先进镍基单晶高温合金蠕变行为的研究进展

Advanced Ni-based single crystal superalloys have long been the candidate materials for applications in the critical components of modern aeroengines and in-land gas turbine engines due to their superior composition compatibility, microstructural stability, and creep, fatigue, oxidation, corrosion resistances at temperatures up to 1 000℃ and beyond. During services, Ni-based single crystal superalloys are mainly subjected to creep and fatigue deformation caused by the centrifugal forces generated from turbine blade rotation. In addition, the increasing need for the higher turbine entry temperature in modern aeroengines also poses a greater challenge to the temperature and load tolerance of advanced Ni-based single crystal superalloys. In the past decades, considerable endeavors have been made aiming at promoting the creep resistance of Ni-based single crystal superalloys. The addition of refractory elements such as W, Cr, Mo, Re, etc. has led to lower diffusion rates and consequently elevated solid solution strengthening levels in superalloys. A significant number of γ'-forming elements added to Ni-based single crystal superalloys, e.g. Al, Ti and Ta, have been proved able to impart strengthening via the ordered compound, i.e., γ'-Ni₃(Al, Ti, Ta) phase, which is known as the precipitation hardening. The coherency strains, existence of order in γ' phase, and differences in elastic moduli and stacking fault energy (SFE) between γ' phase and γ matrix, etc. contribute to the improvement of creep resistance of advanced Ni-based single crystal superalloys. By applying a series of adjusted heat treatments, including ramp solution heat treatment, fast solution heat treatment and melting solution heat treatment, etc., the sizes, morphologies and volume fractions of γ' precipitates have been further optimized, which achieves full potential of the precipitation hardening. Moreover, following from the consideration that the creep resistance of a superalloy will be enhanced due to the increased γ/γ' interface strength, moderate Mo and Re elements have been added, resulting in appropriately increased γ/γ' lattice misfit, improved γ/γ' interfacial dislocation network density and interfacial strength, all of which benefit the creep resistance. Researchers have also successfully reduced the susceptibility to precipitation of topologically close-packed (TCP) phases by adding Pt-group elements, and in consequence, have made further strides in stabilizing the superalloys' microstructure. However, Ni-based single crystal superalloys already have quite high alloying degree. The refractory elements content in CMSX-10 has approached 20.5 wt%, which has almost touched the ceiling solubility in Ni-matrix. Meanwhile, there have been observed a series of negative consequences of alloying, e.g. microstructural instability (the increasing precipitation tendency of TCP phases and solidification defects), and the rise in alloy density and cost. On the other hand, despite of relying on increasing the content of refractory elements and platinum group elements' addition to stabilize microstructures, there lacks other known and effective ways available for designing next-generation Ni-based single crystal superalloys. And current methods seem more bent on running counter to the prevailing ideal of modern industry that advocates low density, low cost and enviro nment-friendliness. This sobering development situation has highlighted the importance of a deep understanding of the intrinsic relationship of composition-microstructure-performance, and the urgency of a breakthrough in the traditional alloy design theory. From the perspective of one of the most important mechanical properties, i.e. creep resistance, this review provides elaborate descriptions about the composition, microstructure and creep behavior, etc., in advanced Ni-based single crystal superalloys. We also discussed carefully the action principles of solid solution elements, and sizes, morphologies and volume of γ' precipitates, as well as γ/γ' interface, stacking fault energy (SFE) and antiphase boundary (APB) energy, etc., on the creep behavior and mechanism. The current existing challenge and future research aspects on the study of creep in new-generation Ni-based single crystal superalloys were stated and prospected.