Slip and Twinning in Sapphire (a-AI,O,)

The plastic deformation of sapphire (a-AI,O,) has been studied under hydrostatic confining pressure at tempera-tures below the ambient pressure brittle-to-ductile transi-tion temperature. Samples oriented for prism plane slip (Type I samples) were deformed via dislocation slip at tem-peratures as low as 200°C. Samples oriented for basal slip (Type I1 samples) could be plastically deformed at tempera-tures as low as 400°C but showed more complicated defor-mation behavior, inasmuch as the sample orientation also allowed for the activation of basal twinning and two of the three rhombohedral twin systems. The temperature depen-dence of the critical resolved shear stress ( I ~ ~ J, InT,, = lnT0- B- T for basal slip was significantly greater than that for prism plane slip (Bbasal>BpriPmplane), causing the latter system to b