When alumina polycrystal is heat-treated at 1400 and 1500℃ with $Al_2O_3-Cr_2O_3$ powder mixtures, the grain boundaries migrate, forming behind them a solid solution enriched with $Cr_2O_3$. The migration rate increases with the heat-treatment temperature and the $Cr_2O_3$ content in the mixture. The driving force for the migration is believed to arise from the coherency strain in the $Cr_2O_3$ diffusion zone in front of the migrating grain boundaries. The migrating grain boundaries develop strong faceting with flat segments meeting at sharp edges. Because of the elastic anisotropy of alumina, the diffusional coherency strain energy $G_c(n)$ varies with the surface orientation n of the retreating grain, but the variation is smooth at the minimum in the polar plot of $G_c(n)$. Therefore, smoothly curved boundaries meeting at sharp edges are predicted for the growth shape contrary to the observed flat facet boundaries. It is suggested that the grain boundary mobility may also vary with the grain surface orientation and therefore influence the growth shape.
When pure silver is heat-treated at 800℃ in $N_2$ or $O_2$ after heat-treating at 800℃ for 48 h in $N_2$ and water-quenching, the grain boundaries migrate. On slow-cooling and slow-heating in $N_2$, the grain boundaries don't migrate. Therefore, the driving force the migration is believed to arise from the residual stress of the surface which is generated by the water quenching. The boundary migration is observed in the specimen which is heat-treated at 520℃ for 1 h in $O_2$ after slow-cooling and slow-heating in $N_2$. Its driving force is believed to be the difference of the surface energy change between the neighboring grains when the atmosphere is change from $N_2$ to $O_2$. It can be explained that the quantity of the surface energy change on changing the atmosphere is sufficient to drive the boundary migration.
