Dislocation mobility and crack tip plasticity at the ductile-brittle transition

Abstract

Recent ideas on the mechanisms controlling the initial stages of plastic flow at cracks in intrinsically brittle materials, at the ductile-brittle transition, are reviewed. New experiments on precracked specimens of Si are reported, which confirm earlier work that the ductile-brittle transition is controlled by dislocation velocity. A model has been developed in which at the ductile-brittle transition dislocations from a discrete number of sources at the crack tip are emitted sufficiently rapidly to shield the most vulnerable parts of the crack, furthest away from the sources, at a rate such that the local stress intensity factor remains below K1c for values of the applied stress intensity factor K above K1c. Computer modelling of the dynamics of dislocation generation from the crack tip, in mode III loading, suggests that a sharp transition is obtained if crack tip source activity begins at values of K close to K1c. It is suggested that crack tip sources can be formed when existing dislocations are attracted to the crack tip, where they transform into efficient shielding sources. The model explains the observed strain rate dependence of the transition temperature (Tc) on the activation energy controlling dislocation velocity, and predicts a dependence of Tc on dislocation density. The predictions of the model are in good agreement with observed values of Tc for Si. General conclusions are drawn about the factors which control yielding in precracked specimens of intrinsically brittle materials at the ductile-brittle transition