High-resolution transmission electron microscopy was used to study the structure of annealing twins in -phase material. A basal twin was found where each of the 2 twin boundaries with the matrix had a different 2-dimensional structure. The energies of alternative twin boundary structures were calculated by using an ionic model. It was found that the relative differences in the energies of the boundary structures were appreciable. The relative energies of steps at the boundary, which were required for twin formation and termination, exhibited large differences for opposite growth directions in the basal plane. It was suggested that the differing twin boundary structures were the result of the anisotropic growth of alumina. This was related to the lack of centro-symmetry in the trigonal corundum structure. The results were considered to be important for the understanding of annealing twin formation and growth mechanisms in sapphire, and could be extended in order to predict anisotropic growth phenomena in other non-centrosymmetrical structures.

W.D.Kaplan, P.R.Kenway, D.G.Brandon: Acta Metallurgica et Materialia, 1995, 43[2], 835-48