Nucleation and Concurrent Anomalous Grain Growth of α‐Al₂O₃ During γ ‐ α Phase Transformation

Nanocrystalline Al₂0₃ thin films (50 nm in thickness) have been synthesized by rf reactive sputtering deposition and subjected to annealing at temperatures ranging from 800° to 1200°C. TEM analysis indicated that the as‐deposited alumina films contained both amorphous phase and metastable γ phase. Structural texture evolved in the films annealed at 800°C for 24 h; the texture had a [00l] preferred orientation and occurred along the {400} and {440} planes of γ Al₂0₃, . In the films annealed a t 1200°C for 2h, nucleation and concurrent anomalous grain growth of α‐Al₂0₃took place in a fine‐grained, polycrystalline γ‐Al₂0₃ matrix. The anomalously grown γ‐AL₂O₃ grains were primarily [0001]‐oriented single crystals with grain sizes varying from 3 to 15 pm, while the γ‐Al₂O₃ matrix had an average grain size of 50 nm. The γ‐Al₂O₃ matrix was also strongly textured along the [001] axis and exhibited a heavily faulted, layered micro‐structure. Most of these layers were oriented along the {220} crystallographic planes. Periodic superstructure was identified in the layered γ‐ Al₂O₃. The formation of layered structure in γ‐ 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 γ‐ Al₂O₃. The nucleation of α‐ Al₂O₃ appears to occur along the {220} crystallographic planes of γ‐ Al₂O₃. The explosive grain growth of α‐ Al₂O₃ during the γαphase transformation is explained by a mechanism involving interface boundary migration and lattice epitaxy. The orientation relationships between γ‐ and α‐ Al₂O₃ are determined. Copyright © 1991, Wiley Blackwell. All rights reserved