The brittle-to-ductile transition - I: A cooperative dislocation generation instability

Abstract

Most materials (with the possible exception of f.c.c. metals) exhibit the well-known brittle-to-ductile transition (BDT) with increase in temperature. The variation of the fracture toughness, (K), with temperature is usually gradual in the brittle regime, but a dramatic rapid increase occurs over a narrow temperature range where the sharp crossover from brittle to ductile behavior takes place. The authors describe a new model that explains the dual characteristics of the BDT, namely, the massive dislocation generation at the transition temperature and the strain-rate dependence of this behavior. In the first part, the authors present a statistical mechanics based two-dimensional (2D) model of spontaneous dislocation generation leading to the BDT. The onset of ductile behavior corresponds to a thermally-induced cooperative instability of many small dislocation dipoles in the presence of an applied stress.