Via in situ uniaxial tensile tests in a high-resolution transmission electron microscope,we directly observed a cross-over of plastic deformation mechanisms in a nanocrystalline (nc) Cu thin film containing nano-twin lamellae.For a certain twin lamellae length,the twin lamellae with a larger thickness emit dislocations inclined (Schmidt-factor dislocations,i.e.,S-dislocations) toward the twin boundaries.Upon decreasing the twin lamellae thickness to a critical value,such as approximately 15nm,the plasticity switches toward emission of twinning partial dislocations (T-dislocations) parallel to the twin planes that cause migration of the twin boundaries.The critical twin thickness value also depends on the length of the twin.These results provide direct evidence for thestrengthening and softening mechanisms in nano-twinning structured metals.
