Nanocrystalline gold with small size: inverse Hall-Petch between mixed regime and super-soft regime

Molecular dynamics simulations were used to study the atomic mechanisms of deformation of nanocrystalline gold with 2.65-18 nm in grain size to explore the inverse Hall-Petch effect. Based on the mechanical responses, particularly the flow stress and the elastic-to-plastic transition, one can delineate three regimes: mixed (10-18 nm, dislocation activities and grain boundary sliding), inverse Hall-Petch (5-10 nm, grain boundary sliding), and super-soft (below 5 nm). As the grain size decreases, more grain boundaries present in the nanocrystalline solids, which block dislocation activities and facilitate grain boundary sliding. The transition from dislocation activities to grain boundary sliding leads to strengthening-then-softening due to grain size reduction, shown by the flow stress. It was further found that, samples with large grain exhibit pronounced yield, with the stress overshoot decrease as the grain size decreases. Samples with grain sizes smaller than 5 nm exhibit elastic-perfect plastic deformation without any stress overshoot, leading to the super-soft regime. Our simulations show that, during deformation, smaller grains rotate more and grow in size, while larger grains rotate less and shrink in size.