Grain size effect of V-shaped multiple twins in annealed nanocrystalline Cu

V-shaped multiple twins have been observed in annealed nanocrystalline Cu by high-resolution transmission electron microscopy. The density of V-shaped multiple twins depends on the grain size remarkably. With decreasing grain size, the amount of V-shaped multiple twins increases initially and then decreases, reaching the maximum at a critical grain size of 40 nm and exhibiting an obvious grain size dependence. The formation mechanism of V-shaped multiple twins is proposed based on cooperative grain rotation and grain boundary migration. However, the amount of V-shaped multiple twins decreases significantly below the critical grain size, which is attributed to the grain rotation-mediated single crystal growth.