Microstructure evolution and stability of a fourth-generation Ni-based single-crystal superalloy under ultra-long-term aging

This study investigates the microstructure evolution of a fourth-generation Ni-based single-crystal superalloy during ultra-long-term aging up to 3000 h at 950 °C and 1050 °C. The γ′ phase coarsens following LSW theory, with coarsening rates of 2.94 × 10^–5 μm³/h (950 °C) and 6.22 × 10^–5 μm³/h (1050 °C), respectively. After long-term aging at 950 °C and 1050 °C for 3000 h, nearly no TCP phase was precipitated in the alloy, demonstrating outstanding microstructural stability attributed to the Ru-induced inverse distribution effect and an increased solid solution threshold of the matrix. During the long-term aging process, the γ matrix channels broaden and secondary γ′ phases precipitate internally. Because elements Ru and Re inhibit the uphill diffusion process of the γ′-forming elements, the size of the secondary γ′ phase precipitated after aging the alloy at 1050 °C for 2000 h was only ~ 12 nm. Although there is no external stress, during the aging process, a heterogeneous dislocation network forms at the alloy interface to release the mismatch stress, comprising regular dislocation walls and disordered dislocation forests. Meanwhile, several dislocations cut into the γ′ phase to form super-dislocations. These findings provide new insights into the long-term stability and deformation mechanisms of advanced Ru-containing superalloys under service-like thermal exposure.