Deformation Twinning in Sapphire

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. 

Rhombohedral Twinning Rhombohedral twinning is the deformation mode with the lowest CRSS in sapphire [105], and can occur at temperatures as low as —196 °C [106]. 

Fault vector Fault plane Fault energy ( m Method 

The twin law is as follows Ki = 0112 Tij = (0111) K2 = 0114 TI2 = (0221) and s = 0.202 [106]. The dislocation model suggested for rhombohedral twinning [107] is based on the double cross-sUp mechanism proposed by Pirouz [108] for silicon (see also Lagerlof et al. [109]). Briefly, twin nucleation involves a pinned zonal dislocation undergoing the following dissociation  

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