Several different dislocation models were proposed to explain basal and rhombohedral twinning in sapphire (α-Al2O3). To provide evidence for these models, sapphire samples deformed under hydrostatic confining pressures and containing both basal and rhombohedral twins were characterized by using conventional transmission electron microscopy techniques. It was found that perfect prism dislocations with a [10•0] Burgers vector had dissociated into three 1/3[10•0] partials in such a way that these dislocations acted as twinning partials for a basal twin and that all three partials lay in the twin boundary. For the case of rhombohedral twinning, a twin was found which had one flat boundary and one boundary that showed wavy curvature. These transmission electron microscopic observations were only consistent with the type of double cross-slip twinning mechanism first proposed for Si.

Considerations of the Double Cross-Slip Mechanism for Basal and Rhombohedral Twinning in Sapphire (α-Al2O3). K.P.D.Lagerlöf, J.Castaing, A.H.Heuer: Philosophical Magazine A, 2002, 82[15], 2855-67