The current experimental methods are not able to reveal the actual processes of atomic movements during twinning and thus are incapable of clarifying the underlying mechanisms of annealing twinning, which are still not clear at present. We developed a method of molecular dynamics simulation to study the mechanism of annealing twinning in copper at an atomic-level. The simulation revealed that a annealing twin can be developed quickly from a pair of grains with σ3 misorientation interfaced by a (5?1?1?)/ (1?11) asymmetric boundary. The twinning proceeds by a mechanism in which every three adjacent (5?1?1?) atomic layers merge into a (1?1?1?) layer in the (5?1?1?) side, while the atomic arrangement in the (1?11) side remains unchanged. Such twinning takes place readily upon annealing at temperatures ranging from 700 to 1300 K, without requiring any extra driving force, indicating that annealing twinning in copper is indeed a thermally activated process with an activation energy estimated to be 0.1 eV. Similar annealing twinning is also observed in another two pairs of grains with σ3 misorientation interfaced by (8?22)/(022) and (2?44)/(2?00) asymmetric boundaries, respectively, yet their twinning rates are much slower than that of the (5?1?1?)/ (1?11) grain pair, suggesting a different mechanism governing the process. The simulation also suggested that annealing twinning may involve two separate steps of which one is the formation of grain pairs with σ3 misorientation and the other is the fine-tuning through which the grain pairs with σ3 misorientation are converted into ideal annealing twins, which can grow larger with grain growth.
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