DISLOCATION DYNAMICS AND BRITTLE-TO-DUCTILE TRANSITIONS

The validity of different types of models for crack-dislocation interactions and their effect on fracture is discussed. The brittle-to-ductile transition (BDT) in precracked silicon and sapphire can be described by a two-stage dynamic model based on dislocation motion to and around the crack tip. In silicon and germanium, where the stress-temperature-velocity relations for the test materials are known, the model allows accurate prediction of the transition temperature. Computer simulations of the dynamic development of near-crack tip dislocation arrays then allows prediction of the shape of the BDT given the pre-existing dislocation arrangements in the crystal; we have performed experiments to confirm these predictions. In sapphire, where these data are not available, the model allowed prediction of dislocation velocities from fracture tests. These predictions were then checked by measuring dislocation velocities and were found to be a close fit to experiment. We have also studied the brittle-ductile transition in semibrittle materials (b.c.c. metals and oxides), where pre-existing dislocations appear to control the transition behaviour via slow, stable crack growth. In molybdenum the stable crack growth is relatively small in extent and our model predicts the fracture stress-temperature curve to good accuracy.