Stick–slip dynamics of coherent twin boundaries in copper

The migration kinetics of coherent twin boundaries (CTBs) and the underlying atomistic mechanisms are determined through molecular dynamics (MD) computer simulations. Details of motion dynamics and associated effective migration of CTBs are examined for nanotwinned copper crystals under externally applied shear loading. The present study reveals that the magnitude and direction of the resulting CTB migration velocity is dependent on the shear-loading orientation. It is found that 〈112〉-type shearing on {1 1 1} twin boundaries maximizes their transverse migration velocity. Shearing at directions which remain parallel the TB plane but are inclined to the 〈112〉-direction results in a smaller degree of coupling, and finally to twin boundary sliding alone when the shear direction is along 〈110〉. It is found that the dynamics of CTB motion can be described as a two-step “stick–slip” process. Analysis of atomic configurations indicates that the “stick” phase of the dynamics is associated with accumulated strain in the crystal, and that such strain is suddenly released by the nucleation of 1/6 [1 1 2]-type twinning partial dislocations. In atomic layers adjacent to the twin boundary, coordinated shuffling of atoms is found to take place immediately before dislocation nucleation. The “slip” phase of the dynamics is shown to be controlled by fast propagation of nucleated twinning partial dislocations and their spreading along the twin boundary.