It was found that, when an alumina polycrystal was heat-treated with Al2O3-Cr2O3 powder mixtures at 1400 or 1500C, the grain boundaries migrated and left a Cr2O3-enriched solid solution behind them. The migration rate increased with heat-treatment temperature and with the Cr2O3 content of the mixture. The driving force for migration was suggested to arise from coherency strains in the Cr2O3 diffusion zone ahead of the migrating grain boundaries. The latter developed strong faceting, with flat segments meeting at sharp edges. Because of the elastic anisotropy of alumina, the diffusional coherency strain energy, G, varied as a function of the surface orientation of the retreating grain. The variation was smooth at a minimum in the polar plot of G. Therefore, smoothly curved boundaries which met at sharp edges were predicted for the growth shape; contrary to the observed flat facet boundaries. It was suggested that the grain boundary mobility could vary with the grain surface orientation, and therefore influenced the growth shape.

S.C.Han, D.Y.Yoon, M.K.Brun: Acta Metallurgica et Materialia, 1995, 43[3], 977-84