Size-dependent dislocation-twin interactions

Dislocation-twin interactions critically control the plastic deformation and ultrahigh strength of nanotwinned metals. Here, we report a strong twin-thickness dependence of dislocation-twin interaction mechanisms from the tensile deformation of face-centered cubic metallic nanocrystals by in situ nanomechanical testing. Direct observations at atomic scale reveal that the predominant dislocation-twin interaction abruptly changes from dislocation transmission on the {111} slip planes to the unusual (100) slip plane of the twin, when the twin thickness is smaller than 4 layers. Using atomistic simulations, we find that the energy barrier for {100} slip transmission mechanism gradually decreases, with decreasing twin thickness, below the energy level required for normal (111) slip transmission, which remains identical for all twin sizes. Our in situ observations and simulations provide atomistic insights into a fundamentally new mechanism of plasticity in nanotwinned metals, down to the lowest twin size limit.