Size dependence and associated formation mechanism of multiple-fold annealing twins in nanocrystalline Cu

The formation mechanisms and grain size dependence of annealing coherent multiple-fold twins, such as twofold and fivefold twins, were investigated in nanocrystalline Cu with zero applied stress by a combination of transmission electron microscopy and molecular dynamics (MD) simulation. It was found that the formation frequency of twofold and fivefold twins with coherent twin boundaries (CTB) increases with decreasing grain size (d), reaching a maximum frequency at the critical size of 35 nm, followed by a reduction at d < 35 nm. Numerous stacking faults (SFs) ribbons are also observed and associated with the formation of multiple-fold twins. Dislocation mediated grain rotation and SFs overlapping become the dominant formation mechanisms of multiple-fold twins, which are demonstrated by experiment and MD simulation. The competition between grain growth by GB migration and the transformation of GBs to intersectant CTBs via grain rotation causes the inverse grain size dependence.