Twin-size effects on the deformation of nanotwinned copper

The strength and ductility of nanolayered materials are determined by a delicate balance between dislocation and interface motion under applied mechanical loading. We present here studies of dislocation and interface motion in single crystal and nanotwinned copper, utilizing molecular dynamics simulations. Motion of twin boundary interfaces themselves is dictated by the stress state at the interface, and is found to be maximum for applied shear loads parallel to the boundary. The mechanism of twin boundary migration is shown to be a result of Shockley partial nucleation and growth at twin interfaces. The stress state at twin boundaries is found to play a significant role in determining the deformation mode. While deformation twinning is found to be the dominant mode under tensile loading, shear loading is found to favor twin boundary migration. The influence of the twin lamella thickness on the deformation behavior of nanotwinned Cu is determined under constant applied strain rate and constant applied stress, and the conditions for dislocation confinement within nanotwins are determined. The stacking fault density and the number of nucleated dislocations are compared for different size lamellae of twin structures. The present simulations reveal the origins of strengthening caused by nanotwins as the restriction of dissociated dislocation loop motion in narrow channels. A critical twin thickness for the maximum strength in twinned copper is found to be around 4 nm.