Fundamental differences in the plasticity of periodically twinned nanowires in Au, Ag, Al, Cu, Pb and Ni

The role played by nanoscale twins is becoming increasingly important in order to understand plasticity in nanowires synthesized from metals. In this paper, molecular dynamics simulations were performed to investigate the synergistic effects of stacking fault energy and twin boundary on the plasticity of a periodically twinned face-centered cubic (fcc) metal nanowire subjected to tensile deformation. Circular nanowires containing parallel (1 1 1) coherent twin boundaries (CTBs) with constant twin boundary spacing were simulated in Au, Ag, Al, Cu, Pb and Ni using different embedded-atom-method interatomic potentials. The simulations revealed a fundamental transition of plasticity in twinned metal nanowires from sharp yield and strain-softening to significant strain-hardening as the stacking fault energy of the metal decreases. This effect is shown to result from the relative change, as a function of the unstable stacking fault energy, between the stress required to nucleate new dislocations from the free surface and that to overcome the resistance of CTBs to the glide of partial dislocations. The relevance of our predictions to realistic nanowires in terms of microstructure, geometry and accuracy in predicting the generalized planar and stacking fault energy curves is also addressed. Our findings show clear evidence that the plastic flow of twinned nanowires under tension differs markedly between fcc metals, which may reconcile some conflicting observations made in the past.