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 degrees C. Plastic deformation of sapphire occurs via basal and prismatic slip during loading at high temperature (above 1030-1100 degrees C, depending on orientation), The BDT temperature, T-c, varies with strain rate and crystallographic orientation of the fracture plane, The activation energy of the process controlling the BDT in sapphire, derived from the strain rate variation of T-c, is approximately 3.2 eV, close to that for dislocation glide, In one specimen orientation, ''warm-prestressing'' increases the room-temperature fracture stress.