Deformation and Failure Processes Operating in Ultra-Fine Grain Metals

Abstract

Nanograined metallic materials typically have higher yield and tensile strength, lower stiffness and better resistance to environmental degradation and wear than their large-grained analogs. We currently lack an understanding of how the fundamental deformation processes change as a function of grain size and therefore we interpret the macroscopic response by scaling-down the processes that operate in large-grained systems. The disparity between model predictions and experiment suggest we have yet to include the necessary physical processes. Molecular dynamics computer simulations of deformation and failure processes in nanograined materials have been used extensively to determine the deformation processes. It has been suggested that for high stacking-fault energy materials such as Al, the deformation mode changes from perfect to partial dislocation processes at a grain size of about 18 nm and to grain boundary mediated processes at smaller grain sizes. A similar transition occurs in low stacking-fault energy materials except no perfect dislocations are involved in the deformation mechanism. These results have been summarized in the form of a deformation map[1]. Farkas et al.[2], using MD simulations, have shown that both intergranular and transgranular fracture can occur and that the fracture process is accompanied by dislocations and grain boundary mechanisms. Intergranular fracture occurs through the linkage of nanocracks that form at triple junctions. They also show that grain boundaries perpendicular to the crack propagation direction can arrest the crack. These deformation and failure processes remain to be validated experimentally.