A new mechanism for nucleation of {1122}(1123) twinning via interaction of {1121}(1126) 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 {1122 }(1123 ) twinning which is an important mode in some HCP metals such as titanium and zirconium. Our simulations show that a coherent {1122 } twin boundary can be formed as a result of twin-twin interaction between co-zone {1121 } twin variants. During deformation, three co-zone {1121 } twins form first and then interact. Two of the {1121 } twin boundaries (TBs) merge into a coherent {1122 } 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 {1121 } and {1122 } twins have the same (0001) K2 plane. The migration of {1122 } TB is found to be mediated by the single-layer twinning dislocations.