Migration of grain boundaries in alumina induced by chromia addition

Abstract

When alumina polycrystal is heat-treated at 1400 and 1500°C 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 raise 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 vary with the grain surface orientation and therefore influence the growth shape.