The effect of rhenium on the microstructure stability and γ/γ′ interfacial characteristics of Ni-based single crystal superalloys during long-term aging

The effect of 2 wt% Re addition on the microstructure stability and γ/γ′ interfacial characteristics of designed Ni-based single crystal (Ni-SX) superalloys during long-term aging up to 2000 h under 900 °C and 1000 °C was investigated. Re addition significantly affected the γ′ precipitate morphology, γ/γ′ lattice misfit and microstructural stability during long-term aging. Results showed that Re addition significantly enhanced the γ′ precipitates stability: higher volume fraction and smaller coarsening rate. Adding Re delayed the dissolution of the γ′ precipitates at high temperature, suppressed the variation of composition in the γ matrix and attenuated temperature sensitivity of the lattice misfit. Thus, this further reduced the interfacial energy of thermodynamic driving force for coarsening. Through the comparison with available commercial alloys, the Re-containing Ni-SX superalloys possessed minimizing lattice misfit and interfacial energy. More importantly, as the slowest alloying element in terms of diffusion coefficient, Re reduced the effective diffusion coefficient of the alloy, effectively hindering the matrix-diffusion-controlled coarsening of γ′ precipitates. Finally, the γ′ morphological evolution during aging process was discussed, the bifurcation point of the morphological transitions, spheroid-to-cuboid transition and cuboid-to-rod-like transition, were determined based on dimensionless parameter L and shape parameter η.