Steps and {11(2)over-bar1} secondary twinning associated with {11(2)over-bar2} twin in titanium

{11 (2) over bar2} twinning commonly takes place in alpha-titanium (alpha-Ti). High-resolution transmission electron microscopies (HRTEM) explored various steps along {11 (2) over bar2} coherent twin boundary. Topological model of {1122} twin revealed twinning disconnections (TDs) that are represented by (b(i), ih({11 (2) over bar2})) corresponding to a step height ih({11 (2) over bar2}) 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 (b(3), 3h({11 (2) over bar2})) is the elementary TD and (b(1), h({11 (2) over bar2})) is the reassembly TD. Steps observed in HRTEM thus can be treated as a reassembly of (b(3), 3h ({11 (2) over bar2})) TDs and (b(1), h({11 (2) over bar2})) TDs. In addition, Electron Backscatter Diffraction (EBSD) maps revealed {11 (2) over bar2} -> {11 (2) over bar1} double twins in alpha-Ti. Using two-dimensional and three-dimensional atomistic simulations, we demonstrated the nucleation of (b(1), h({11 (2) over bar2})) TD and {11 (2) over bar2}->{11 (2) over bar1} double twin through the interaction between basal dislocation and {11 (2) over bar2} twin. Our results enrich the understanding of {11 (2) over bar2} twinning including TDs, steps, and {11 (2) over bar1} secondary twins in hexagonal metals. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.