle fatigue properties of the alloy are improved.

Manufacturing of aerospace engine components by very high strainingBE during concurrent cooling/straining methods require knowledge of the materialMaterial response to solution heat treatmentHeat treatment. This paper presents evaluation of the shear-spinningSpinning forming process by experimenting, with different heat treatmentHeat treatment temperatures and time at temperatureTemperature, to increase knowledge of re-crystallisation of the strained grains. SpinningSpinning and shear-spinningSpinning has the potential to replace conventional sheet metal forming methods for small to medium volumes by reducing cost, increase flexibility, lower forming forces and reduced development time. The shear spinningSpinning performed resulted in an engineering strain of 176% in the axial direction and −55% in the sheet thickness direction. The hardnessHardness increased from HV_{1.0 kg}228 to HV_{1.0 kg}446. The heat treatmentHeat treatment experiments showed that the lower end solution treatmentSolution treatment is sufficient to reach full re-crystallisation in Alloy 718Alloy 718 even at a time as low as 20 min. Low to medium temperatures resulted in significant grain sizeGrain size reduction due to re-crystallisation. Using the higher end of the temperatureTemperature range the original grain sizeGrain size and hardnessHardness could be re-established. Low solution temperatureTemperature resulted in an increase of delta phase in the grain boundariesGrain boundaries and inside the grains which was not found at the higher temperatures.

The low cycle fatigueLow cycle fatigue behaviors of Udimet 720Li (U720LiU720Li) and the related microstructure evolutionMicrostructure evolution have been investigated at 725 °C under strain control tests. The interrelationships between microstructureMicrostructure factors and properties were analyzed using transmission electron backscatter diffraction (t-EBSDEBSD) in scanning electron microscopy (SEM) and transmission electron microscopy (TEMTEM). For comparison, LCF behaviors at 650 °C were also presented to find the inferior aspects of U720LiU720Li alloy at relatively higher service temperatures. The results show that recrystallizationRecrystallization occurs during the LCF testing at 725 °C. The extent of recrystallizationRecrystallization increases with the strain. The LCF property at 725 °C is weakened to a large extent after recrystallizationRecrystallization, which could be the significant factor that causes LCF degradation. Combination t-EBSDEBSD and TEMTEM are beneficial for characterizing and analyzing the microstructure evolutionMicrostructure evolution in terms of primary γ′ precipitates and dislocations, proposing that gliding dislocations could concentrate into walls to form sub-grain boundariesGrain boundaries, with the combination of primary γ′ precipitates to form sub-grains.

In the present work, stress relaxationStress relaxation behavior of three typical nickel-base Superalloys: Alloy 718Alloy 718, WaspaloyWaspaloy and AEREX 350, were investigated in a temperatureTemperature range of 600–800 °C for up to 10 h. The effects of stress relaxationStress relaxation parameters on behavior were analyzed and stress relaxationStress relaxation characteristics of the three superalloys are compared and mechanisms revealed by FESEM and TEMTEM observation. The study results show that the stress relaxationStress relaxation property of Alloy 718Alloy 718 is very sensitive to temperatureTemperature. It is very stable at 650 °C, but decreases extremely with temperatureTemperature increasing to 750 °C as a result of serious microstructureMicrostructure degeneration. Moreover, stress relaxationStress relaxation stability is related with initial stress, and appropriate increase of initial stress under normal service temperatureTemperature can increase the stress relaxationStress relaxation limit of Alloy 718Alloy 718. The stress relaxationStress relaxation resistance of WaspaloyWaspaloy decreases with increasing temperatureTemperature. The increase of initial stress and initial strain is beneficial for stress relaxationStress relaxation resistance of WaspaloyWaspaloy, but the influencing degree is related with temperatureTemperature. Furthermore, WaspaloyWaspaloy with heat treatmentHeat treatment A (1020 °C × 4 h/AC + 845 °C × 4 h/AC + 760 °C × 16 h/AC) shows better stress relaxationStress relaxation resistance than that with heat treatmentHeat treatment B (1080 °C × 4 h/AC + 845 °C × 24 h/AC + 760 °C × 16 h/AC). In addition, stress relaxationStress relaxation stability of AEREX 350 is the best among the three superalloys in the temperatureTemperature range of 600–800 °C on the whole. The combined effect of γ′ phase and η phase guarantee the stress relaxationStress relaxation property of AEREX 350.

In order to collect accurate information about the widely reported OxidationOxidation Assisted Intergranular CrackingIntergranular cracking (OAIC) mechanism of the superalloy 718Superalloy 718 at 650 °C in air environment, we investigated a new tensile testTensile test procedure which enables to have access to a quantitative semi-continuous assessment of the damaging process of the alloy 718Alloy 718 during a standard tensile testTensile test. Tensile tests were carried out on a solutionized and aged alloy 718Alloy 718 by varying the deformation modeDeformation mode of the alloy during the ongoing experiment. The semi-continuous quantification of the intergranular damage was performed after a fractureFracture surface analysis of samples tested in both Dynamic Strain Ageing (DSA) domain and DSA-subdomain where Portevin-Le Chatelier (PLC) effect occurs. This innovative testing method allows to characterize the intergranular damage that occurs when the materialMaterial is stressed with an unserrated DSA deformation modeDeformation mode as the alloy 718Alloy 718 is only sensitive to OAIC in this domain. In these conditions, the onset of the intergranular crackingIntergranular cracking process was found to be around 10% of total strain. An increase in the crackingCracking kinetic when necking occurs was also noticed. A close connection between the intensity of the intergranular damage and the cumulated strain in DSA deformation modeDeformation mode before the onset of the damaging process was found. The gathered information represents a significant improvement in the understanding of still debated OAIC or Stress Corrosion CrackingCracking (SCC) mechanisms.

Grain sizeGrain size optimization is an important method to improve the fatigueFatiguecrack growthCrack growth resistance of superalloys. For ATI 718Plus®alloyATI 718Plus® alloy, the effect of grain sizeEffect of grain size on the fatigue crack growth rateFatigue crack growth rate (FCGR) is more complex, because of the complex effects of η-Ni₃Al_0.5Nb_0.5 precipitated at the grain boundariesGrain boundaries. In the past, the fine-grain ATI 718Plus®alloyATI 718Plus® alloy was found to exhibit better dwell fatigue crack growth rateDwell fatigue crack growth rate (DFCGR) resistance than that of the coarse-grain alloy at high temperatureTemperature. An attempt to explain this phenomenon was based on the grain boundary oxygen diffusionOxygen diffusion/oxidationOxidation viewpoint; however, the mechanism remained unclear until now. Based on this, we investigated the relationship between the grain sizeGrain size and the DFCGR of ATI 718Plus®alloyATI 718Plus® alloy. The DFCGR of ATI 718Plus®ATI 718Plus® alloys with different grain sizes were tested in air at 704 °C. Furthermore, the microstructuresMicrostructures and crack growthCrack growth paths were analyzed, and the plastic zonePlastic zone near the crack tip was calculated for specimens with different grain sizes. The better effect fine grain ATI 718Plus®ATI 718Plus® has on DFCGR is related to more η-Ni₃Al_0.5Nb_0.5 phase precipitationPrecipitation and reduced oxygen diffusionOxygen diffusion along grain boundariesGrain boundaries, as well as the reduction of the plastic zonePlastic zone near the crack tip and the increase in secondary crackingCracking with decreasing grain sizeGrain size.

The increasing operating temperatures of turbine designs for power generation applicationsApplications present challenges in selecting materials. With steamSteam temperatures approaching 700 °C, and beyond, ferritic or martensitic steels are inadequate. Ni-based superalloysNi-based superalloys perform much better under these conditions and possess mechanical propertiesMechanical properties suitable for turbine and boiler applicationsApplications. However, only limited Ni-based superalloysNi-based superalloys are available for large turbine castings where a combination of high-temperatureTemperaturestrengthStrength (in particular, yield stress), long-term creepCreep resistance, toughness, and weldabilityWeldability are essential properties. Research was undertaken at the National Energy Technology Laboratory to produce a cast INCONEL® 740H alloy with mechanical propertiesMechanical properties comparable to those of the wrought product. Since thermo-mechanical processing is not an option in castings, agingAging trials were performed to alter the grain boundary morphology after a computationally designed homogenizationHomogenizationheat treatmentHeat treatment. Thermodynamic simulations were used throughout to produce various grain boundary microstructuresMicrostructures. Early investigations on cast INCONEL® 740INCONEL® 740/740H revealed less than desirable ductility. The modified microstructuresMicrostructures altered the tensile and creep propertiesCreep properties, and reinforced the possible use of cast INCONEL® 740H in power plants. The benefits associated with the alternate heat treatments were found to originate from the grain boundary phases present in the alloy controlled through the targeted agingAging treatments. Those results and the relations between thermal processing, microstructureMicrostructure, mechanical propertiesMechanical properties and failure mechanisms of cast INCONEL® 740H will be discussed.

High temperatureTemperatureNi-based superalloysNi-based superalloys are largely used in components of aircraft engines which are subjected to high thermal (above 700 °C) and structural loads. Investment castingCasting has been traditionally implemented for the manufacturing of these components with different levels of complexity and size. However, investment castingCasting of large and complex parts can entail some difficulties which lead to higher castingCasting defect ratios and the need of repairing. Therefore, it is essential to develop easy to repairRepairNi-based superalloysNi-based superalloys castings, i.e., castings with an enhanced weldabilityWeldability or reduced weld crackingCracking susceptibility. Improvement of weldabilityWeldability will also enable the combination of small size castings with forged and rolled components in welded structures reducing overall manufacturing costs and foundry capacity requirements. CrackingCracking susceptibility of Ni-based superalloySuperalloy castings is much greater in comparison with equivalent wrought components due to coarser and non-homogeneous microstructuresMicrostructures, grain boundary segregations and particular chemistry which promote several crackingCracking mechanisms during weldingWelding. A methodology based on VarestraintVarestraint test (hot crackingHot cracking test) is introduced to evaluate crackingCracking susceptibility of Ni-based superalloysNi-based superalloys with a reduced number of samples. This reduction is particularly important when considering castings since practical conclusions can be drawn with minimum materialMaterial usage. Influence of Varestraint testingVarestraint testing parameters, machine performance and total crack length (TCL) measurement procedure on scattering of results are assessed. Conditions to reduce scattering and improve repeatability as well as reproducibility of results are defined, which are essential to compare weldabilityWeldability performance between different alloys and microstructuresMicrostructures.

Nickel based superalloySuperalloyInconel 625Inconel 625 welds were fabricated by depositing its filler in AISI 304LAISI 304L substrate using a single-V groove configuration and employing SMAW (Shielded metal arc weldingWelding) as well as GMAW (gas metal arc weldingWelding) process. FatigueFatiguecrack growthCrack growth and fracture toughnessFracture toughness characteristics of SMA weld Inconel 625Inconel 625 compact tensionTension specimens of thickness 25 mm in as welded and after post weld thermal agingPost weld thermal aging treatments(temperatures ranging from 650 to 850 °C and duration of 10 and 100 h) were investigated on the basis of curves plotted between crack length and number of cycles. Fatigue crack growth rateFatigue crack growth rate was examined in delta k range of 21–39 MPa√m. Varying degree of precipitationPrecipitation in these welds was observed due to thermal agingAging which influenced their fracture toughnessFracture toughness significantly. The fracture toughnessFracture toughness values for SMA weld Inconel 625 specimens in as welded condition was found to be J_max (213.183 kJ/m2) and J_Q (5.618 kJ/m2) Whereas, the treatment (650 °C/100 h) specimen exhibited stable crack growthCrack growth during the J-integral test and J_max increased to (221.58 kJ/m2) and J_Q (12.56 kJ/m2).

Comparative investigations were carried out where multipass multi-layers of Inconel 625Inconel 625 (UNS N06625) weld claddings were overlaid on 12 mm thick austenitic stainless steel plates (AISI 304LAISI 304L) using SMAW (shielded metal arc weldingWelding process) and GMAW (gas metal arc weldingWelding) process. The specimens extracted from the cladded plates were subjected to four different post weld thermal agingPost weld thermal aging treatments (650 °C/10 h, 650 °C/100 h, 850 °C/10 h and 850 °C/100 h). The pitting corrosionPitting corrosion of Inconel 625Inconel 625 clads was evaluated using PAP (Potentiodynamic anodic polarization) techniquePAP (Potentiodynamic anodic polarization) technique and the results were generated as pitting curves which depicted their behavior. Pitting potential (E_pitt) for these welds varied from 31.77 to 922.7 mV. Under all conditions, the GMA welds showed better pitting performance than the SMA welds. AgingAging treatment of 650 °C/10 h improved the pitting resistance of both the welds, as pitting potential of 922.7 mV (which was recorded as the highest value of E_pitt among all the welds) was observed in case of GMA weld and 384 mV for the SMA weld. PrecipitationPrecipitation strengthening occurred under this agingAging condition which was attributable to the presence of Laves phaseLaves phase, γ′ and γ″-phase, besides carbides in Inconel 625Inconel 625 weld/clad metal. However, a loss of pitting resistance was observed under agingAging conditions 850 °C/10 h and 100 h, which could be attributed to the segregation behavior of alloying elements as well as the formation of delta phase which promoted the tendencies for pit nucleation and their subsequent growth in the matrix of the weld metal.

Fabrication and weldingWelding of structural components for the hot section of aero-engines continues to be of high importance to the manufacturing industry of aero-engines. This paper discusses and reviews the literature on hot crackingHot cracking and strain age crackingStrain age cracking, crackingCracking phenomena that can occur during weldingWelding or subsequent heat treatmentHeat treatment of precipitationPrecipitation hardened Ni- and Fe-Ni-based superalloys. The influence of chemical compositionComposition in terms of i.e. hardening elements and impurities, microstructureMicrostructure of base materialMaterial and weld zone, together with weldingWelding processes and corresponding parameters and heat input are discussed and related to the crackingCracking susceptibility of different nickel based superalloys.

The effect of base metal conditions on the weld crackingCracking response of cast ATI 718Plus®ATI 718Plus® was investigated in this study, comparing as cast microstructureMicrostructure with pseudo hot isostatic pressing (HIPHIP) heat treatments at 1120, 1160 and 1190 °C for dwell times of 4 and 24 h. Linear grooves have been filled using multipass manual gas tungsten arc weldingWelding (GTAW) to simulate repair weldingRepair welding conditions. Metallographic investigation revealed cracks in both base metal heat affected zone and fusion zone layers. The heat treatmentHeat treatment temperatures chosen below, at and above incipient laves meltingMeltingtemperatureTemperature of ATI 718Plus®ATI 718Plus® were found to have an effect on weld crackingCracking behaviour, with an increased average total crack length in the base metal heat affected zone for both 1160 and 1190 °C as compared to the as cast condition and the 1120 °C homogenizationHomogenization treatment. The increase in crackingCracking susceptibility shows a correlation with the amount of Nb-rich secondary phases, with lower amounts leading to crack concentration to solidificationSolidificationgrain boundariesGrain boundaries present from the castingCasting process, increasing the average crack length.

This study investigates the effect of eta phaseEta phase on hot crackingHot cracking susceptibility of ATI 718Plus®ATI 718Plus®. Two heat treatmentHeat treatment conditions of 950 °C/1 h and 950 °C/15 h having different amounts of eta phaseEta phase were tested by longitudinal Varestraint testingVarestraint testing method. The heat treatmentHeat treatment at 950 °C/15 h exhibited the highest amount of crackingCracking. This was related to the higher amount of eta phaseEta phaseprecipitationPrecipitation during the long dwell heat treatmentHeat treatment which aided to extensive liquation during weldingWelding.

CastingCasting and solidificationSolidification simulations, heat transfer models, hot isostatic pressHot isostatic press consolidation simulations, hot working simulations, phase equilibrium calculations… the list is quite long of digital technologies embraced by ATI and other producers of specialty materials and components. Through virtual experimentation, product development cycle time and cost is reduced, the potential impact of process upsets can be determined without testing, and sensitivity to process variation can be assessed prior to ever making any metal, among other efficiency and cost benefits. Nevertheless, effective, efficient development of superalloys manufacturing processes requires that these digital experiments are complemented by careful physical experiments at the pilot scale in order to manage risk and expense of scale-up. Pilot scale development provides critical insight into factors not easily computed, such as the integrity of a VIM cast electrode, the variation in segregation during VAR meltingMelting, and crackingCracking due to grain structure or surface condition during hot working. ATI integrates use of physical experiments conducted in pilot research facilities and computational methods to facilitate both new product development and development of the manufacturing method. Such an approach has been instrumental in the development of ATI 718Plus®alloyATI 718Plus® alloy and René 65René 65 alloy and an array of new products including cast-and-wrought, powderPowder, and titanium alloys and components. This paper will discuss the advantages of using physical experiments to streamline the development of new products and manufacturing methods and some of the ways ATI capitalizes on the combined approach.

The final heat treatmentHeat treatment of large aerospace components of alloy 718Alloy 718 typically comprises solution treatmentSolution treatment followed by oil quenching and agingAging. Such operations give rise to small, but non-uniform, plastic strains that can produce significant levels of residual stressResidual stress. During oil quenching, in particular, materialMaterial elements may undergo not only concurrent cooling and strainingBE during concurrent cooling/straining, but also reversed-plastic flow. Therefore, constitutive relations for the simulationSimulation of residual-stress evolution should include a description of such transient phenomena. To meet this need, a series of experiments involving reversed strainingBE during concurrent cooling/straining of short-gage-length samples of 718718 was performed at temperatures between 427 and 871 °C and strain rates of 0.0001 or 0.003 s−1. These experiments revealed a measurable Bauschinger EffectBauschinger effect (BE) over a range of temperatures. The temperatureTemperature at which the BE disappeared was a function of the applied strain rateStrain rate. A similar BE was also observed in experiments involving both reversed strainingBE during concurrent cooling/straining and continuous cooling.

The requirements for rotating jet engine components have placed increasing demands on materials suppliers in the aerospace industry. Higher temperatureTemperature demands of blade and rotor diskDiskapplicationsApplications in the compressor section have led to a need for alloys with properties superior to ATI 718Plus®ATI 718Plus®™ while retaining the ability to be processed by the conventional cast plus wrought process. René 65René 65 was developed for rotor diskDiskapplicationsApplications. The alloy’s property capability also makes it an excellent candidate for compressor blade applicationsApplications. Processing parameters become critical throughout different stages for this alloy. This study is focused on the hot rolling process of René 65René 65 for blade applicationsApplications, with rolling temperatureTemperature and strain rateStrain rate as the parameters of interest. The results show the significant influence of starting temperatureTemperature and strain rateStrain rate for hot rolling of René 65René 65 bar when processing through a continuous rolling mill. Slight changes in rolling temperatureTemperature and strain rateStrain rate could ultimately have a major impact on resulting microstructureMicrostructure and mechanical propertiesMechanical properties.

For high performance air craft engine disks, high-performance Ni-based wrought superalloys (e.g. U720LiTM, AD730TM) with over 40vol% of the γ′ phase at 600–700 °C have been developed. Applying them to gas turbines is expected to improve the efficiency. However, due to their low workabilityWorkability, it is difficult to make large size components. We have developed a new innovative process (the MH process) to improve the workabilityWorkability of high-performance Ni-based wrought superalloys. The strengthening mechanism of these γ′ precipitationPrecipitation-typed superalloys is widely known to be enhanced by the coherent interface between the γ phase and γ′ phase. We focused on the incoherent γ′ phase that precipitated on the γ phase grain boundary during forgingForging and it was found that the incoherent boundary shows no strengthening effect. Formation of γ/incoherent γ′ two-phase microstructureMicrostructure would dramatically improve their workabilityWorkability. Cold working was achieved by applying the MH process to the U720-type alloy AD730. The MH process made it possible to fabricate cold rolled sheets, cold drawn wires and the forged turbine blades. It also exhibited excellent ductility at high temperatureTemperature; we obtained 99% reduction of area at over 920 °C and 500% elongation at 950 °C by applying the MH process. It was demonstrated that U720LiU720Li can be processed by die forgingForging at low temperatureTemperature and high strain rateStrain rate without using superplastic forming. In this study, we investigated how the heat treatmentHeat treatment condition affects the formation of unique microstructuresMicrostructures with good hot and cold workabilityWorkability.

Critical engine disks are typically die forged products due to the high demand on quality and reproducibility. Therefore, it is state of the art that the die forgingForging process is well controlled by a closed loop controlled hydraulic press combined with a tight monitoring of process parameters, e.g. deformation velocity, press load, transfer time etc. Based on the geometry of the diskDisk, for example, in the case of low pressure turbine disks that have a large inner diameter, die forgingForging is not always the most cost efficient process. In these circumstances, ring rollingRing rolling of a pre-shape for final die forgingForging or direct ring rollingRing rolling of the diskDisk by using contoured rolls can result in a much higher materialMaterial yield and reduced production cost and time. For critical components, that require a fine and homogeneous microstructureMicrostructure to fulfill the required mechanical propertiesMechanical properties, ring rollingRing rolling has not been used due to the lack of process control and the difficulties to simulate the process. By using an improved process control of the ring rollingRing rolling mill, in combination of an enhanced process monitoringProcess monitoring, a suitable microstructureMicrostructure can be achieved during ring rollingRing rolling. Results of rolled rings with rectangular and shaped contours made from IN 718IN 718 will be presented and the necessary process control and process monitoringProcess monitoring will be described. In addition, microstructureMicrostructure results of rolled rings will be compared with simulationSimulation results.

The development of equipment and technology makes it possible to manufacture a Ø2000 mm Alloy 706Alloy 706 disc 800 MN hydraulic press in China. Generally, high temperatureTemperature or low strain rateStrain rate is needed during the closed-die forgingForging process to lower the requirement for press capacity; however, it will result in coarse unrecrystallized grains which are not beneficial for mechanical propertiesMechanical properties. In the investigation of hot deformation behavior by thermo-simulationSimulation compression tests in Alloy 706Alloy 706, it is found that abnormal dynamic recrystallizationRecrystallization (DRX) behavior occurs in a high temperatureTemperature and low strain rateStrain rate condition. Electron backscatter diffraction (EBSDEBSD) investigation indicates that the fraction of twin boundaries in the abnormal microstructureMicrostructure is 80% lower than that in the normal microstructureMicrostructure, and apparent textureTexture can be detected. Subscale disc forgingForging experiments were carried out to investigate the effects of abnormal microstructureMicrostructure on mechanical propertiesMechanical properties of Alloy 706Alloy 706. The microstructure evolutionMicrostructure evolution mechanism of abnormal DRX is validated and the influence on mechanical propertiesMechanical properties is discussed.

The development of innovative materials for aeronautics requires use of modern research methods to characterize their structure on the level from micro- to nanoscale. Allvac® 718Plus™Allvac® 718Plus™ (718Plus) is a relatively new nickel-based superalloyNickel-based superalloy that has high strengthHigh strength, is corrosion resistant and has improved higher temperatureTemperature performance compared to the 718718, while retaining the excellent processing characteristics of this alloy. The aim of this study was the application of analytical electron microscopy and tomographic techniques to perform the qualitative and quantitative characterizationCharacterization of structural elements in the wrought and cast 718PlusCast 718Plus. Determination of crystallographic structure of selected phases was performed using electron diffraction method supported by JEMS software. 3D imaging of microstructural elements and chemical compositions of selected phases were performed using STEM-EDX tomographySTEM-EDX tomography technique using a probe Cs corrected Titan3 G2 60-300 with ChemiSTEM™ system. Tomographic reconstruction was performed using Simultaneous Iterative Reconstruction Technique (SIRT) method on a tilt series, which allowed visualizing the three-dimensional distribution of selected elements (Al, Cr) in the analysed volume. The shape and distribution of γ′ particles in the reconstructed volume, as well as complex particle compositionComposition of η-phaseη-phase were analyzed using STEMSTEM-EDX and FIB-SEM tomographyFIB-SEM tomography techniques. 3D imaging of precipitates in the interdendritic region (Laves and η phases) was performed using FIB-SEM tomographyFIB-SEM tomography technique. The 3D visualization of reconstructed space was performed using ImageJ and Avizo Fire software. The study showed that advanced microscopic techniques and test methods in conjunction with tomography technique permits to obtain complementary information about the microstructureMicrostructure of 718Plus superalloySuperalloy.

The computed tomographyComputed tomography (CT) application presented herein aims at measuring cracks three dimensionally in a non-traditional fractureFracture mechanics Superalloy 718Superalloy 718 specimen exposed to supercritical CO_{2Supercritical CO2} (sCO₂). Superalloy 718Superalloy 718 is one materialMaterial of choice for manufacturing components for Concentrated Solar Plant (CSP) units using sCO₂ power cycle. To quantify the low cycle fatigueLow cycle fatigue (LCF) capability of the Superalloy 718Superalloy 718materialMaterial exposed to sCO₂, at 550 °C, 200 bar internal pressure, hollow tensile specimens were designed and LCF experiments at different loading levels were conducted. The same hollow specimen was considered to monitor fatigueFatiguecrack growthCrack growth (FCG) while the crack surface is exposed to sCO₂ environment. By interrupting the LCF test procedure and conducting ex situ volumetric CT scans, the location, size and shape of several cracks were identified and, incremental crack advancement was measured.

Alloy 718Alloy 718 has been applied in a lot of applicationsApplications such as aerospace, power generation plant, automobile and Oil & Gas. Most products of Alloy 718Alloy 718 are finally produced by machiningMachining process because there are a lot of complex product shapes. Therefore, improvement of the machinabilityMachinability can contribute to the manufacturability. One of the important factors for the machinabilityMachinability is large carbideCarbide because carbideCarbide has very high hardnessHardness. Distribution and size of carbides strongly influence on cuttingCutting tool life. New developed Alloy 718Alloy 718 has low carbon content to suppress the formation of NbC type carbides. Developed alloy extremely improves turning tool life in comparison with conventional Alloy 718Alloy 718. Moreover, developed alloy satisfies chemical compositions and mechanical propertiesMechanical properties of AMS 5663N and API 6ACRA.

Alloy 718Alloy 718, a high-strengthStrength Ni-base superalloySuperalloy, is used for the gas turbineGas Turbine disks of aircraft engines and thermal power generation plants. However, macrosegregationMacrosegregationdefectsDefects often form in larger ingots, making it difficult to apply Alloy 718Alloy 718 to large-size industrial gas turbineGas Turbine disks. In order to improve the macrosegregationMacrosegregation properties of Alloy 718Alloy 718, the chemical compositionComposition was modified using calculated phase diagrams. The macrosegregationMacrosegregation defect type of Alloy 718Alloy 718 is a sinking type. According to the difference in density, ∆ρ, calculated using the Scheil-Gulliver equation, it is found that W, which contributes to the same solid solution strengthening as Mo, could strongly enhance floating-type macrosegregationMacrosegregationdefectsDefects. Therefore, it is possible to suppress sinking-type macrosegregationMacrosegregationdefectsDefects by W addition. A new high-strengthStrength Ni-base superalloySuperalloy (named FX550) for large-size gas turbineGas Turbine disks has been developed by substituting W for Mo on the basis of Alloy 718Alloy 718. The microstructureMicrostructure, mechanical propertiesMechanical properties, and macrosegregationMacrosegregation properties of a forgingForging were investigated. According to the experimental results, the microstructureMicrostructure, microstructureMicrostructure stability, and tensile strengthStrength of FX550 were confirmed as being the same as those of Alloy 718Alloy 718. The macrosegregationMacrosegregation properties of FX550 were also similar to those of Alloy 706Alloy 706, which is used for large-size gas turbineGas Turbine disks. This paper presents an alloy design methodAlloy design method for improving the macrosegregationMacrosegregation properties of Alloy 718Alloy 718. In addition, the trial status of a large-size diskDisk is described.