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Deformation twinning is an important deformation mode of nanocrystalline metals. In current study, we investigate the scratching-induced deformation twinning in nanocrystalline Cu by means of molecular dynamics simulations. The tribological behavior, the deformation mechanisms, the formation mechanism of deformation twins, and the grain size dependence of the propensity of deformation twinning are elucidated. Simulation results demonstrate that deformation twinning plays an important role in the plastic deformation of nanocrystalline Cu under nanoscratching, in addition to dislocation activity and grain boundary-associated mechanism. The nucleation of initial twinning partial dislocations originates from the dissociation of lattice partial dislocations that emit from grain boundary triple junctions, and subsequent twin boundary migration is resulted from the glide of lattice partial dislocations emitted from twin boundary-grain boundary intersections on the twin plane. It is found that the propensity of deformation twinning in nanocrystalline Cu under scratching has strong dependence on both grain size and stress state. These findings will advance our understanding of the tribological behavior of nanocrystalline Cu and provide design and fabrication guidelines for nanocrystalline Cu based micro/nanosystems.