Diffusion-induced grain-boundary migration during internal reduction of chromium-doped Al2O3

Diffusion-induced grain-boundary migration is observed during internal reduction of chromium-doped Al2O3. Chemical diffusion of oxygen along grain boundaries is fast compared with volume diffusion; therefore the internal reduction front penetrates from the grain boundaries into the grains, leading to chromium precipitation and formation of a thin, coherently strained chromium-depleted oxide layer. Migration of the grain boundary to either side of the boundary occurs, while the global energy in the overswept area decreases owing to oxygen potential relaxation, precipitate coarsening and elastic strain relaxation. The role of multiple factors, namely the chemical composition of the starting mixed oxide, chemical composition inhomogeneities, crystallographic orientation of adjacent grains and of the boundary plane, kinetics and mechanism of the internal reduction itself, on the direction and velocity of boundary migration is experimentally studied and interpreted.