Grain coarsening in nanocrystalline copper with very small grain size during tensile deformation

Molecular dynamics (MD) simulation has been used to study tensile deformation of nanocrystalline copper with mean grain sizes from 2.1 to 11.5 nm. The flow stress, yield stress and Young's modulus are proportional to the reciprocal of the square root of grain size. They decrease as the grain size is reduced. Grain rotation and grain boundary migration are observed, which are two major causes of grain coarsening during the deformation. Notable grain coarsening is only observed at mean grain sizes below a critical grain size which is dependent on the strain rate. This coarsening is due to the extensive ordering of atoms in grain boundaries induced by grain boundary migration. Deformation twin forms much easily in very small grains.