An investigation of deformation mechanisms for grain size strengthening using molecular dynamics

Abstract Molecular dynamics simulations of compression tests were performed for aluminum considering different polycrystalline systems and different average grain sizes. The simulations spanned a grain size range between 5 and 100 nm and the mean flow stress for each sample was determined. The simulations show there is a transition from grain refinement hardening for grain sizes larger than ~ 20 nm to grain refinement softening for smaller grain sizes. The grain size for the maximum flow stress depends on the testing strain rate. The results allowed an estimation of the grain refinement hardening coefficient, equivalent to the Hall–Petch coefficient, and the results are in good agreement with predictions from the model of conventional grain boundary sliding. The stress state in each atom was tracked and used to calculate the local effective stress distribution. It is shown that stress concentrations up to 5 times the applied external stress can develop near grain boundaries and these high stresses are associated with the homogeneous nucleation of dislocations. These dislocations glide across the grains and are absorbed at the opposite grain boundaries without the formation of any dislocation substructures within the grain interiors.