Limits of hardness at the nanoscale: Molecular dynamics simulations

Contrary to the often reported findings from molecular dynamics computer simulation that metals soften as their grain sizes fall below 10-15 nm, we do not observe such softening in nanocrystalline specimens when they are first thermally relaxed. We offer a simple model that illustrates that the increased hardening is a consequence of grain-boundary relaxation, which suppresses grain-boundary sliding and forces the material to deform by dislocation glide. These observations provide an explanation for why some experiments observe an inverse Hall-Petch relationship at grain sizes below 10-20 nm while others do not.