MODELING OF DISLOCATION MOBILITY CONTROLLED BRITTLE-TO-DUCTILE TRANSITION

The phenomenon of brittle-to-ductile transition (BDT) is known to be c ontrolled by the competition between cleavage fracture and dislocation activity at crack tips, but the transition could be determined by one of the two successive processes, dislocation nucleation or dislocatio n motion. In this paper a model is developed to study the BDT controll ed by dislocation mobility. The model material is assumed to undergo e lastic rate-dependent plastic deformation, with the plastic strain rat e scaled with dislocation velocity. An isotropic plasticity theory is used. The BDT is assumed to occur when the crack tip is shielded by th e surrounding plastic zone such that it never reaches the critical str ess intensity for cleavage fracture. The crack tip shielding due to pl astic deformation is evaluated using the finite element method. The di mensionless groups that affect the BDT are identified, and a parametri c study is performed to reveal the effects of various dimensionless pa rameters. Numerical results using the specific material properties of Si single crystals are compared with experimental data. Good agreement s are obtained. Some interesting features of the BDT behaviour are pre dicted by our computer simulations which require further confirmation by experimental studies.