Role of Elemental Sublimation during Solution Heat Treatment of Ni-Based Superalloys

The role of elemental evaporation on the microstructural stability of blade surfaces has been investigated on solutioned and aged samples of Ni-based single-crystal superalloys. Evaporation of Ni and Cr at the casting surface during solution heat treatment leads to the formation of a Ni- and Cr-depleted layer at the surface. Nucleation and growth of γ′ phase occur within this layer through subsequent long-range diffusion of Re, Ta, and W between the γ′ layer and the substrate. Beyond a critical Ni and Cr loss, incipient melting initiates at the surface and principally γ′ and TCP phases are stabilized with de-stabilization of γ phase. Nucleation of TCP phases occurs at grain boundaries arising from cellular recrystallization during the ramp-up cycle. Therefore, on quenching, a range of microstructures are observed at the casting surface.