The formation of α-Al₂O₃from θ-Al₂O₃: The relevance of a "critical size" and: Diffusional nucleation or "synchro-shear"?

The coarsening of θ-Al₂O₃ crystals to a 'critical size' is often interpreted as the first step in the shear nucleation of α-Al₂O₃. The existence of this so-called critical size has also been used to explain the observation that α-Al₂O₃ "nuclei" are generally twice as large as the crystals in the θ-Al₂O₃ matrix. This paper discusses the important issues in the nucleation of α-Al₂O₃ from θ-Al₂O₃. A few key experiments are also presented to clarify the nucleation process. It is concluded that a critical θ-Al₂O₃ crystal size is not a prerequisite for α-Al₂O₃ nucleation, but is primarily a result of the incubation time required to produce α-Al₂O₃ nuclei by diffusional nucleation. It is proposed that the large observed α-Al₂O₃ crystal size also does not result from a shear nucleation event in a 'critical size' θ-Al₂O₃ crystal, but is due to the intrinsically low α-Al₂O₃ nucleation density, together with rapid growth of α-Al₂O₃ after nucleation.