NUCLEATION AND CONCURRENT ANOMALOUS GRAIN-GROWTH OF ALPHA-AL2O3 DURING GAMMA-]ALPHA PHASE-TRANSFORMATION

Nanocrystalline Al2O3 thin films (50 nm in thickness) have been synthesized by rf reactive sputtering deposition and subjected to annealing at temperatures ranging from 800-degrees to 1200-degrees-C. TEM analysis indicated that the as-deposited alumina films contained both amorphous phase and metastable gamma-phase. Structural texture evolved in the films annealed at 800-degrees-C for 24 h; the texture had a [001] preferred orientation and occurred along the {400} and {400} planes of gamma-Al2O3. In the films annealed at 1200-degrees-C for 2 h, nucleation and concurrent anomalous grain growth of alpha-Al2O3 took place in a fine-grained, polycrystalline gamma-Al2O3 matrix. The anomalously grown alpha-Al2O3 grains were primarily [0001]-oriented single crystals with grain sizes varying from 3 to 15-mu-m, while the gamma-Al2O3 matrix had an average grain size of 50 nm. The gamma-Al2O3 matrix was also strongly textured along the [001] axis and exhibited a heavily faulted, layered microstructure. Most of these layers were oriented along the {220} crystallographic planes. Periodic superstructure was identified in the layered gamma-Al2O3. The formation of layered structure in gamma-Al2O3 is attributable to the change of stacking sequence of atomic layers along the <220> orientations. An atomic model is presented to explain the formation of layered structure in gamma-Al2O3. The nucleation of alpha-Al2O3 appears to occur along the {220} crystallographic planes of gamma-Al2O3. The explosive grain growth of alpha-Al2O3 during the gamma --> alpha phase transformation is explained by a mechanism involving interface boundary migration and lattice epitaxy. The orientation relationships between gamma- and alpha-Al2O3 are determined.