The formation and anisotropic/isotropic diffusion behaviors of vacancy in typical twin boundaries of α-Ti: An ab initio study

The distinct atomic structure of twin boundary (TB) significantly affects the physical and mechanical behaviors of materials. In this work, the local atomic structure, the formation and diffusion of vacancy in (10 (1) over bar2), (11 (2) over bar1), (11 (2) over bar2) and (10 (1) over bar1) TBs of alpha-Ti were investigated in details using ab initio calculations. Our calculations illustrated that most atoms at or close to the TBs have lower atom coordination number (CN), thereby experiencing the positive changes of local volume and tension strain. The CN and strain state significantly affect the vacancy formation. It's shown that vacancy can easily form in these TBs, with the lower formation energy in the range of 1.26-1.59 eV. Our calculations further shown vacancy diffusion are somewhat anisotropic in (10 (1) over bar2), (10 (1) over bar1) and (11 (2) over bar2) TBs planes. Vacancy prefers to diffuse along the direction of [(1) over bar2 (1) over bar0] in (10 (1) over bar2) and (10 (1) over bar1) TBs, and along the twinning direction of [11 (2) over bar(3) over bar] in (11 (2) over bar2) TB, with the migration barriers of similar to 0.43, similar to 0.27, and similar to 0.31 eV, re- spectively. Vacancy diffusion in (11 (2) over bar1) TB plane is isotropic, with the migration barrier of similar to 0.25 eV. Our calculations demonstrated that the fast diffusion channel mediated by vacancy for metallic atoms transport in these TBs can be mainly ascribed to the lower migration barrier and formation energy of vacancy in TBs.