Abstract：For metals with the hexagonal close-packed (hcp) crystal structure, deformation twinning plays an important role in their plastic deformation. Due to the complex lattice structures of hcp metals, the atomic motions in twinning are very complex because not all of the atoms move in the direction of twinning shear. Therefore, the glide-shuffle mechanisms associated with the motion of twinning dislocations (TDs) are responsible for the twinning in hcp metals. In this paper, we focus on the mechanisms of TDs for different types of twinning, such as {101 ̅2}, {101 ̅1},{112 ̅2}, and {102 ̅1}. It is revealed that the lattice shuffles are needed for almost all hcp twinning mechanisms, except for the one-layer TD in {112 ̅2} twinning. This one-layer TD can be achieved by pure shear via the glide of partial dislocations along the twinning plane. Therefore, the twinning mechanisms in hcp metals can be classified as the glide-dominated, shuffle-dominated and glide-shuffle mechanisms. For a twinning with a determined invariant plane k_1 (and the shear direction η_1), different types of TDs with different shears and shuffles can be involved, while the second undistorted plane k_2 and the conjugate shear direction η_2 are altered accordingly, resulting in the corresponding tensile or compressive deformation of twinning. The prediction of possible twinning modes is based on the hypothesis that both the twinning shear and the shuffle magnitudes should be small. The {101 ̅2} twinning is the most easily activated twinning mode in hcp metals due to its small twinning shear and atomic shuffles. Furthermore, the activation of various twinning mechanisms under specific temperature and stress conditions can be responsible for the improvement of plasticity in hcp metals and alloys. The comprehensive understanding of twinning mechanisms will be helpful for further experimental and theoretical exploration on designing alternative and more innovative hcp-type structural materials with superior mechanical properties.
2021, Vol. 42 
