Micro-pillar plasticity controlled by dislocation nucleation at surfaces

A model of micro-pillar plasticity controlled by the nucleation of dislocations at free surfaces was developed using methods of dislocation dynamics innovated by Johnston and Gilman 50 years ago. It is shown that the stress dependence of the rate of nucleation can be related to the dependence of the flow strength on the diameter of the pillar. The steady state flow stress in this model depends only on the kinetics of nucleation, whereas the transient deformation behavior depends on the initial dislocation density and the dislocation mobility, as well as the rate of nucleation. The model can describe not only the dependence of the strength on pillar diameter, but also recent experiments on fcc gold crystals, wherein pre-straining leads to softening and annealing leads to hardening. The model does not account for the stochastic, jerky nature of plastic flow in micro-pillars and is not meant to apply to pillars large enough to support substructure formation.