Interface-controlled phase transformation and abnormal grain growth of α-Al 2O 3 in thin γ-alumina films

Abstract

Abnormal grain growth of α-Al 2O 3 took place in a γ-Al 2O 3 matrix at 1200°C; anomalously large α-Al 2O 3 grains were observed to grow within a fine-grained, polycrystalline γ-Al 2O 3 matrix. The grain sizes of these γ-Al 2O 3 grains varied from 3 to 20 μm, while the nominal grain size of γ-Al 2O 3 was about 50 nm. These large α-Al 2O 3 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 γ-Al 2O 3 matrix was strongly textured with 〈001〉 preferred orientations. The γ → α phase transformation accompanied by the abnormal grain growth of α-Al 2O 3 is analyzed in light of a γ- α interphase boundary migration mechanism. Orientation relationships between γ- and α-Al 2O 3 were determined and are presented. A phenomenological equation is formulated to describe the driving force for the interface-controlled abnormal grain growth of α-Al 2O 3. Qualitative calculations demonstrate that the driving force for the abnormal grain growth is primarily attributed to the free energy change as a result of γ → α transformation.