In alumina-anorthite liquid systems, grains have polyhedral shape with atomically flat interfaces. Because of their flat interfaces, it is predicted that they grows by nucleating steps in their interfaces. When grain growth occurs by 2-D nucleation and defect-associated process, the relationship between the growth rate v and the driving force F can be non-linear, and it can lead the grain growth behavior to be abnormal grain growth. In order to determine the v(F) relationship, we observed the migration of the basal planes of a sapphire single crystal in alumina-15wt% anorthite, and determined the driving force for the migration of the basal planes by measuring sizes of matrix grains. In an early stage of the coarsening of grains, the driving force was high, and the migration rate of basal planes varied linearly as the driving force for the growth. As the driving force decreased, however, the relationship between the growth rate and the driving force of a single crystal became non-linear, and the migration of the basal planes depended on their dislocation densities. This kind of behavior proves that the basal planes of a sapphire single crystal migrate by step growth mechanism and abnormal grain growth occurs owing to step growth mechanism.
