Deformation twinning mechanisms from bimetal interfaces as revealed by in situ straining in the TEM

During in situ transmission electron microscopy indentation of layered Cu/Nb composites fabricated by accumulative roll bonding, we observed the nucleation and growth of deformation twinning in Cu. Combining dislocation theory and interface defect structure analysis, we propose that the processes of nucleation and thickening of deformation twins proceed as follows: (1) nucleation and emission of a nanometer thick twin nucleus due to the dissociation of interfacial misfit dislocations or the interaction between a run in Nb slip dislocation and the interface or both; (2) emission of a second twin nucleus by the same mechanism, but at some distance along the interface from the first; (3) in the region in-between them, twin thickening occurs by relatively rapid emission of oppositely signed twinning dislocations in order to reduce the backstresses due to the twins in processes 1 and 2; and finally (4) the continuation of this "alternating emission" growth mechanism such that eventually the entire domain in-between the two initially separated fine twin nuclei transforms into a thick twin.