Intermetallics is composed of two or more metals or of a metal and a nonmetal. The coexistence of covalent and metal bond makes the intermetallic compound have long-term ordered superlattice structure, which greatly reduces dislocation mobility at high temperature, thus exhibiting good high-temperature strength. Typical structural intermetallics such as Ti-Al, Ni-Al and Nb-Si, have the advantages of excellent high-temperature strength and low density, which are very suitable for high-temperature structural parts of aerospace. However, the application of such materials is limited by low fracture toughness at room temperature and poor oxidation resistance at high temperature, which attracts more and more attentions and brings challenges in this field. In this paper, the research and application status in high-temperature strengthening, toughening, oxidation resistance and preparation technology of Ti-Al, Ni-Al, Nb-Si intermetallics-based alloys are introduced.

Single crystal superalloy is the key material used in the hot section of aeroengines and industry gas turbines. The research, development and application of these alloys is generally a mirror of the industry base of a country. The recent progress in research and development of single crystal superalloys is briefly reviewed in the present paper. Some new ideas in alloy development and the design methods are summarized. The deformation behaviors, damage and failure mechanisms of single crystal superalloys during creep, fatigue, oxidation and hot corrosion have been overviewed. The role of typical defects such as low angle grain boundary, recrystallization and micro-porosity is also discussed. The recent progress in the directional solidification processes and typical parameters of high rate solidification, gas cooling casting, liquid metal cooling and fluidized bed cooling are introduced. Fundamental correlations of processing parameters to defect formation and microstructure evolution during manufacture of single crystal blade is discussed. Additionally, the future opportunities and challenges are also explored.

The newly developed high-performance cast-wrought alloys have been widely applied in advanced turbine engines in recent years, particularly, served as turbine disc and compressor disc. The significant breakthrough has been made on the processing techniques for these highly alloyed disc alloys, including the triple-melting of the large-scale ingots with the diameter more than 500 mm, conversion of the large-scale ingot and the processing of fine-grained billets with the diameter more than 300 mm, customization of the microstructures and properties of disc forgings. The chemical compositions can be well controlled on vacuum arc remelting (VAR) ingots, including the ultra-low carbon content lower than 0.011%. The white spots and freckles have been found to be the primary defects on large-scale VAR ingots, which is triggered by the improper re-melting parameters. The metallurgical defects can be identified on fine-grained billets via supersonic inspection. The investigations have shown that the existence of the micro-duplex structure opens a window for the processing of these highly alloyed disc alloys with up to more than 60% mass fraction of precipitates. Via a multicycle thermal-mechanical processing technique, the hot working process of the disc alloys, can be achieved in a cost-effective way, and the un-recrystallized grains can also be eliminated efficiently. The above-mentioned techniques have greatly promoted the development and application of the high-performance disc alloys, such as GH4065, GH4720, GH4175 and GH4975, et al. These alloys can provide a promising solution of high reliability combined with low life cycle cost for military and commercial gas turbines. Nonetheless, in order to maximize the advantageous of cast-wrought disc alloys on reliability and cost-effective ratio, the comprehensive understanding about the prevention and identification of metallurgical defects, increase in the yield rate of materials during conversion, processing of fine-grained billets and dual-property disc forgings, is still needed.

Wrought superalloys are high temperature alloys produced by casting-forging-hot rolling-cold drawing, including disc, plate, bar, wire, tape, pipe etc. These products are widely used in aviation, aerospace, energy, petrochemical, nuclear power and other industrial fields. In this paper, domestic progress of wrought superalloys in recent ten years was reviewed, including advances in fabrication process, research in new alloys (GH4169G, GH4169D, GH4065 and GH4068 alloy et al.) and new techniques (deforming of FGH4096 alloy, nitriding of NGH5011 alloy and 3D printing of In718 alloy et al.).

The research progress in powder metallurgy (PM) superalloys and manufacturing technologies are reviewed. The key control factors of Ar gas atomization (AA) powder manufacturing are introduced, including the aspects of the equipment development, atomization process, particle size, oxygen content, powder morphology and inclusion control. For the turbine disk manufacturing technology, the research progress of dual-property turbine disk, dual-alloy integral turbine wheel technologies and isothermal forging die materials are summarized. In the field of basic research, high-throughput experiment, advanced characterization and creep behavior of PM superalloys were introduced. According to the current major demand for aero-engines and 3D printing, the future of PM superalloys manufacturing technology is prospected.

Equiaxed grain cast superalloys are widely used in aeroengine and other fields due to their low manufacturing cost and excellent mechanical properties at medium and low temperatures. Aeroengine casing is a typical complex thin-walled equiaxed superalloy castings used at medium and low temperatures. The complex thin-walled superalloy investment castings with the complex structures, the accurate size and the lightweight are the key components for advanced aeroengines. The coordinated control of the precise forming and the solidification microstructure for these castings is very difficult. Correspondingly, the requirements for materials, casting technologies, structure controls and mechanical properties in superalloy integral structure castings are becoming increasingly higher. In this paper, the development and application of polycrystalline superalloys, solidification and forming, the simulations and the new technologies are reviewed.

Significant interior residual stresses, which were generated during the manufacture process, could affect the machining dimension precision and structural stability during the subsequent machining process and service operation in the superalloys component, such as turbine disk. In this paper, the neutron diffraction method and contour method are described for measuring the distribution of interior residual stresses. The distribution, evolution of interior residual stress, and its mechanism are analyzed during quenching, ageing heat treatment and machining process in superalloys disk forging. The residual stresses are mainly generated by the temperature gradient formed during rapid cooling after solution heat treatment. After quenching, the residual stresses in hoop direction and radial direction of disc forging are significant, and its distribution along the profile is characterized by "internal tension and external pressure". The magnitudes of the residual stresses are equivalent to the yield strength of as-quenched alloys at room temperature. Quenching-induced residual stresses are partially relieved during the ageing process due to plastic strain and creep-controlled dislocation rearrangement. The precipitation behavior of γ″ or γ′ phase during heat treatment has a significant interaction with the distribution and magnitude of residual stress. During the machining process, part of the residual stresses contributing to the equilibrium of the internal forces are removed along with the material. Additional moment caused by re-balance of residual stresses results in the serious consequences of distortion in the remaining body.

Ni-based superalloy turbine blades have been widely used in aerospace and industrial engine. Numerical simulation techniques can optimize the superalloy directional solidification process and enhance the rate of finished products. This paper summarized the existing macroscopic and microscopic numerical models in the superalloy blade directional solidification process. Simulations have been done on the temperature field evolution, grain structure and dendrite morphology in typical HRS and LMC directional solidification conditions, and the resulting microstructure features were investigated. In particular, the application of varying withdrawal rate in directional solidification of the superalloy blade was introduced. And the advantages of the varying withdrawal rate technique were emphasized by comparing it with the constant withdrawal rate method. The simulation results indicate that by applying varying withdrawal rate, the convex or concave shape of the mushy zone can be change to flat shape, so that parallel columnar grains can be obtained with enhanced high-temperature performance of the turbine blade.

The superalloy is one of key metal materials, representing the level of scientific and technological development. Nickel-based superalloy is the most important which has been widely used for rotating component of aerospace. Nickel-based GH4169 alloy shows excellent combination properties including good fatigue property, excellent oxidation and corrosion resistance, as well as the microstructure stability during long-term ageing. The using amount of GH4169 alloy is about 45% of total wrought superalloys. For satisfying high performance of aero-engine, both strength and ductility of GH4169 alloy at high temperature are required to be simultaneously improved for safety servicing. It is an effective method to strengthen alloys by adding alloying elements. The alloying element addition ratio of GH4169 alloy is more than 40%, which unavoidably leads to hard deforming and plasticity declining, so that it restricts the further application of the alloy. Therefore, it is key to find methods realizing strengthening-toughening and without any losing of hot-deforming ability. In this work, the plastic deformation behavior and strengthening-toughening mechanisms of GH4169 alloy with multi-field coupling (electric-pulse current (EPC)/temperature/stress) were investigated. The results show that the deformation resistance of GH4169 alloy decreases and plastic deformation ability increases with multi-field coupling. The thermal vibration of atoms enhances and thus leads to decreasing of Peierls force with multi-field coupling, which is the essential factor on decreasing of deformation resistance and increasing of plastic deformation coordinate ability. When the alloy aged with electric-pulse treatment (EPT)/temperature coupling, the ultimate strength, yield strength and fracture elongation increase simultaneously at elevated temperatures. The vacancy concentration increases of the alloy aged with EPT/temperature coupling. Vacancy induces ultrafine nm-sized γ" phase to precipitate during tensile deformation at high temperature, which is the key factor on strength and ductility improvement. At the same time, because of the EPT/temperature coupling ageing, part of γ" phases precipitate around dislocation, while, due to the increasing of γ" phase size, the ductility of the alloy will be improved. With the multi-field coupling treatment, the strengthening-toughening of GH4169 alloy can be realized depended on an appropriate distribution of two kind sizes of γ" phase.

Ni-based single crystal superalloys have excellent comprehensive properties and become the preferred material for advanced aeroengine turbine blades. DD6 alloy which has been widely used in China and DD5 alloy are the second generation single crystal superalloy, and their chemical compositions and mechanical properties are quite different. In the past few decades, high cycle fatigue failure has become one of the main causes of turbine blade failure. More and more attention has been paid to the high cycle fatigue properties of single crystal superalloys. Therefore, it is important to study the high cycle fatigue behavior of single crystal superalloys, especially the second generation single crystal superalloys. In order to compare high cycle fatigue performance, two typical second generation single crystal (SC) superalloys DD6 and DD5 with [001] orientation were subjected to high cycle fatigue (HCF) loading at temperatures of 760 and 980 ℃ in ambient atmosphere. The results demonstrate that the fatigue limit of DD6 alloy is 414 and 403 MPa at temperatures of 760 and 980 ℃, respectively. DD6 alloy exhibits an excellent HCF performance under a condition of stress ratio of -1 regardless of medium or high temperature. Analysis on fracture surfaces of DD6 and DD5 alloys at 760 and 980 ℃ demonstrate that quasi-cleavage mode is observed. In addition, different types of dislocation structures were developed during the cyclic deformation. When the stress amplitude is low, dislocation movement in the γ matrix by bowing and cross slip is the main deformation mechanism and shearing γ' particles by dislocation pairs occurs occasionally under high stress level. The analysis shows that the carbon content of DD5 alloy is eight times than that of DD6 alloy, which makes the carbide content much higher than DD6 alloy, and there are significant differences in carbide morphology. In the process of fatigue fracture, carbide plays two roles of secondary crack initiation position and crack propagation channel, which greatly accelerates the fatigue crack growth rate. In the end, the fatigue resistance of DD5 alloy is reduced.

Single crystal nickel-based superalloys have been widely used in high temperature structural materials applications including blade parts of aero-engines and gas turbines due to their excellent mechanical properties in service. Although commercial single crystal superalloy blades are in [001] orientation, misorientation deviations are inevitable in industrial productions and work blades frequently have to endure complex stress states caused by their complicated shapes and temperature gradients. Therefore, it is of great significance to study the creep behavior of single crystal superalloys with different orientations for the design of engine blades. The anisotropic creep properties of a nickel-based single crystal superalloy with different orientations near <111> were investigated under 760 ℃ and 650 MPa. It is found that specimens with the smallest deviation from <111> orientation exhibit best creep strength because of the relatively low Schmid factors of both {111}<110> and {111}<112> slip systems. With the increase of orientation deviate from [\bar{1}11] to [011], creep properties decrease more significantly compared with the deviation from [\bar{1}11] to [001]. All samples deviate from <111> within 20° exhibit poor strain hardening. While orientations toward [\bar{1}11]-[001] boundary have a distinct incubation creep stage with relatively low initial creep rate. Further dislocations and lattice rotation analysis showed that the dominant slip systems are {111}<110> for specimens with minimum deviations. The stress is almost uniformly distributed in three γ matrix channels, which lead to a homogeneous deformation behavior. As the orientation deviation increases, {111}<112> slip systems begin to play a leading role during creep process. While the generation of <112> dislocations is closely related to the reaction and decomposition of <110> dislocations. Specimens on [\bar{1}11]-[011] boundary have coplanar double slips for {111}<110> slip systems resulting in a high initial creep rate and poor strain harding. Meanwhile, Schmid factors of {111}<112> slip systems increase rapidly with the increase of orientation deviation from [\bar{1}11] to [011], which lead to a significantly degradation on creep properties. While as for orientations along [\bar{1}11]-[001] boundary, Schmid factors increase in a relatively gentle way with the number of dominant slip systems reduced from 6 to 2. Multiplication of dislocations and the formation of <112> dislocation ribbons are impeded, resulting in a comparatively long incubation creep stage.

Since nickel-based superalloys are more and more used as fasteners, it is necessary to investigate the stress relaxation behavior and mechanism of nickel-based superalloy. In present work, the stress relaxation mechanism for four typical nickel-based superalloys (GH4169, GH4169D, GH4738, GH350) for fasteners under service condition was investigated. The stress relaxation tests were carried out according to GB/T 10120-2013 in the temperature range of 600~780 ℃ and initial stress range of 260~510 MPa, and the stress relaxation curves were recorded. The microstructure was studied by FESEM and TEM. The results show that the stress decreases fast in the initial stage of stress relaxation test and then trends to be steady. The stress relaxation stability decreases with increasing temperature. There is no apparent change in the microstructure after stress relaxation test. TEM observation shows that the major mechanism of stress relaxation is the movement of dislocations, and the stress relaxation properties of different alloys depend on the inhibition of dislocation movement. The species, size, shape and distribution of phases determine the ability to hinder dislocation movement and the stress relaxation property of different alloys. GH4169 alloy gets the stress relaxation property mainly by γ' phase, γ'' phase and δ phase hindering the movement of dislocations. In GH4169D alloy, both γ' phase and η phase participate in the stress relaxation process. γ' phase in GH4738 alloy can effectively impede the movement of dislocations and provide good stress relaxation property. The combined effect of γ' phase and η phase guarantees the stress relaxation stability of GH350 alloy.

Welding is an important joining method to fabricate the dissimilar welding integral blisk structure of single crystal and polycrystalline superalloy. The microstructure and properties of the welded joint are the key factors to determine the reliability of the integral blisk structure of dissimilar superalloys. The single crystal superalloy of DD407 and polycrystalline superalloy of IN718 were butt welded by continuous fiber laser system. The evolution of microstructure and composition segregation of the welded joints fabricated under the optimized welding parameters as-welded (AW) and after post weld heat treatment (PWHT) were investigated. The high temperature tensile deformation behavior of the welded joint after PWHT was also examined. The results show that the microstructures of fusion zone (FZ) in the welded joint consist of planar crystal, cellular crystal, columnar crystal and equiax crystal. The difference of the dendrite microstructures between the two sides of the weld centerline is very obvious. In terms of the joint as-welded, the microhardness of the FZ is low and there exists obvious micro-segregation. After PWHT, the micro-segregation has been improved and the microhardness increases significantly in the FZ which is much more than those of both base metals (BMs) of DD407 and IN718 alloys. There exists local hardening zone in the heat-affected zone (HAZ) of DD407 single crystal alloy and narrow softening zone and grain boundary liquation phenomenon in the HAZ of IN718 polycrystalline alloy. The ultimate tensile strength and elongation of the welded joint after tensile test at 650 ℃ are 1111 MPa and 9.42%, respectively. And the tensile specimen of the welded joint fails in the BM of IN718 polycrystalline alloy. The main deformation mode of the laser welded joint at high temperature includes the multi-slips of dislocation in the BM and FZ of single crystal alloy, and the dislocation slip and grain-boundary sliding in the BM of polycrystalline alloy. The tensile fracture surface is characterized by multi-source cracking, and the dimple and crystal sugar shaped facture surface exist simultaneously in the crack source area, which is a mixed fracture of microvoid aggregation and intergranular fracture. So the tensile fracture mechanism contains micro-void accumulation fracture and inter-granular fracture. The grain boundary liquefaction in the HAZ of IN718 polycrystalline does not affect the short-time high temperature mechanical properties of the welded joints.