THE EFFECT OF ELASTIC-ANISOTROPY ON THE DIRECTION AND FACETING OF CHEMICALLY-INDUCED GRAIN-BOUNDARY MIGRATION IN AL2O3

The coherency strain energy of a Fe2O3 diffusion zone is calculated to be maximum at the (0001) surface of Al2O3 grain. An Al2O3 single crystal has been diffusion bonded at its (0001) surface with a polycrystalline Al2O3 piece to form (0001) grain boundaries. When this specimen is heat-treated at 1600-degrees-C in the presence of Fe2O3, all of the (0001) grain boundaries migrate to the direction of the single crystal, producing behind them zones enriched with Fe2O3. The migrating (0001) boundaries show faceting due to the appearance of boundary segments with different orientations moving more slowly than the initial (0001) segment. These results are consistent with the coherency strain theory for the initiation and continuation of the migration.