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

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Abstract
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.

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