Slip and twinning dislocations in sapphire (α-Al2O3)

The 1/3 (1010) partial dislocation plays a crucial role in the plastic deformation of sapphire (α-Al2O3). During deformation at high temperatures, basal slip (1/3(1120) (0001)) has the lowest critical resolved shear stress; 1/3 (1120) perfect dislocations undergo dissociation (which is probably restricted to the dislocation core) to 1/3(1010) and 1/3 (0110) half-partial dislocations. These partials glide on an electrically neutral motion plane within a puckered cation array. The 1/3(1010) partial also acts as the twinning partial when basal twinning occurs at intermediate temperatures, say 600–1000°C. Twinning occurs when a pinned screw partial sweeps out on this same motion plane, forms a complete loop of a microtwin and then cross-slips onto the next available motion plane to permit twin thickening. New transmission electron microscopy evidence is presented, confirming several predictions of this new model of basal twinning. Prism plane slip ((1010) {1210}) is actually the preferred slip system at lower temperatures (below about 600°C), in spite of the very large Burgers vector of the (1010) dislocation, 0.822 nm. This occurs because this dislocation dissociates into three colinear 1/3(1010) partials, separated by two relatively low-energy stacking faults. (The stacking-fault energy in sapphire is much lower on prism planes than on basal planes.) The motion plane for prism plane slip is between two puckered oxygen layers but also permits dislocation motion with no net change.