Quantitative description of plastic deformation in nanocrystalline Cu: Dislocation glide versus grain boundary sliding

Uniaxial plastic deformation of polycrystalline Cu with grain sizes in the range of 5-20 nm was studied by using molecular dynamics computer simulations. We developed a quantitative analysis of plasticity by using localized slip vectors to separate the contributions of dislocation activity from grain boundary sliding. We conclude that the competition between these two mechanisms depends on strain rate and grain size, with the dislocation activity increasing with grain size but decreasing with increasing strain rate. For samples with a 5 nm grain size, dislocations contribute ≈ 50% of the total plastic strain during steady state deformation at a rate of 1× 108 s-1, but this fraction decreases to 35% at a rate of 1× 1010 s-1. When the grain size is increased to 20 nm, dislocations account for 90% of the strain, even at 1× 1010 s-1. During the initial stages of plastic deformation, grain boundary sliding initially decreases with strain owing to strain-induced relaxation processes within the grain boundaries. The grains also rotate a few degrees during straining to 20%; the rate of rotation (per unit strain) slightly decreases with strain rate. Lastly, we computed the amount of forced atomic mixing during plastic deformation. The mean square separation distance between atom pairs within grain interiors increases with strain at a rate proportional to their distance apart (i.e., the mixing is superdiffusive), but for pair separations greater than the grain size, this rate becomes independent of the separation distance.