First-principles study of four mechanical twins and their deformation along the c-axis in pure α-titanium and in titanium in presence of oxygen and hydrogen

Interstitial solutes such as oxygen and hydrogen have an important and complex role on the mechanical properties of titanium. With ab initio calculations, we get the atomic structures and energies of the three most common twin boundaries (TBs) observed in alpha-Ti (hexagonal close packed), viz. {10 (1) over bar2}, {11 (2) over bar1}, and {11 (2) over bar2}, together with the {10 (1) over bar1} TB. The segregation energies of O and H to the four TBs are evaluated and their behaviour under deformation along the c-axis is investigated, in pure Ti and in presence of segregated O or H. All TBs decrease the theoretical ultimate tensile stress of alpha-Ti. Remarkably, the {10 (1) over bar2} and {11 (2) over bar2} TBs are unstable for a c-axis deformation as small as about 2%. O and H segregate to all four TBs except for O to the {11 (2) over bar2} TB case. As such, they slightly enhance the {10 (1) over bar2} and {11 (2) over bar2} TB limited stability under c-axis deformation. They decrease the {10 (1) over bar1} TB stability under high deformation and so does O for the {11 (2) over bar1} TB. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.