Brittle–Ductile Transition and Dislocation Mobility in Sapphire

The brittle‐to‐ductile transition (BDT) of Precracked sapphire in four‐point bending was studied as a function of orientation and strain rate from room temperature to 1500°C. Plastic deformatiom of sapphire occurs via basal and prismatic slip during loading at high temperature (above 1030–1100°C, depending on orientation). The BDT temperature, T c , varies with strain rate and crystallographic orientation of the fracture plane. The activation, derived from the strain rate variation of T c , is approximately energy of the process controllingh the BDT in sapphire, derived from the strain rate variation of T c is sapproximately 3.2 eV, close to that for dislocation glide. In one specimen orientation, “warm‐prestressing” increases the room‐temperature fracture stress.