Extended zonal dislocations mediating {11(2)over-bar2}⟨11(23)over-bar⟩ twinning in titanium

Using molecular dynamics (MD) simulations, the {11 (2) over bar2}< 11 > twinning mechanism in titanium (Ti) was studied by analyzing the interfacial structure at the twin boundary (TB). The simulation results reveal interesting twin growth controlled by interfacial dislocations at the TB. The elementary twinning dislocations (b(T)) nucleate and glide in pairs but separately and sequentially on two neighboring planes, significantly different from conventional zonal dislocations, which spread over two or more twinning planes with each plane comprising one Burgers vector of an elementary twinning dislocation. The twin growth can be approximately described as b(T)(1) (similar to 1/3 x 1/6 < 20 >) + b(T)(2) (similar to 1/3 x 1/6 < 02 >) -> 2b(T) (similar to 1/3 x 1/3 < 11 >). These two separate elementary twinning dislocations amount to a net Burgers vector 2b(T) approximate to 0.16nm along the twinning vector eta(1) = 1/3 < 11 >, with the components in the in-plane direction perpendicular to eta(1) canceled out. These results support the classical twinning theory in which a homogeneous shear and local shuffling have to be involved. A mechanism taking into consideration local structure of the twinning plane for such extended zonal dislocations is discussed.