Ni-based single crystal superalloys have been widely used to produce turbine blades for advanced aero-engines because of the super temperature-related microstructural stability and comprehensive mechanical properties. However, due to effects of the high temperature and complicated stresses in service, the microstructures of superalloys might gradually evolve and fail in different modes. The present paper reviews the progress of microstructural stability and mechanical behavior including the γ’ phase rafting, TCP phase precipitation, high temperature creep, low cycle fatigue and thermomechanical fatigue of single crystal superalloys. The addition of Ru improves the creep life of superalloys, but also indirectly promotes the occurrence of “topological inversion”. On the other hand, with the increase of aging temperature and time, the contents of refractory elements in m phase rise significantly. With the increase of applied tension stress, more m phase precipitate from the γ matrix, whereas inverse tendency is shown under compression stress. Numerous planar defects are formed during precipitation of m phase, and these defects promote the nucleation of P and R phases. During high temperature and low stress creep, an important dislocation a<010> superdislocation is observed, which moves in the γ’ phase slowly by a combination of slide and climb. Under very high temperature, incubation with accelerating creep rate occurs before the primary stage, which relates to the extending process of the γ width. At last, the stacking fault energy is significantly reduced after Ru additions, and thus a series of complex deformation mechanisms occur during low cycle fatigue, e.g. stacking faults penetrating γ/γ’ interface, trailing a/6<112> Shockley dislocations shearing into the γ’ phase. During thermomechanical fatigue, the life of superalloys is influenced by the site of crack initiation, microstructural evolution and oxidation resistance.

Single crystal (SC) superalloy is a kind of complex structure and multi phase materials. With the increase of the degree of alloying and the content of refractory elements, or the more complicated structure and larger size of the casting made of SC superalloy, it is essential important to suppress the formation of solidification defects to improve the quality and performance of the blades. The microstructure and solidification defects of single crystal alloy are not only related to the composition of the alloy, but also depend on its solidification characteristics and technological conditions. The paper first summarizes the research progress of the solidification characteristics for advanced SC superalloys, focusing on analysis of the effects of solidification characteristics and processing parameters on the formation and its mechanics for two typical directional solidification defects, crystallographic orientation deviation and stray grains. Then some methods and approaches to suppress such defect formation for complex single crystal blade have been reviewed.

Based on the analysis of solidification processing in complex turbine blades, a new idea of 3-dimensional and precise control of single crystal (SC) growth was proposed. A series of new techniques were presented,exhibiting the new development in the production of SC blades of superalloys. The heat conductor (HC)technique was developed to minimize the hot barrier effect which hindered the lateral SC growth. This method promotes the successful transition of SC growth from the blade body into the platform extremity prior to the nucleation of stray grains. To achieve symmetric thermal conditions for solidifying the SC blades, the PHC (parallel heating and cooling) system has been employed. With this technique, both sides of a shell mold can be both symmetrically heated in the heating zone as well as cooled in the cooling zone. The negative shadow effect in the current Bridgman process and the related defects are hence removed. With the H&D (dipping and heaving) technique using thin shell, the main problems of the Bridgman process, such as the ineffective radiative heat exchange and the large thermal resistance in thick ceramic molds, can be effectively resolved. This technique enables the establishment of a high temperature gradient at solidification front. By combining targeted cooling and heating technique, a 3-dimensionalcontrol of SC growth in large components can be achieved.

The elimination of the segregation improves the thermo plasticity of superalloy ingot during the homogenization process, but coarser grain structure and high-temperature oxidation caused in further homogenization have an adverse impact on the thermo plasticity. The inheritance of coarse grain structure in the followed hot working process increases the tendency of cogging crack and makes the grain refining harder, leading to a lower yield of the final workpiece. The microstructure characteristics and their hot deformation behaviors of GH4740H, GH4738, GH3625 and 690 alloys under different homogenizations were investigated by means of microstructure analysis methods and crack propagation testing. The experimental results show that the reasonable homogenization processing needs to take into account the segregation elimination arising thermo plasticity addition, more to consider grain coarsing and severe oxidation leading to decrease plasticity. Based on the residue dendrites can provide more recrystalazation nucleation sites, the partial homogenization possessing probably exists rationality. This research work provides an exploratory study for the improvement of the homogenization-cogging process of superalloy.

Microelement Hf added in Ni-based powder metallurgy (PM) superalloy can modify microstructure and improve mechanical properties, such as stress-rupture life, creep resistance and crack growth resistance, and also benefit to eliminate notch sensitivity. So systematically studying the effect of microelement Hf on PM superalloy microctructure will help to comprehend its corresponding mechanism. The effects of microelement Hf on the morphologies, chemical compositions and content of γ’ phase and MC carbide in FGH97 PM Superalloy were investigated by means of SEM and physiochemical phase analysis. The results showed that Hf facilitated the precipitations of γ’ phase and MC carbide, and changed chemical compositions of γ’ phase and MC carbide, the effect of Hf on the size and morphology of MC carbide was not obvious, while Hf greatly affected the size and morphology of γ’ phase and accelerated the splitting of γ’ phase from one instable cubic γ’ particle to stable octet of cubes. As Hf affected the lattice misfit of γ’/γ phase (d), modifying Hf content changed the critical splitting size of γ’ phase (D_c). The relationship between D_c and Hf content (w(Hf)) was found to be D_c=315.4+640.2w(Hf)-358.2[w(Hf)]². With Hf content increased, the absolute value of d decreased and D_c increased. Cubic γ’ particle split into an octet of cubes when γ’ phase grew up to the critical splitting size.

Much attention has been paid to the development of more advanced materials for high-pressure compressor and turbine discs of gas turbine engines. A high performance wrought superalloy GH4065 for disc applications has been recently developed based on the comprehensive evaluation of a series of model alloys with characteristic chemical composition, lattice parameter, particularly γ’ volume fraction. The concentration of major alloying elements of GH4065 is closely similar with René 88 DT and specifically optimized considering the demands of ingot metallurgy technologies. Therefore, GH4065 can be considered as an ingot metallurgy version of powder metallurgy René 88 DT. Large scale vacuum arc remelting (VAR) ingots of GH4065 alloy with diameter up to 508 mm have been produced via standard triple melting techniques. Micro-scale segregation of alloying elements on large VAR ingot has been effectively suppressed due both to optimized alloying elements concentration and to improved melting techniques. Ultra-low carbon content (less than 0.02% in mass fraction) significantly decreases the dendritic segregation tendency of certain alloying elements and promotes the uniformity of microstructures. VAR ingot of GH4065 exhibits extraordinary hot plasticity, ingot conversion can be accomplished using conventional open die forging procedure. Fine and uniform γ+γ’ duplex structures can be obtained on billets and disc forgings via a newly developed multi-cycle thermomechanical processing method. The flow stress data show that the formation of γ+γ’ microduplex results in a significant decrease of flow stress in comparison with γ’ dispersion structures under exactly the same deformation conditions. The distribution of strain rate sensitivity m in relationship with temperature and strain rate accurately identifies a specific domain within which γ+γ’ microduplex exhibits superplasticity. Full-scale turbine discs of GH4065 alloy with diameter of 630 mm achieve an optimal combination of creep resistance, fatigue lifetime and ductility. GH4065 discs exhibit extraordinary microstructural and property stability during prolonged thermal exposure, which means that dendritic segregation has been successfully restricted to an acceptable level. The results reveal that highly alloyed disc alloys produced via ingot metallurgy techniques exhibit lower costs and higher productivity, and can still meet the ever increasing demand of high performance gas turbine engines.

GH4169 alloy is widely used to make aero engine, gas turbine as it is one of the most important superalloy. γ’' phase is the main strengthen phase, however, the metastable γ’' phase will transform to stable d phase during aging or servicing for certain time. d phase is significance in the alloy, its precipitating and distributing behavior have an effect on the properties of the alloy. In recent year, researchers pay more attention on electric field treatment (EFT), this is because of high energy density, accurate controlling, clean and safety. EFT is one of the most important energy field except temperature field and stress field. In this work, EFT was performed on GH4169 superalloy to investigate the influence of EFT on precipitation behavior of d phase in the alloy, and the mechanism of the effect of EFT on the phase transformation was also discussed. The results show that d phases precipitate on the grain boundaries after EFT with 8 kV/cm at 850 ℃ for 15 min, and large amounts of γ’' phases precipitate inside the grains. With the increasing of EFT time, both the volume fraction and the size of d phase increase, at the same time the size of γ’' phase increases. The volume fraction of d phase is less and the size of d phase is smaller, and the volume fraction of γ’' phase is higher by EFT, compared with that by aging treatment (AT) for the same time. In addition, the Nb content on the grain boundary decreases and both Fe and Cr content increase, meanwhile the lattice parameters of c decreases and a, b increase. The vacancy concentrations can be accelerated by EFT, so that the diffusion of Fe and Cr atoms can be promoted. Meanwhile, the Nb atoms in d phases on the grain boundaries can be displaced by Fe atoms and Cr atoms, therefore the Nb atoms are dissolved into the grain. The nucleation rate of γ’' phases increases with the increasing of vacancy concentrations. The vacancies relax coherent distortion between γ phases and γ’' phases, and suppress γ’' phases to transform to d phases. Thus the stabilization of γ’' phases is enhanced.

Service safety of turbine blades in aircraft engines are threatened by microstructural and property degradation instantly caused by overheating during service. Systematic investigations about microstructural degradation during overheating exposures and its influence on mechanical properties of turbine blades during service are limitedly reported. In this work, microstructure and mechanical properties of GH4033 alloy, which was sectioned from the shank of a serviced 2^nd stage turbine blade in an aircraft engine, were studied after overheating at 900~1100 ℃ for 3 min. Microstructural degradation during overheating exposures as well as its influence on room temperature hardness and stress rupture life at 700 ℃, 430 MPa were analyzed. The results of microstructural characterization indicated that the coarsening and dissolution of γ’ precipitates were introduced by overheating exposures, and all of the γ’ precipitates dissolved at 980 ℃ for 3 min. Gradual dissolution of grain boundary (GB) carbides was observed with the increase of overheating temperature. Complete dissolution of GB carbides at 1100 ℃ resulted in grain growth. The room temperature hardness after overheating exposures decreased grossly with the dissolution of γ’ phase. Due to the dissolution and re-precipitation of γ’ phase as well as the dissolution of GB carbides, the stress rupture life under 700 ℃, 430 MPa of GH4033 alloy was initially increased and then decreased significantly.

The Ni-based single crystal superalloys are widely used in key hot section parts of advanced aero engine due to the superior high temperature mechanical properties. Multi-axial stresses resulting from complex temperature and stress state happen frequently in blades during service, thus the mechanical properties of three orientations need to be studied. However, most of these works are conducted in the first and second single crystal superalloys and there is rare report concerning the third single superalloys. Therefore, in this work the microstructures and tensile properties of the third generation single crystal superalloy DD9 with [001], [011] and [111] orientations were investigated by OM, SEM, TEM and tensile testing machine at 760 and 1100 ℃. The results show that as-cast dendritic structures and heat treated γ’ of DD9 alloy with three orientations are different on the section perpendicular to the crystal growth direction. With rising of temperature, the ultimate tensile strength and yield strength decrease and tensile anisotropy drops obviously. The ultimate tensile strength and yield strength of DD9 alloy with [001] orientation are higher than those with [011] and [111] orientation except that the yield strength with [001] orientation is slightly lower than that with [011] orientation. With temperature increasing, the fracture characteristic transforms from quasi-cleavage at 760 ℃ to dimple at 1100 ℃. At 760 ℃, very high density dislocations appear in the matrix channels with [001], [011] and [111] orientations, but some stacking faults are present only in γ’ particles with [001] orientation. At 1100 ℃, the high density dislocation networks resulted in the matrix channels and particles of the alloy with [001] and [111] orientations, while a large number of deformation twins are found in samples with [011] orientation.

The effect of Hf on the as-cast, heat-treated microstructures and stress rupture properties under 1100 ℃ and 140 MPa was investigated in four second generation Ni-based single crystal superalloys DD11 with various levels of Hf (0~0.80%, mass fraction) additions. The results indicate that increasing Hf addition resulted in decreasing the solidus and liquidus temperatures, while it enhanced the volume fraction of (γ+γ’) eutectic and MC carbide as well as solidification segregation. The number of micropores reduced significantly and the volume fraction of residual (γ+γ’) eutectic and MC carbide increased after heat treatment as Hf content increased. Compared to the Hf-free alloy, the stress rupture life was observed to increase in the alloys with 0.40%Hf, but dropped in the alloy containing 0.80%Hf. Hf addition increased the elemental partitioning ratio of Re, Mo, Cr, resulting in increasing γ/γ’ misfit and decreasing the spacing of γ/γ’ interfacial dislocation networks. The solution strengthing effect was also improved with the enhanced concentration of Re, Mo and Cr in γ phase in Hf-modified alloys. However, when the Hf content was 0.80% in DD11 alloy, the stress rupture properties was decreased obviously due to high volume fraction of residual (γ+γ’) eutectic and MC carbide in heat-treated microstructures.

Ni-based single crystal (SX) superalloys are widely used for production of blades in gas turbines and aircraft engines for their superior mechanical performance at high temperatures. To obtain high cooling efficiency, most of the SX blades consist of thin wall with cooling holes. However, thermal fatigue cracks are usually observed in blades with this kind of structures. Thus, it must be valuable to investigate the crack initiation and propagation around a hole during thermal fatigue tests in a SX superalloy. In the present work a second generation SX Ni-based superalloy was used. Plate specimens that parallel to directional solidification (DS) direction and along (100) or (110) planes were prepared. A hole with diameter of 0.5 mm was drilled vertical to the surface in the middle of the plate by electro-discharge machining (EDM). Thermal fatigue tests were performed between room temperature and 1100 ℃. Effect of crystal orientation on the crack initiation and propagation was investigated and the reasons were analyzed. It was found that a thin recast layer was produced around holes of EDM drilled. The thickness of the recast layer was 15 mm in the maximum. Crystal orientation has great effect on the crack initiation sites and propagation kinetics. After 80 cyc thermal fatigue tests, in (110) specimens cracks initiated at the edge of the holes that vertical to the DS direction, then grew quickly and propagated along directions about 45° from the DS direction. After 200 cyc tests, cracks developed to more than 2 mm in length. While in (100) specimens no cracks could be observed even after 200 cyc thermal fatigue tests. This difference was mainly due to the combined effects of different thermal stress caused by the anisotropy of single crystals and of the microstructure characteristics.

According to the requirement of high-pressure turbine guide vane during service, the aim of this work is to design a single crystal Ni₃Al-based alloy named IC21 with low density, low cost, and high strength which can be used as high-pressure turbine guide vane material. The mass fraction of the Re has been limited less than 1.5% on purpose. The single crystal bars of IC21 were prepared by high rate solidification method. The density of IC21 is 8.0 g/cm³ and the incipient melting temperature was identified by metallography. After standard heat treatment, the distribution of the g' precipitates is uniform with the average size of about 420 nm, and volume fraction of 80%. The tensile and yield strengths at 1100 ℃ are 490 and 470 MPa, respectively. Moreover, IC21 shows superior creep properties, the stress-rupture life at 1100 ℃,140 MPa is 170.5 h and at 1150 ℃,100 MPa still remains 110.0 h. The microstructure stability of IC21 alloy at 1080 ℃ for as long as 1000 h were evaluated. The results show that no precipitated phase exists during thermal exposure at 1080 ℃, which exhibits good stability. The oxidation kinetic curves of IC21 alloy follows a parabolic rate law in different oxidation stage during cycle oxidation for 100 h in air. IC21 alloy has a good high temperature oxidation resistance, the strengthening mechanism are attributed to high volume fraction of g' phase, large negative misfit and well-established interface networks.