INTERFACE-CONTROLLED PHASE-TRANSFORMATION AND ABNORMAL GRAIN-GROWTH OF ALPHA-AL2O3 IN THIN GAMMA-ALUMINA FILMS

Abnormal grain growth of alpha-Al2O3 took place in a gamma-Al2O3 matrix at 1200-degrees-C; anomalously large alpha-Al2O3 grains were observed to grow within a fine-grained, polycrystalline gamma-Al2O3 matrix. The grain sizes of these alpha-Al2O3 grains varied from 3 to 20 mum, while the nominal grain size of gamma-Al2O3 was about 50 nm. These large alpha-Al2O3 grains were predominantly single crystals with [0001] orientations, although some grains were noted to consist of a few subgrains. On the other hand, the polycrystalline gamma-Al2O3 matrix was strongly textured with [001] preferred orientations. The gamma --> alpha phase transformation accompanied by the abnormal grain growth of alpha-Al2O3 is analyzed in light of a gamma-alpha interphase boundary migration mechanism. Orientation relationships between gamma- and alpha-Al2O3 were determined and are presented. A phenomenological equation is formulated to describe the driving force for the interface-controlled abnormal grain growth of alpha-Al2O3. Qualitative calculations demonstrate that the driving force for the abnormal grain growth is primarily attributed to the free energy change as a result of gamma --> alpha transformation.