With the continuous increase of turbine inlet temperature of advanced aero-engine, the protective coating technology plays a vital role in improving the oxidation and corrosion resistance of turbine blade materials to ensure the safe performance of turbine blades. However, an intrinsic physical and chemical property mismatch exists between protective coating and superalloy. Interfacial reaction leads to the degradation of interfacial microstructure and mechanical properties. It is the key factor to restrict the application of coating. In this paper, the evolution and diffusion behavior of typical coating/superalloy interface microstructure and its influencing factors are summarized. The influence of interfacial behavior on microstructural stability and mechanical properties of superalloys with coatings is also discussed. The control methods of coating/alloy interface are introduced from three aspects, including the optimization of microstructure composition, design of interfacial diffusion-resistant layer, and development of a new type of interfacial stabilizing coating. Furthermore, the key characteristics of the compatibility of the coating/superalloy interface are summarized, which will promote systematic studies on the effect of the interface on the coating/alloy properties, the combination of multiple methods to control the interface, and the computer-aided coating design.

Single crystal Ni-based superalloys are key materials used in the hot section of aeroengines and industrial gas turbines. In service, single crystal blades face harsh environments, including high temperatures, complex stresses, oxidation and hot corrosion. Therefore, they must meet strict technical specifications, such as impurity, defects and dimensional control. Single crystal components should be manufactured using complex technologies within a highly narrow processing window. The present paper reviews recent progress in the research and development of alloy design, microstructure and property evolution and characterization, evaluation in near-service conditions, and single crystal manufacture. Further, the development of “next generation” high-temperature structural materials, such as refractory high-entropy alloys, is briefly discussed.

Recently, with the development of aviation engines and ground-based gas turbines, the demands for the environmental resistance and temperature-bearing capacity of their key hot-end components have considerably increased. Compared to Ni-based superalloys, novel γ′-strengthened Co-based superalloys are more advantageous owing to their corrosion resistance and melting temperature. To facilitate the development of these alloys, research on their alloying principles, alloy design, and creep mechanisms is summarized in this paper based on domestic and international results. Furthermore, herein, the key scientific problems in the development of such alloys are discussed, and the possible development trends and challenges in the future are surveyed.

Residual stress exists in an equilibrium state inside an object without external forces, mainly due to uneven plastic deformation during object preparation. Superalloys exhibit low stacking fault energy and face difficulty in recovery. Therefore, compared with the residual stress in other metal materials, the residual stress in superalloys accumulates easily and is difficult to release and control, causing various problems in their subsequent processing and service. Starting from the formation and evolution mechanism of residual stress in superalloy forgings, this article reviews the research progress regarding the casting, forging, heat treatment, machining, and welding processes involved in residual stress characterization, numerical simulation, optimization control, etc. and focuses on analyzing the interaction behaviors between multiscale residual stress and precipitation phase transformation in superalloys. Further, this article analyzes the impact of residual stress on the service performance of superalloy forgings; the possibility of reasonable preset and utilization of residual stress is envisioned based on this.

The breakthrough application of triple melt technology (vacuum induction melting (VIM) + electroslag remelting (ESR) + vacuum arc remelting (VAR)) for fabricating GH4169 alloy facilitated the optimization of the entire production process of GH4169 disks. This paper summarizes the research progress on the chemical composition, triple melting, homogenization treatment, cogging, disk forging, residual stress control, and quality control system of GH4169 alloy. The breakthrough and large-scale application of triple melting technology have resulted in improved purity of the GH4169 alloy and reduced occurrence probability of metallurgical defects. In addition, the microstructural uniformity and yield of forging bars have been improved by the combination of fast (upsetting and drawing) and radial forging. Furthermore, deformations occurring during the machining and operation of GH4169 disks have been reduced using residual stress control technology. Results related to ultrahigh strength, ultralarge scale, high corrosion resistance, and hydrogen embrittlement characteristics of GH4169 alloy are discussed, and potential future research directions are outlined here.

There has been rapid development in the turbine power systems of aeroengines and gas turbines. Consequently, the application of surface impact strengthening technology for the surface strengthening of superalloys used in turbine rotors and its corresponding mechanisms have attracted wide attention. However, it is difficult to prevent the recovery and recrystallization of the surface hardened layer of superalloys serviced at high temperatures. This leads to the degradation of both the surface strengthening/toughening and fatigue resistance. This is the main hurdle restricting the wide application of surface impact strengthening technology for key components of advanced superalloys. In this paper, the progress made in surface impact strengthening mechanisms and the applications of nickel-based superalloys in recent years are summarized. The effect of surface impact strengthening on the surface strength, toughness, and fatigue resistance of nickel-based superalloys is analyzed. The evolution of the microstructure of the hardened surface of the alloys during long-term aging at high temperatures, and its effect on high-temperature stability are explored. The paper aims to provide essential and important information for developing surface impact strengthening mechanisms of nickel-based superalloys and improving the fatigue resistance of turbine rotors of aeroengines and gas turbines.

Superalloys are widely used in aerospace industry owing to the excellent mechanical properties and microstructure stability at high temperatures. However, the recent developments in the aerospace industry have piled higher demands on superalloys, especially for damage tolerance at high temperatures. The fatigue crack growth rate (FCGR) is an important parameter that describes damage tolerance. Although several domestic studies on FCGR in superalloys have been reported, systematic understanding is still lacking and urgently required. Hence, this study investigated the phenomenon of sensitive temperature for rapid fatigue crack propagation in several nickel-based superalloys and reasons for its emergence by adopting a systematic program of experiments and simulations. The fatigue crack propagation behavior of FGH4096, FGH4097, and FGH4098 powder-metallurgy nickel-based superalloys, and GH4720Li and GH4738 wrought nickel-based superalloys were systematically investigated in a wide temperature range of 550-800^oC using a fatigue crack propagation test. The fatigue crack propagation paths and crack microstructures after the fatigue crack propagation tests were observed. The results clearly demonstrated that the relationship between fatigue life and temperature is nonlinear. A sensitive temperature for rapid fatigue crack propagation for all investigated nickel-based superalloys was also observed, where the fatigue crack propagation rate markedly increased and fatigue life dramatically shortened. Microstructure evolution and mechanical property degradation at high temperatures were not found to be the major reasons behind the occurrence of sensitive temperature of rapid fatigue crack propagation. However, a comparison of fracture morphologies and fatigue crack propagation paths at different temperatures combined with the analysis of oxidation damage components revealed the high-temperature oxidation damage of grain boundary as the major reason for the occurrence of sensitive temperature. The contributions of fatigue damage and oxidation damage at different temperatures were compared using the classical linear superposition damage component model. The results showed that the contribution of oxidation damage increased markedly with increasing temperature. As a result, the fatigue life decreased dramatically at high temperatures and the fatigue propagation rates increased rapidly. Furthermore, the effect of O on the grain boundary strength in the Ni and NiCr system at different temperatures was investigated by molecular dynamics simulations. The grain boundary separation work decreased with increasing temperature and after which the value decreased dramaticalloy. It was concluded that the accumulation of O on the grain boundary resulted in a decrease in the grain boundary separation work and weakened the grain boundary.

DD6 is a second generation single crystal superalloy independently developed in China, which offers advantages such as high-temperature strength, stable structure, and satisfactory casting process performance. Currently, it is being widely used in development of aviation engine turbine blades. Sand blasting can be performed for surface cleaning and adjusting surface roughness of single crystal turbine blades and is an essential process in manufacturing of single crystal blades. Although sand blasting has been widely used in the manufacturing process of DD6 alloy turbine blades, only few reports studied the impact of sand blasting on the surface integrity and fatigue performance of DD6 alloy. Therefore, research is required to provide a theoretical basis for the safe service of DD6 alloy turbine blades. In this work, several specimens after a standard heat treatment are blasted with white corundum sand with 150, 124, and 100 μm diameters at 0.5 MPa to study the effect of sand blasting on the surface integrity of DD6 alloy; the rotary bending high cycle fatigue properties of the specimens without and with sand blasting (blasting with white corundum sand with 150 μm diameter) were tested at 760 and 980^oC, respectively, to study the effect of sand blasting on the alloy's fatigue property. The results show that sand blasting destroys the surface integrity of the single crystal superalloy, resulting in irregular pits on the surface caused by the cutting by sand particles while changing the surface morphology as well; the surface roughness and microhardness increase with sand particle size increase; after sand blasting, many dislocations slip in the γ phase, and the dislocation density near the surface is high. Additionally, many dislocations shear the γ′ phase, forming antiphase domain boundaries and stacking faults; sand blasting results in deformation strengthening and residual stress; blasting with 150 μm diameter sand at 0.5 MPa has a small effect on the rotary bending high cycle fatigue properties of DD6 alloy at 760^oC, but it considerably reduces the alloy's fatigue properties at 980^oC in the low stress amplitude region, decreasing the fatigue strength of the alloy by 7.3%. The combined action of the notch effect, oxidation damage, deformation strengthening, and residual compressive stress leads to the changes in fatigue life without and with sand blasting.

γ' precipitate strengthened cobalt-based alloys exhibit superior comprehensive properties and are potential candidates for the anticipated next-generation superalloy. The phase field method, which considers the combined effect of multiple energy fields, effectively elucidates the processing and mechanism of microstructure evolution. By using the ternary elastoplastic phase field model coupled with CALPHAD and crystal plasticity model, the γ' evolution of Co-9Al-xW (x = 8, 9, and 10; atomic fraction, %) alloys during creep processes is simulated herein. The corresponding rafting behaviors and creep properties are evaluated from the perspective of the changes in second-order moment invariant map (SOMIM) and stress/strain fields. The results show that as the W content increases, the volume fraction of the γ' phase increases, the plastic strain in the γ matrix reduces, and rafting occurs with accelerated rate, which enhances the creep property. Further, the SOMIM analysis shows that the raft structure leads to a steady creep behavior in 9W and 10W alloys. In addition, the alloy with a high W content has a high misfit stress in the γ matrix, which leads to a low plastic strain.

Ni-based single crystal superalloys are widely used for turbine engine blades because of their excellent high-temperature mechanical properties. Thermo-mechanical fatigue (TMF) is a complex deformation process that combines strain and temperature effects. This process is also considered as a deformation method related to the working conditions of aviation turbine blades. Therefore, understanding the deformation mechanism of materials undergoing TMF is important for extending the service life of aviation turbine blades. Here, third-generation and fourth-generation single crystal superalloys that experienced TMF deformation are investigated by SEM and TEM, including aberration-corrected STEM. The results show the formation of deformation twins on different {111} planes of the single crystal superalloys. In addition, a large number of recrystallized grains are found in parallel twin lamellae or around the intersection of twin lamellae. The grain boundary of recrystallized grains is primarily composed of twin boundaries, low-angle grain boundaries, and large-angle grain boundaries generated by twin intersections. Furthermore, the twinning boundaries after deformation are analyzed using aberration-corrected TEM. Consequently, the process of twinning-induced dynamic recrystallization is comprehensively understood, which improved the TMF fracture mechanism of single crystal high-temperature alloys. These results improve the understanding of the deformation mechanism of single crystal superalloys under service conditions.

Developing superalloys and improving their temperature capability are extremely crucial for the advancement of aero-engines. The powder metallurgy (PM) technology can prevent the macroscopic segregation caused by casting and create a high-alloying aero-engine turbine disk alloy having remarkable microstructural homogeneity and superior thermal capability. PM superalloys have been developed into the 3rd generation alloys for decades, and alloys such as René104 already entered service. The chemical composition of the 4th generation PM superalloy is still being researched with the aim of increasing the temperature capability for disk applications to 815^oC. In this work, the remarkable creep resistance and creep strengthening mechanism of a novel high-W and high-Ta type PM superalloy GNPM01 was examined. The creep deformation mechanism of GNPM01 alloy and the segregation of elements on deformation defects were investigated using advanced spherical aberration-corrected scanning transmission electron microscopy. The results reveal that the creep resistance of GNPM01 alloy is considerably higher than that of the 3rd generation PM superalloy. The temperature capacity of GNPM01 alloy is approximately 40^oC greater than that of FGH4098 alloy under the creep condition of 600 MPa and 1000 h. The creep strength of GNPM01 alloy is approximately 160 MPa higher than that of the FGH4098 alloy at 815^oC. In the experimental conditions, the creep deformation behavior was dominated by deformed microtwins, and the GNPM01 alloy clearly slowed down the widening of extended stacking faults and the thickening of microtwins during the creep deformation. It was discovered that the element enrichment of Co, Cr, and Mo existed in the microtwins, and the phase transformation of the twin-structure in γ' phase was disordered because of the segregation of Co, Cr, and Mo by atomic-level energy dispersive X-ray spectroscopy. The isolated superlattice stacking faults in FGH4098 alloy also occurred in the disordered phase transitions. The disordering of superlattice stacking fault or microtwin structure was due to the segregation of Cr, Co, and Mo, which also resulted in the a / 6<112> Shockley partials shearing γ′ phase without producing high-energy nearest-neighbor Al—Al bonds. The segregation disordered the L1₂ structure resulted in reduced pinning of partials by the ordered γ′ phase, which increased the creep rate of the alloy. During the GNPM01 alloy creeping at 815^oC, solute atoms W, Ta, and Nb segregated at the isolated superlattice extrinsic stacking fault (SESF) had ordered atomic occupancy. The fault-level local phase transformation occurred in isolated SESF, forming the [(Ni, Co)₃(Ti, Nb, Ta, W)] ordered η phase that can effectively inhibit the formation and expansion of microtwins, thus lowering the creep rate of GNPM01 alloy.

Hot cracking is a prevalent defect in metallurgy that often occurs during the laser additive repair of single crystal superalloys. The understanding of the cracking mechanism is vital for defect prevention. Consequently, this study entails combining experimental analysis and theoretical calculations to investigate the hot cracking mechanism in a second-generation single crystal superalloy, DD432, during laser additive repairing. The incident of hot cracking was observed predominantly at high-angle grain boundaries (HAGBs). High-magnitude stress concentrations were identified on both sides of the crack, accompanied by an extensive distribution of MC-type carbides in the crack initiation region. Hot cracking depended on factors such as liquid film stability, stress concentration, and MC-type carbide precipitates. The stability of the liquid film depended on dendrite coalescence undercooling, which in turn was related to the angle of grain boundaries. According to Rappaz's theory of dendrite coalescence undercooling, the calculated dendrite coalescence undercooling at HAGBs was 395 K. This figure was substantially higher than the 38 K liquid film undercooling found within a single dendrite, and far exceeded the undercooling at a low-angle grain boundary (3.6°) with a value of 56 K. The elevated level of stress concentration served as a driving force for crack initiation and propagation. MC-type carbide precipitates promoted crack initiation through a pinning effect on the liquid feed, thereby weakening the interface bonding strength with the substrate.

The K439B alloy is a novel equiaxed superalloy and is used for producing hot section components that need to resist high temperatures in aero engines and gas turbines as its temperature capacity exceeds 800^oC. In this study, the evolution of the microstructure and mechanical properties of K439B equiaxed superalloy after being subjected to long-term aging at 800^oC for 3000 h was examined. The predominant deformation mechanisms affecting room-temperature tensile and stress rupture properties at 815^oC and under 379 MPa stress following different aging durations for the K439B alloy were investigated.Results indicate that for heat-treated alloy, the morphology of the γ' phase is spherical, MC carbide is generated in the interdendritic region and grain boundaries, while M₂₃C₆ carbide is in the grain boundaries. During long-term aging at 800^oC, γ′ precipitates conform to the Ostwald ripening mechanism for growth and tend to take a cubic form; the coarsening rate of the γ′ phase is calculated to be 71.7 nm³/h; Additionally, the MC carbide deteriorates while the content of M₂₃C₆ carbide gradually increases. After long-term aging for 3000 h, the precipitated grain boundary phase comprises MC carbide, γ′ phase, and M₂₃C₆ carbide; the orientation relationship between γ′ phase and M₂₃C₆ carbide can be described as [111] _γ' //[111] _M{}_{{}_{\mathrm{23}}}_C{}_{{}_{\mathrm{6}}} and (\mathrm{2}\overline{\mathrm{2}}\mathrm{0}) _γ′ //(\mathrm{2}\overline{\mathrm{2}}\mathrm{0}) _M{}_{{}_{\mathrm{23}}}_C{}_{{}_{\mathrm{6}}}. The heat-treated alloy demonstrates room-temperature tensile and yield str-engths of 1159.0 MPa and 911.5 MPa, respectively. Meanwhile, the stress rupture life at 815^oC and under 379 MPa stress is 150.4 h. As the size of γ′ precipitates increases, the dominant deformation mechanism shifts from dislocation slipping in the matrix to dislocation cutting through the γ′ phase after long-term aging, resulting in superior stacking faults appeared in the γ′ phase. Consequently, the room-temperature tensile strength and stress rupture life show reduction at 815^oC and under 379 MPa stress.

Oxygen content of Ni-based superalloy powders is higher than those of their bulk alloy counterparts due to the larger specific surface area of the former, which is detrimental to the performance of powder metallurgy (PM) and additive manufacturing (AM) superalloys. Therefore, at present, research in this field is primarily focused on understanding the mechanism of oxygen content increase of the powders and approaches of oxygen decrease. Storage and degassing treatment are typical processes of increasing and decreasing of oxygen content in superalloy powders, respectively. Studying the effects of these processes is of great significance for guiding the optimization of powder treatment processes and further improving alloy properties. The original surface state of powders with different narrow particle size ranges, as well as the effects of oxygen increasing/decreasing processes, i.e. storage and degassing, on the microstructure and mechanical properties of alloys were investigated using field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and temperature programmed desorption with mass spectrometry (TPD-MS). The results indicate that the surface composition of the original powders with different particle sizes has no significant difference, all samples exhibit NiO/Ni(OH)₂, TiO₂, CoO, and Cr₂O₃ on their surfaces. The average thickness of the surface oxide layer for 0-15 μm fine and 150-180 μm coarse powders is 3.32 and 10.90 nm, respectively. The oxygen content of the 0-15 μm fine powders and 150-180 μm coarse powders gradually increases in ambient air environment and stabilize at about 250 × 10^-6 and 40 × 10^-6, respectively, within 3-10 d. The oxygen content of the bulk alloy consolidated from the post-storage powders (0-53 μm) increased compared to that of the alloy from pre-storage powders, and the tensile strength at room temperature, 650^oC, and 750^oC showed minor changes, but the ductility decreased and the stress rupture properties of the alloy at 650^oC, 890 MPa and 750^oC, 530 MPa decreased. During the heating process from room temperature (~25^oC) to 1000^oC, the gas desorption occurred on the 0-15 μm fine powders, with desorption peaks of CO₂, H₂O, and H₂ observed. The gas desorption mainly occurred on the powders surface in the range of 100-600^oC, and the desorption peaks are mainly located within 300-600^oC. However, the desorption peaks were not obvious during the heating of the 150-180 μm coarse powders. The oxygen content of the alloy consolidated from powders with particle size range of 0-53 μm decreased from 195 × 10^-6 in the initial state to 113 × 10^-6 after the (300^oC + 600^oC) combined degassing process. Alloys prepared from powders that underwent combined degassing exhibited higher mechanical properties, with the performance improvement mainly reflected in the ductility index of the alloy. The oxygen increase mechanism of superalloy powders mainly includes surface oxidation and surface adsorption, while the oxygen decreases mainly due to the desorption of oxygen-bearing gases on the powder surface. The temperatures of the peak position in the desorption curves of superalloy powders were selected to accurately customize the holding temperature of the degassing process. As a result, through multi-stage degassing treatment at 25^oC + 150^oC + 310^oC + 470^oC, the oxygen content of the powders (0-53 μm) stored in ambient air was further reduced to within (87-96) × 10^-6.

While using traditional methods of directionally solidifying superalloy castings, the liquid density at the lower region of the mushy zone gradually lowers than the top. This is due to a strong segregation of alloying elements. The gravitational force then exacerbates this density inversion, leading to upward convection from the mushy zone to the liquid ahead of the solidification front. This process, known as solutal convection, results in several solidification defects such as freckle defects, an upward accumulation of γ/γ' eutectic, and seeding process issues. As higher-generation single crystal superalloys continue to develop, the problems of element segregation and solutal convection become more pronounced. Traditional measures, such as adjusting process parameters, struggle to effectively alleviate these issues. Given that these problems largely arise from gravity-induced fluid flow, this work aims to investigate the role of gravity on solidification structure and propose appropriate solutions. To achieve this, the conventional pull-down and novel pull-up methods were adopted to perform directional solidification experiments with superalloys. The influence of gravity on solidification behavior is starkly different in these two experiments. In the pull-down process, dendrites grow upward, against gravity, leading to a variety of solidification defects such as freckles on the casting's lateral surface and an upward accumulation of γ/γ' eutectic on the upper surface of the single crystal turbine blade castings. Stray grains also formed in the remelting region during seeding. These phenomena are caused by the density inversion of the remaining liquid between dendrites, resulting in a top-heavy and bottom-light hydrodynamic state. Liquid convection in the mushy zone was then unavoidable under gravity in the pull-down process. In contrast, the pull-up process had dendrites growing downwards, in line with gravity, leaving the least dense liquid at the top of the mushy zone. In this top-light and bottom-heavy state, gravity stabilizes the segregated residual liquid in the mushy zone, thereby preventing solutal convection. Consequently, freckle defects were eliminated, and the γ/γ' eutectic structure was evenly distributed, not accumulated, on the upper surface of the single crystal blade's platform. Additionally, the stability of remelting and epitaxial growth of seed crystals was ensured by eliminating liquid convection. By using this pull-up process, the negative effects of gravity on the directional solidification of superalloys were removed, and all gravity-related solidification defects consequently disappeared. This novel pull-up process could potentially be developed into a new production process for single crystal superalloy castings, significantly improving casting quality. However, it should be noted that this new pull-up process is more complex in comparison to the conventional method. Although this work lays the groundwork for this process, further technological enhancements are required before this method can be applied to industrial production.