Sample shape and temperature strongly influence the yield strength of metallic nanopillars

For bulk face-centered cubic metals in conventional tests, the effects of sample shape on the yield stress are negligibly small, and the strength dependence on temperature is very weak. Using molecular dynamics simulations, here we demonstrate marked differences in yield stress for Cu pillars with different cross-sectional geometries, or tested at different temperatures, when the sample dimensions are on the nanometer scale. The origin of this change in yield behavior from conventional bulk Cu is explained by analyzing dislocation emission from the surfaces and corners as the mechanism to mediate plasticity. Based on a systematic characterization of the surface structure, atomic-level stresses and strains, a local strain criterion is proposed for yielding in such nanoscale objects.