High-temperature and low-stress tertiary creep and the transition from rafting to topological inversion in single-crystal Ni-base superalloys

Abstract When negative misfit single-crystal Ni-base superalloys are subjected to high-temperature and low-stress <001> tensile creep, small coherent γ′-cuboids separated by an interconnected γ-channel network directionally grow into rafts, perpendicular to the direction of the applied stress. This first stage of microstructural evolution is followed by a second stage, where the rafted microstructure evolves such that the γ′-phase becomes the continuous phase and isolated γ-islands form. One objective of the present work is to study the elementary microstructural processes which characterize the transition from a rafted to a topologically inverted microstructure. The other objective is to explore the role of topological inversion during tertiary creep. It is shown that as the microstructure evolves, the total γ/γ′-interface area decreases. This reduces the area covered by dislocation networks, which form at γ/γ′-interfaces to shield the misfit. In a scenario where the climb processes associated with misfit-mediated coupled knitting-in/out reactions between γ- and γ′-dislocations and these dislocation networks are rate controlling (Parsa et al. in Acta Mater 264:119576, 2024), the decrease of γ/γ′-interface area, as the microstructure evolves towards topological inversion, contributes to the increase of creep rate during tertiary creep.