Slip and twinning dislocations in sapphire (α-Al2O3)

The 1/3[10 (1) over bar 0] partial dislocation plays a crucial role in the plastic deformation of sapphire (alpha-Al2O3). During deformation at high temperatures, basal slip (1/3(11 (2) over bar 0)(0001)) has the lowest critical resolved shear stress; 1/3[11 (2) over bar 0] perfect dislocations undergo dissociation (which is probably restricted to the dislocation core) to 1/3[10 (1) over bar 0] and 1/3[01 (1) over bar 0] half-partial dislocations. These partials glide on an electrically neutral motion plane within a puckered cation array. The 1/3[10 (1) over bar 0] partial also acts as the twinning partial when basal twinning occurs at intermediate temperatures, say 600-1000 degrees 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 ([10 (1) over bar 0] {1 (2) over bar 10}) is actually the preferred slip system at lower temperatures (below about 600 degrees C). in spite of the very large Burgers vector of the [10 (1) over bar 0] dislocation, 0.822 nm. This occurs because this dislocation dissociates into three colinear 1/3[10 (1) over bar 0] partials, separated by two relatively low-energy slacking 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.