Steps and {112 ̄1} secondary twinning associated with {112 ̄2} twin in titanium

{112¯2} twinning commonly takes place in α-titanium (α-Ti). High-resolution transmission electron microscopies (HRTEM) explored various steps along {112¯2} coherent twin boundary. Topological model of {112¯2} twin revealed twinning disconnections (TDs) that are represented by (b_i, ih_{112¯2}) corresponding to a step height ih_{112¯2} and a shear vector b_i. Atomistic simulations were conducted to study the energies and kinetics of TDs. Combining microscopies and atomistic simulations, we concluded that (b₃, 3h_{112¯2}) is the elementary TD and (b₁, h_{112¯2}) is the reassembly TD. Steps observed in HRTEM thus can be treated as a reassembly of (b₃, 3h_{112¯2}) TDs and (b₁, h_{112¯2}) TDs. In addition, Electron Backscatter Diffraction (EBSD) maps revealed {112¯2}→{112¯1} double twins in α-Ti. Using two-dimensional and three-dimensional atomistic simulations, we demonstrated the nucleation of (b₁, h_{112¯2}) TD and {112¯2}→{112¯1} double twin through the interaction between basal <a> dislocation and {112¯2} twin. Our results enrich the understanding of {112¯2} twinning including TDs, steps, and {112¯1} secondary twins in hexagonal metals.