Influence of cooling rate on the formation of bimodal microstructures in nickel-base superalloys during continuous two-step aging

The formation and evolution of bimodal microstructures during the continuous two-step aging in nickel-base single crystal superalloys are simulated through phase-field method. The change of γ′/γ lattice misfit from the first to the secondary aging temperature has been considered in present model. The focus of this study is to explore the influence of cooling rate (after the first aging) on the evolutions of primary and secondary γ′ precipitates. It is demonstrated that the primary γ′ precipitates formed in the first aging become more cubic during the secondary aging. With the cooling rate increasing, the precipitation number and the growth rate of secondary γ′ precipitate are increased during the followed secondary aging. And the size of secondary γ′ precipitate is also the biggest for the case after the high-speed cooling based on the contributions of the greatest chemical driving force and massive coalescences of neighboring secondary γ′ precipitates. Besides, the precipitation of secondary γ′ phase from the γ-channel occurs only when the γ-channel width is greater than a critical value, and in present work, the critical value is about 35 nm, 52.5 nm and 87.5 nm, respectively, for the case after high-speed, medium and low-speed cooling.