Probing Mechanical Properties of Nanostructured Materials via Large Scale Molecular Dynamics Simulations

Abstract

The rapid development of synthesis and characterization of nanostructured materials as well as unprecedented computational power have brought forth a new era of materials research in which experiments, simulation and modeling are performed side by side to probe the unique mechanical properties of nanoscale materials [1,2]. We have recently reported a dislocation source controlled mechanism in the newly-developed nano-twinned metals in which there are plenty of dislocation nucleation sites while dislocation motion is not confined. We show that dislocation nucleation plays the governing role in the strength of such materials, resulting in their softening below a critical twin thickness. Large-scale, fully 3-D, molecular dynamics simulations and a kinetic theory of dislocation nucleation in nano-twinned metals show that there exists a transition in deformation mechanism, which occurs at a critical twin boundary spacing where the strength is maximized, from the classical Hall-Petch type of strengthening due to dislocation pile-up and cutting through twin planes to a dislocation nucleation governed softening mechanism with nucleation and motion of partial dislocations parallel to the twin planes (twin boundary migration). The simulations indicate that the critical twin boundary spacing for the onset of softening in nano-twinned Cu and the maximum strength depend on the grain size: the smaller the grain size, the smaller the critical twin spacing, and the higher the maximal strength of the material.