Effects of ultra-high-temperature thermal cycles on the microstructure evolution and creep behavior of a high γ′ volume fraction nickel-based single-crystal superalloy

Thermal cycling induces various microstructural changes in superalloys, which differ from those observed under isothermal testing. In particular, creep life under cyclic conditions is considerably diminished compared with isothermal testing. This study investigated the creep behavior of a high-γ′ volume fraction Ni-based single-crystal superalloy at 850°C/500 MPa after thermal cycling from 25°C to 1200°C. This study offers a unique perspective on the intrinsic behaviors of the alloy. Results revealed that as the number of thermal cycles increases, creep life at 850°C/500 MPa initially increases and then decreases. It was found that the microrafting structure formed by the secondary γ′ phase in the γ channel and the stable dislocation networks contribute to the creep performance of the superalloy. In addition, a relationship is established between the critical resolved shear stress (CRSS) and the changes in the size of the γ/γ′ phases. As the number of thermal cycles increased, the microstructural evolution of the alloy was accelerated. The reduction in creep life of the superalloy at 850°C/500 MPa is primarily attributed to two factors: the reduced CRSS required for dislocations to penetrate primary γ′ precipitates and the diminished effectiveness of dislocation network strengthening at γ/γ′ phase boundaries. This study establishes a notable theoretical and experimental foundation for understanding and predicting the creep behavior of superalloys under thermal cycling conditions.