Strain rate sensitivity of unequal grained nano-multilayers

► Unequal grained nanoscale Cu/Ta multilayers were fabricated. ► The hardness and strain rate sensitivity of the multilayers were examined. ► A transitional point of strain rate sensitivity was observed. ► Deformation mechanism is dependent on both strain rate and grain size. ► Dislocation dominates at high strain rate whilst grain boundary dominates at lower.