The crystallography of basal and rhombohedral annealing twins in polycrystalline alumina ceramics has been analysed by selected area electron diffraction, and secondary grain boundary dislocations (SGBDs) within those boundaries have been analysed. The twins arise from misorientations of the high coincidence fraction ∑, and the SGBDs have Burgers vectors from the displacement shift complete lattice (DSCL). The basal twin is unique to the rhombohedral system. A O lattice calculation is used to show that the rhomhohedral twin is similar to the ∑ = 7 〈210〉 boundary in hexagonal metals.
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Shiue et al. (1984) studied this question.
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