The origin and stability of nanostructural hierarchy in crystalline solids

The structural hierarchy exhibited by materials on more than one length scale can play a major part in determining bulk material properties. Understanding the hierarchical structure can lead to new materials with physical properties tailored for specific applications. We have used a combined experimental and phase-field modeling approach to explore such a hierarchical structure at nanoscale for enhanced coarsening resistance of ordered γ′ precipitates in an experimental, multicomponent, high-refractory nickel-base superalloy. The hierarchical microstructure formed experimentally in this alloy is composed of a γ matrix with γ′ precipitates that contain embedded, spherical γ precipitates, which do not directionally coarsen during high-temperature annealing but do delay coarsening of the larger γ′ precipitates. Chemical mapping via atom probe tomography suggests that the supersaturation of Co, Ru, and Re in the γ′ phase is the driving force for the phase separation, leading to the formation of this hierarchical microstructure. Representative phase-field modeling highlights the importance of larger γ′ precipitates to promote stability of the embedded γ phase and to delay coarsening of the encompassing γ′ precipitates. Our results suggest that the hierarchical material design has the potential to influence the high-temperature stability of precipitate strengthened metallic materials.