Three-dimensional atomic scale characterization of { 11 2 ¯ 2 } twin boundaries in titanium

The { 11 2 ¯ 2 } < 11 23 ¯ > compression twin can accommodate a considerable amount of strain under c-axis compression in Ti. However, unlike the tensile { 10 1 ¯ 2 } 〈 1 ¯ 011 〉 twin, the structure of { 11 2 ¯ 2 } < 11 23 ¯ > compression twins has not been completely characterized. Here, we apply a combined technique of HR-TEM characterization, topological analysis, and atomistic simulations to explore the facets that bound the { 11 2 ¯ 2 } < 11 23 ¯ > twin in Ti. In addition to the currently known facets (CTB and B-Py), six new facets are observed and categorized for the first time from atomic-scale TEM observations along five crystallographic directions. The six new facets are ( 11 2 ¯ 0 ) // ( 11 2 ¯ 6 ) , PrPr1, PyPy1, ( 2 11 ¯ 1 ) // ( 1 ¯ 2 1 ¯ 1 ) , ( 1 1 ¯ 04 ) // ( 01 1 ¯ 1 ¯ ) , and ( 01 1 ¯ 0 ) // ( 2 11 ¯ 4 ) . Results from the topological and computational analysis are in reasonable agreement with and support the HRTEM observations. Specifically, (1) the observed facets align with low-index interfaces in both twin and matrix domains, (2) the facets with lower surface energies are found to form extended interfaces, and (3) high-surface-energy facets are found at the twin tip region and explained by the fact that the energy of the combined facet and facet junction configuration is energetically preferred in the twin tip region. These results not only provide a comprehensive understanding of the 3D structure of { 11 2 ¯ 2 } < 11 23 ¯ > compression twins in Ti, but also validate the MD procedure and the Ti interatomic potential employed. This is extremely important for future study of { 11 2 ¯ 2 } twin mobility and interactions with other defects, both features that remain extremely challenging to capture in experiments.