Sapphire basal twinning

M.L. Kronberg, Plastic deformation of single crystals of sapphire Basal slip and twinning, Aeta Met. 5, 507-529 (1957). 

Among the oxides described in this chapter, deformation twinning is most important in the case of sapphire. Two twinning systems are known, on the rhombohedral and basal planes, and dislocation models for each have been suggested and confirmed using TEM. 

The availability of sizable single crystals has led to a significant literature on the deformation of sapphire of various orientations, and at various temperatures. As already noted, the first such study was by Wachtman and Maxwell in 1954 [6], who activated (0001) 1/3 (1120) basal slip at 900 °C via creep deformation. Since that time, it has become clear that basal slip is the preferred slip system at high temperatures, but that prism plane slip, 1120 (1100), can also be activated and becomes the preferred slip system at temperatures below 600°C. Additional slip systems, say on the pyramidal plane 1012 1/3 (1011), have very high CRSSs and are thus difficult to activate. Both, basal and rhombohedral deformation twinning systems, are also important in AI2O3 (these are discussed later in the chapter). 

---