Effect of static overheating thermal exposure and thermal cycles on microstructure evolution and stress rupture property of a Ni-based single-crystal superalloy

This study explores the microstructure evolution of a Ni-based single-crystal superalloy under 1200 °C isothermal exposure and thermal cycling ranging from 25 °C to 1200 °C, focusing on its impact on stress rupture performance at 760 °C/750 MPa and intermediate-temperature deformation behavior and mechanisms. Results reveal that compared to the overheating thermal exposure, the overheating thermal cycling significantly accelerates the coarsening of γ and γ′ phases due to increased internal stresses and enhanced elemental diffusion. Additionally, the overheating thermal cycling induces the formation of dislocation networks, reducing lattice misfit and weakening misfit strengthening. Stress rupture life decreases from 214.5 h in the standard heat-treated state to 189 h (overheating thermal exposure) and 99 h (overheating thermal cycling), primarily due to γ′ coarsening and reduced strengthening effects. Under 760 °C/750 MPa testing, the γ′ phase size significantly influences deformation mechanisms: stacking fault activation dominates when γ′ size is below 720 nm, APB shearing is predominant for sizes above 1030 nm, and both mechanisms contribute for sizes between 720 and 1030 nm. Furthermore, Lomer-Cottrell dislocations are discovered which deposit on the (001) plane through dislocation reactions; this process further reduces plasticity and impacts deformation behavior.