Atomistic simulations of diffusion in γ′ -strengthened Co-based superalloys and its connection to selective alumina formation in early-stage oxidation

Abstract

The suppressed diffusion of Al in ${\ensuremath{\gamma}}^{\ensuremath{'}}$ precipitates of Co-based superalloys is suspected to be one of the reasons for the selective formation of alumina during the early-stage oxidation above $900{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. In order to scrutinize this assumption, the diffusion properties of Al, Co, and W in a prototype structure of the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ phase, $\mathrm{L}{1}_{2}\ensuremath{-}{\mathrm{Co}}_{3}(\mathrm{Al},\mathrm{W})$, are investigated in a combined density functional theory (DFT) and kinetic Monte Carlo (KMC) study. A particular challenge in simulating diffusion in this ordered phase is the representation and evaluation of numerous migration pathways on the $\mathrm{L}{1}_{2}$ sublattices. With specified diffusion models for the $\mathrm{L}{1}_{2}$ structure and random solid solution ($\ensuremath{\gamma}$ phase), our KMC results reveal that Al diffusivity in the $\ensuremath{\gamma}$ phase with the same stoichiometric composition is more than two orders of magnitude higher than in the $\mathrm{L}{1}_{2}$ ordered ${\ensuremath{\gamma}}^{\ensuremath{'}}$ phase at $900{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Furthermore, in our rapid thermal exposure experiments for a model Co-based superalloy, the formation of alumina nuclei is observed preferentially in the $\ensuremath{\gamma}$ channels while the Al content is comparable in both channels and precipitates before oxidation. Combining these insights from experiment and simulation, the sluggish diffusion of Al in ${\ensuremath{\gamma}}^{\ensuremath{'}}$ appears to be a key factor in the selective formation of alumina observed in these systems.