A new mechanism for nucleation of {112¯2}〈112¯3¯〉 twinning via interaction of {112¯1}〈112¯6¯〉 twin variants in hexagonal close-packed metals

Twin nucleation in high symmetry cubic structures is closely related to the activities of dissociated lattice dislocations. However, in low symmetry hexagonal close-packed (HCP) metals, the nucleation mechanisms for deformation twinning remain largely unclear. In this work, we conduct atomistic simulations and uncover a new mechanism for nucleation of $\left\{11\overline{2}2\right\}〈11\overline{2}\overline{3}〉$ twinning which is an important mode in some HCP metals such as titanium and zirconium. Our simulations show that a coherent $\left\{11\overline{2}2\right\}$ twin boundary can be formed as a result of twin-twin interaction between co-zone $\left\{11\overline{2}1\right\}$ twin variants. During deformation, three co-zone $\left\{11\overline{2}1\right\}$ twins form first and then interact. Two of the $\left\{11\overline{2}1\right\}$ twin boundaries (TBs) merge into a coherent $\left\{11\overline{2}2\right\}$ TB. This nucleation process does not involve any lattice dislocations or twinning dislocations. Lattice correspondence analyses indicate that such a nucleation process is feasible because all these $\left\{11\overline{2}1\right\}$ and $\left\{11\overline{2}2\right\}$ twins have the same (0001) K₂ plane. The migration of $\left\{11\overline{2}2\right\}$ TB is found to be mediated by the single-layer twinning dislocations.