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 { 11 2 ¯ 2 } 〈 11 2 ¯ 3 ¯ 〉 twinning which is an important mode in some HCP metals such as titanium and zirconium. Our simulations show that a coherent { 11 2 ¯ 2 } twin boundary can be formed as a result of twin-twin interaction between co-zone { 11 2 ¯ 1 } twin variants. During deformation, three co-zone { 11 2 ¯ 1 } twins form first and then interact. Two of the { 11 2 ¯ 1 } twin boundaries (TBs) merge into a coherent { 11 2 ¯ 2 } 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 { 11 2 ¯ 1 } and { 11 2 ¯ 2 } twins have the same (0001) K ₂ plane. The migration of { 11 2 ¯ 2 } TB is found to be mediated by the single-layer twinning dislocations.