The present study focuses on the evolution of prior particle boundaries (PPB) and grain growth in a Ni-based superalloy processed by Spark Plasma Sintering (SPS) and their influence on ductility. Ni-based superalloys made by Powder Metallurgy present powder heredities such as inclusions at prior particle boundaries (PPBs) that are detrimental to mechanical properties because there are preferential sites for crack initiation. Some previous works tried to remove these PPBs in the final microstructure with different techniques, especially using grain growth. The following strategy has been undertaken using a new Ni-based superalloy densified by SPS and heat treated in the super-solvus and the super-solidus domains. Tensile tests and microstructural characterisations (SEM, EBSD and EDS) have been performed on as-SPS and after the two aforementioned heat treatments. Grain growth enables the reduction of PPB structures but does not improve high-temperature ductility.

Low-cycle fatigue (LCF) properties of superalloys govern turbine blade durability during service. This work investigated the LCF behavior of a novel high-Cr CoNi-based single-crystal (SX) superalloy at 760 degrees C under Delta a/2 = 0.6% and 0.8%, focusing on the crack initiation and propagation along with deformation mechanism. The results indicate that the investigated superalloy exhibited enhanced LCF resistance compared with representative Ni-based SX superalloys under the same testing conditions. During LCF, the investigated alloy exhibited a stable stress amplitude response. The fatigue cracks initiated from internal micropores and propagated perpendicular to the loading direction. Due to its low stacking fault energy (SFE), the deformation substructures in the investigated alloy were uniquely dominated by stacking faults (SFs) (comprising 51.4% T-like, 32.4% Xlike, and 14.3% individual configurations) during fatigue, particularly in the regions around micropores. The SF interactions enhanced the deformation resistance of gamma ' phase in the investigated alloy. Concurrently, the over-saturation of Co/Cr around the leading partial dislocations (LPDs) of SF interactions induced a localized gamma '->gamma phase transformation. The SF interactions and the localized gamma '->gamma phase transformation are believed to improve the deformation resistance of gamma ' phase, thus effectively alleviating the plastic strain concentration around micropores and delaying crack propagation during LCF at intermediate temperature.

This study investigated the impact of a 4 pct tensile strain at 800 °C, 900 °C, and 1000 °C, applied between solution and aging treatments, on the time required to reach 0.2 pct creep deformation in both single crystal (SX) and polycrystalline (PX) specimens of René 65 alloy. The microstructure was examined using scanning electron microscopy to analyze how pre-deformation (PD) influences mechanical behavior. For SX specimens oriented near the [ 001] crystallographic direction, PD systematically reduced the time to reach 0.2 pct creep deformation. Microstructural changes due to PD were evident only when PD was applied at 1000 °C. No significant differences were observed at lower temperatures. For [ 101] orientated single crystal, the impact of PD varied with the pre-deformation temperature. At 800 °C and 1000 °C, PD resulted in strain hardening. For the PX material, the PD reduced the time to reach 0.2 pct of deformation. The PD changes the microstructure in the vicinity of grains boundaries without significantly modifying the intragranular microstructure.

The influence of prior plastic deformation on the viscoplastic behavior of Ni-based superalloys at high temperature is a critical aspect in the design of turbine disk components, as plastic strains can be introduced during quenching operations or intentionally applied through controlled pre-spinning processes. In this study, the stress relaxation behavior of a polycrystalline Ni-based superalloy (AD730) at 700 degrees C is investigated following various prior deformation conditions, including plastic strain obtained during tensile tests at room temperature (RT) and at 700 degrees C. For both temperature conditions, a significant increase in the viscoplastic strain rate, by up to two orders of magnitude, was observed at 700 degrees C with increasing prior plastic strain. This effect appears to saturate, as demonstrated by multiple stress relaxation tests. Moreover, creep pre-strain was applied followed by stress relaxation, resulting in a similar impact compared to tensile prior plastic strain. A phenomenological constitutive model is developed, based on a two viscoplastic potentials framework that accounts for both hardening and viscous mechanisms under isothermal conditions. The impact of processing-induced plastic strain under anisothermal conditions on relaxation and creep at 700 degrees C is also discussed.

Nickel-based single-crystal (SX) superalloys are key materials for high-pressure turbine blades due to their exceptional mechanical strength and environmental resistance at elevated temperatures. While their high-temperature creep behavior has been extensively studied, their tensile response in the lower temperature range remains less well understood. This study investigates the influence of alloy chemical composition and γ/γ′ lattice misfit on the tensile behavior of 14 SX superalloys from different generations, tested at 650 °C. The lattice misfit was measured using high-resolution X-ray diffraction (HRXRD) for the first time in 14 different Ni-based superalloys, the microstructure and the deformation mechanism were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. A relationship is established between alloying elements, misfit, microstructural parameters (γ′ precipitate size, γ-channel width, dislocations characteristics), and tensile performance. In particular, alloys exhibiting large negative misfit values, such as TMS-238, show homogeneous deformation with high dislocation density and pronounced stacking faults in the γ channels, leading to strong work hardening and reduced ductility. However, TMS-238 shows a low yield strength probably due to the high coherency stresses. In contrast, alloys with moderate misfit display more localized deformation mechanisms. An approach is presented that takes into account the contribution of the coherency stresses to the yield behavior. A higher magnitude of coherency stresses induces early plastic deformation in general leading to lower yield strength. These findings highlight the critical role of the lattice misfit and the associated coherency stresses in designing components made of SX superalloys for advanced turbine applications.

This paper reports on the hot corrosion behavior of the nickel-based single crystal superalloy TROPEA exposed to a deposit of Na2SO4 in air and in SO2-containing atmospheres at 700 °C and 900 °C for 24 hours. In the absence of SO2, TROPEA simply oxidizes at 700 °C despite the Na2SO4 deposit but the latter induces basic fluxing of protective oxides at 900 °C by developing NaTaO3 and Na2CrO4 phases. Yet, no specific role of Pt is found. In the presence of SO2, Pt is shown to enhance hot corrosion at low temperatures through a potential catalytic effect. However, Pt does not seem to exert any specific role at high temperature under the same gas atmosphere. Instead, Cr and Ta develop sulfides that foster the ingress of S into the material. Overall, TROPEA seems to behave similar to other SX superalloys except at low temperature where Pt appears to be particularly detrimental.

A series of experiments investigating the recrystallization (RX) mechanisms of the single-crystalline superalloy AM1 during its manufacture have been carried out. High and very high-temperature tensile tests were conducted to investigate the effect of the level of plastic deformation and the temperature at which the strain was applied. These parameters were then analyzed for their influence on RX mechanisms during the subsequent solution heat treatment. The strain threshold for RX under various temperature ranges has been determined, and it has been shown that RX is more likely to occur within the 900 °C to 1150 °C deformation temperature range. Some correlations between deformation mechanisms and volume fraction of γ′ in this type of alloy over these temperature ranges have been discussed in order to explain these trends. In addition, non-isothermal tensile tests were conducted to reproduce as closely as possible the thermomechanical path experienced during the manufacturing of single-crystalline parts using investment casting. A strong correlation has been demonstrated between the thermomechanical path followed during non-isothermal tests that lead to the appearance of recrystallized grains and the “recrystallization zone” identified from pure isothermal tests. This correlation becomes evident when the thermomechanical path crosses this zone. The effect of non-isothermal thermomechanical loading on microstructure was studied by EBSD analysis. Significant local misorientation was observed around microstructural inhomogeneities such as casting pores and eutectic/casting pore pairs. These local rotations suggest that microstructural inhomogeneities act as stress concentrators leading to the first RX nuclei once a super-solvus solution heat treatment has been applied.

Micropores, carbides and residual eutectics within interdendritic (ID) regions of the nickel-based single crystal (SX) superalloys can initiate microcracks during fatigue, thereby reducing fatigue life. Thus, controlling these microstructural features is critical for enhancing the high-cycle fatigue (HCF) properties of nickel-based SX superalloys at high temperature. In this study, the samples with different microstructures in the ID regions, achieved through various HIP and heat treatment schemes, were subjected to HCF tests at 850 degrees C/+/- 650 MPa, f = 30 Hz. The findings suggest that large micropores of non-HIP samples in the ID regions can easily become the crack initiation site during fatigue. HIP treatment decreased the size and volume fraction of micropores and changed crack initiation sites to oxidized carbides or surface oxides in the HIP-treated samples, thereby increasing fatigue rupture life. Furthermore, the crack initiation site in the HIP-treated sample with fewer carbides was found to be surface oxides, leading to longer fatigue rupture lives with nearly one order of magnitude than that of non-HIP samples. This study reveals the crack initiation and propagation mechanism by HIP treatment during fatigue, and improves the high-temperature HCF properties of nickel-based SX superalloys.

Understanding dislocation evolution during viscoplastic deformation is essential to accurately predict hightemperature superalloy behavior. While creep and monotonic deformation are well studied, the evolution of the dislocation substructures during stress relaxation, particularly following prior plastic straining, remains unclear. This study investigates a single crystal gamma/gamma' AD730TM superalloy deformed at 700 degrees C, focusing on dislocation evolution and redistribution. Transmission electron microscopy reveals that stacking faults formed during plastic straining progressively disappear during relaxation, suggesting thermally activated mechanisms such as dislocation re-association and recovery. After re-association, perfect dislocations spread into the matrix, forming a homogeneous dislocation network. These microstructural evolutions are believed to underlie the macroscopic stress relaxation behavior, which exhibits two regimes in the Norton diagram: an initial high-stress exponent regime linked to rapid dislocation rearrangement, followed by a low-stress exponent regime associated with broader dislocation activity.

The over-aging of a polycrystalline γ / γ' nickel-based superalloy can alter numerous microstructural features that are known to be critical for the mechanical properties of these alloys. The aim of this study is to decouple the impact of different microstructural modifications on the monotonic and viscoplastic properties of the René 65 superalloy, produced by conventional cast-and-wrought processing, after various thermal exposures. Over-aging treatments were carried out at 700 °C, 725 °C, 750 °C, 775 °C, 800 °C, and 850 °C for durations of up to 4 000 h. The evolution of γ' precipitates and the formation of TCP phases during these treatments were quantified. These microstructural changes were identified as being responsible for either the degradation or the improvement of specific monotonic and viscoplastic properties of the alloy. It was shown that the dissolution or progressive transformation of the smallest γ' precipitates, within grains or at grain boundaries, governs the evolution of several properties in tension (20 °C and 700 °C), creep (700 °C), and stress relaxation (700 °C). The wide range of treatments applied allowed, at least partially, to decouple the effects of fine γ' precipitates evolution from those of TCP phase formation. In particular, the stress relaxation tests provided insights into the deformation mechanisms controlling the evolution of the creep rate after over-aging.

Turbine disksTurbine disk are engineered to exhibit exceptional mechanical propertiesMechanical properties across a wide temperature range, from room temperature up to 700–750 °C. However, certain process steps, such as solution heat treatmentHeat treatment (SHT) and quenching, can introduce significant residual stressesResidual stresses. This study investigates the impact of aging heat treatmentHeat treatment parameters on a monocrystalline superalloySuperalloy known as AD730AD730®® (SX AD730®). Specifically, the evolution of intragranular precipitates during aging was analyzed in the absence of grain boundaries, which are known to be beneficial for stress relaxation. This work has investigated the effects of one distinct sub-solvus solution heat treatmentsHeat treatment, followed by air or water quenching, and of different aging temperatures (680 and 850 °C) on the subsequent tensile and creepCreep properties. Relaxation tests on solution-treated SX AD730AD730®® samples provided further insights into the material’s behavior at both 680 and 850 °C. The cooling rate has been demonstrated to affect the size of secondary precipitates. Moreover, the study showed that an increase in temperature can degrade the microstructureMicrostructure, leading to a decline in the material’s properties at elevated temperatures.

The self-heating method, which is based on the measurement of temperature evolution of a specimen during cyclic loading, makes it possible to considerably reduce characterization times. The aim of this paper is to propose a test protocol at very high frequency (20 kHz) and very high temperature (up to 1000 degrees C), as well as an ad hoc analysis method to determine the dissipative sources field responsible for the measured temperature rise. To this end, a method for solving the 1D heat diffusion equation, based on Fourier transforms, is developed. This method is validated using finite element calculations, then applied to experimental results obtained at 850 degrees C on AM1, a single-crystal nickel-base superalloy, which exhibits a single regime of dissipation.

Ex-situ fatigue tests combined with X-ray Computed Tomography (XCT) observation and detailed microstructure characterization were conducted under low and high stress amplitude at 760 degrees C to investigate pore evolution, crack initiation and propagation. It was found that, in the early stages of fatigue loading, the decrease in the quantity of solidification/homogenization-pore (S/H-pore) was attributed to the diffusion of vacancies to sample surface and the effect of local deformation. Some "disappeared" (below the resolution of XCT) S/H-pores reappeared at the same sites as fatigue progressed. The formation of deformation-pores (D-pore) was observed directly, depending on the loading stress amplitude. Under low stress amplitude, the largest S-pore in fatigue sample was the preferred site for crack initiation. Severe local deformation near S-pores induced lattice rotation, the formation of slip band and resulted in micro-crack initiation. New mechanisms of micro-crack formation near the pore in the very early stages of fatigue, involving carbide oxidation and diffusion-induced cavity were also observed. In addition, ex-situ XCT observation revealed that large pores promoted crack propagation, whereas carbides played a lesser role in crack growth. However, at high stress amplitude, cracked carbide at the surface promoted crack initiation and propagation. 3D reconstruction of the crack by XCT indicated that pores had little effect on crack growth at high stress amplitude.

The as-deposited coating-substrate microstructure has been identified to substantially influence the high-cycle fatigue（HCF）behavior of Ni-based single-crystal（SX） superalloys at 900℃,but the impact of degraded microstructure on the HCF behavior remains unclear.In this work,a PtAl-coated third-generation SX superalloy with sheet specimen was thermal-exposed at 1100℃ with different durations and then subjected to HCF tests at 900℃.The influence of microstructural degradation on the HCF life and crack initiation were clarified by analyzing the development of microcracks and coating-substrate microstructure.Notably,the HCF life of the thermalexposed coated alloy increased abnormally,which was attributed to the transformation of the fatigue crack initiation site from surface microcracks to internal micropores compared to the as-deposited coated alloy.Although the nucleation and growth of surface microcracks occurred along the grain boundaries in the coating and the interdiffusion zone（IDZ） for both the as-deposited and the thermal-exposed coated alloys,remarkable differences of the microcrack growth into the substrate adjacent to the IDZ were observed,changing the crack initiation site.Specifically,the surface microcracks grew into the substrate through the cracking of the non-protective oxide layers in the as-deposited coated alloy.In comparison the hinderance of the surface microcracks growth was found in the thermal-exposed coated alloy,due to the formation of a protective Al 2 O 3 layer within the microcrack and the γ' rafting in the substrate close to the IDZ.This study will aid in improving the HCF life prediction model for the coated SX superalloys.

Nickel-based single-crystal (Ni-SX) superalloys under cyclic stress are susceptible to cracking at stressconcentration sites, eventually leading to low-cycle fatigue (LCF) failure. LCF cracks typically originate from intrinsic defects (e.g., voids and carbides) within solidified dendrites. However, systematic quantitative experimental analyses of defect-mediated local damage remain limited. To thoroughly understand the microscopic origins and evolution of LCF damage, correlated 3D mapping of dendrites across various regions is essential. Here, macroscale micro-computed tomography (mu-CT) was initially used to capture internal interdendritic secondary cracks within bulk DD413 superalloy after LCF testing at 760 degrees C. Subsequently, a multimodal methodology combining synchrotron 3D microdiffraction (3D-mu XRD), high-resolution mu-CT, and electron microscopy was established. This approach allowed precise localization of internal damage zones near interdendritic secondary cracks and detailed mapping of the 3D correlated distributions of dendrites, defects, and residual stress/ strain fields within these zones at submicron spatial resolution. Finally, the same approach was applied to specimens subjected to interrupted loading at approximately 40 % of the fatigue life to uncover the early damage states of dendrites. The dendrite cores (DCs) and interdendritic regions (IDs) exhibit microscale heterogeneous mechanical responses: nearly defect-free DCs accumulate local irreversible slip along specific slip systems to generate slip bands, while the IDs containing various defects accommodate local microplasticity through the activation of multiple slip systems around these defects. The local tensile stress near defects in the IDs exceeds that in the DC slip band regions by more than threefold, leading to the generation of local damage zones within the IDs. Chain-like defect distributions facilitate the interconnection of these local zones into a continuous damage region, further elevating the overall tensile stress in the IDs. Additionally, geometrically necessary dislocations alone are insufficient as indicators of LCF damage; both the internal stress state and its magnitude must be considered. These experimental results provide critical data and insights for the development of multiphysics fatigue models.

The present work describes and quantitatively assesses the strengthening mechanisms in Inconel 718 fabricated by arc-based DED (IN718 arc-based DED) through an experimentally-based approach. IN718 arc-based DED (in the as-built condition) showed a typical coarse (millimetric-sized grains) and oriented (cube texture) microstructure with a significant quantity of interdendritic eutectics (Laves and MC-type carbides). After heat treatment (1100 degrees C/2 h + aging), these eutectics were partially dissolved; however, the original grain size and crystallographic texture aspects were not altered. In addition, the heat treatment promoted a notorious gamma '' (Ni3Nb) and gamma ' (Ni3(Al, Ti)) phases content (-17 and 5 %, respectively), which results in superior room temperature tensile strength despite the aforementioned non-optimized microstructure. The grain size, dislocation density, precipitation content and morphology, and alloying elements in solid solution were experimentally measured and utilized as input data for a quantitative assessment of the strengthening mechanisms. This analysis concludes that the linear dependence of the majority of the strengthening mechanisms on the Taylor factor predominantly promoted the yield strength anisotropy. Furthermore, as expected, it is evidenced that the precipitation strengthening mechanism governs the final strength of IN718 arc-based DED.

Ni-based superalloys fabricated by arc-based directed energy deposition (DED) typically exhibit a non-optimized microstructure, characterized by coarse and oriented columnar grains, which limits their performance and industrial applications. In this context, microstructure refinement techniques previously applied to arc-based welding have been tested in additive manufacturing, highlighting interlayer mechanical deformation methods (e.g., rolling and peening). The present work describes the grain size refinement mechanism of in situ interlayer hot forging (HF) and evaluates its effect on the elevated-temperature tensile behavior of Inconel 718 fabricated via DED (IN718 DED). HF induced a dynamically recrystallized zone (similar to 1 mm) on the top layer surface and a deformed zone (similar to 0.5 mm) immediately beneath it, which was not completely remelted by the subsequent layer deposition, thereby generating recrystallized grains during subsequent multiple thermal cycles. During the homogenization heat treatment, the remaining deformed region recrystallized, generating a finer grain size and an almost equiaxed microstructure. HF also improves the yield strength at elevated-temperatures and induces an almost isotropic behavior. For the first time, enabling the IN718 DED to meet the grain size, elongation, and elevated-temperature yield strength requirements (AMS 5662).

The creep and tensile behavior of Waspaloy, a γ′-strengthened nickel-based superalloy, processed by Wire Arc Additive Manufacturing using Cold Metal Transfer (WAAM–CMT) was investigated for repair applications. Mechanical properties of the bulk material were assessed up to 850 °C in both the As-Built (AB) and Post-Weld Heat Treated (PWHT) conditions, with particular attention to the effect of the loading direction. The AB microstructure consisted of large columnar grains with ultra-fine γ′ precipitates and serrated boundaries pinned by carbides, providing considerable hardening despite being out-of-equilibrium. PWHT promoted γ′ coarsening and secondary carbide formation, enhancing yield and tensile strength to levels comparable with coarse-grained wrought Waspaloy. A shallow heat-affected zone (300 to 600 µm) was identified, and digital image correlation analysis of strain localization confirmed the good mechanical strength of the interface. Tensile testing revealed weak anisotropy (< 9 pct) in strength, with AB samples exhibiting higher ductility. PWHT was particularly efficient at elevated temperatures, enabling properties equivalent to fine-grained wrought Waspaloy. Creep testing showed in situ aging and an initial contraction in the AB material. PWHT improved creep resistance at 700 °C to 750 °C but degraded performance at 800 °C to 850 °C, with strong anisotropy in creep life and strain at rupture. These findings establish WAAM–CMT as a promising repair technique for aircraft components.

This work investigates mechanisms of irregular microstructure evolution and creep damage localization induced by the prior room-temperature plastic deformation applied to Ni-based single crystal superalloys, AM1 and CMSX-4 Plus, in between solution and aging treatments. Dislocation climb similar to what occurs during the high-temperature low-stress creep is the main mechanism of the microstructure evolution occurring around pre-deformation slip bands, and subsequently leading to the formation of the bands with coarsened gamma/gamma ' microstructure. Presence of "ready-to-shear" dislocations inside the coarsened gamma ' phase is confirmed by a stereo-pair anaglyphs of scanning transmission electron microscopy. This native dislocation structure before the high-temperature/low-stress creep test is the origin of creep damage localization observed for the pre-deformed specimens. The comparison of two alloys revealed that magnitude of plastic activity (precipitation coarsening and dislocation development) inside the bands with the coarsened microstructure is notably larger for AM1 what consequently explains the creep properties degradation under all creep conditions.