SOFTENING OF NANOCRYSTALLINE METALS AT VERY SMALL-GRAIN SIZES

Nanocrystalline solids, in which the grain size is in the nanometre ra nge, often have technologically interesting properties such as increas ed hardness and ductility. Nanocrystalline metals can be produced in s everal ways, among the most common of which are high-pressure compacti on of nanometre-sized clusters and high-energy ball-milling(1-4). The result is a polycrystalline metal with the grains randomly orientated. The hardness and yield stress of the material typically increase with decreasing grain size, a phenomenon known as the Hall-Fetch effect(5, 6). Here we present computer simulations of the deformation of nanocry stalline copper, which show a softening with grain size (a reverse Hal l-Petch effect(3,7)) for the smallest sizes. Most of the plastic defor mation is due to a large number of small 'sliding' events of atomic pl anes at the grain boundaries, with only a minor part being caused by d islocation activity in the grains; the softening that we see at small grain sizes is therefore due to the larger fraction of atoms at grain boundaries. This softening-will ultimately impose a limit on how stron g nanocrystalline metals may become.