Quantum mechanical study of solid solution effects on dislocation nucleation during nanoindentation

A multiscale quasicontinuum density functional theory method is used to study the solid solution effect on dislocation nucleation during nanoindentation. Specifically, an Al thin film with Mg impurities is considered. We find that the solid solution effect depends sensitively on the local configuration of the impurities. Although a random distribution of the impurities increases the hardness of the material, linear distributions of the impurities actually lower the hardness. In both cases, the strengthening/ softening effects are due to dislocation nucleation. Consistent with recent experiments, the change of the ideal strength is found to be small. Different incipient plasticity behaviors are observed: in the pure material, two full dislocations are nucleated under the indentor. For a random distribution of impurities, two partial dislocations are formed instead. For linear distributions of impurities, only one partial dislocation is formed. Thus the nucleation of dislocations is sensitive to the local distribution of impurities in an alloy.