Probing the formation mechanism of hierarchical microstructures in experimental Ni-based superalloys

Hierarchical microstructures, consisting of a γ matrix with γ′ precipitates that incorporate finer-scale embedded γ precipitate, are known to enhance strength and enhanced coarsening resistance of the ordered γ′ precipitates in various nickel-based superalloys. Despite their potential benefits, the underlying origins and design principles of such hierarchical microstructures remain poorly understood due to the complex multicomponent nature of superalloys. In this study, we employ an integrated approach that combines advanced composition and microstructure analysis and computational thermodynamic and kinetic modeling to investigated the formation mechanism of the hierarchical microstructure in two experimental multi-component nickel-base superalloys. Using chemical mapping via atom probe tomograph and nucleation driving force calculations, we find that the enrichment of γ-stabilizing elements, such as Co and Cr, along with the depletion of Al within the γ′ precipitates, promotes the decomposition of the γ′ phase and the precipitation of the finer γ phase. In addition, kinetic modeling predicts the supersaturation of Co and Cr during cooling, elucidating the influence of heat treatment on solute partitioning. Our findings illustrate how alloy composition and tailored heat treatment strategies discussed can guide the design of a hierarchical γ/γ’/γ microstructure, offering a pathway towards optimizing mechanical properties in nickel superalloys through microstructure design.