First-principles prediction of oxygen diffusivity near the (101¯2) twin boundary in titanium

We study the diffusivity of oxygen interstitials around a ( 10 1 ¯ 2 ) twin boundary in Titanium. First, we identify all possible stable interstitial sites around the twin boundary and compute the corresponding site energies and transition energy barriers for jumps between these sites, using density functional theory. We show that the site energies and the barriers are consistently lower than in bulk, suggesting the higher tendency of oxygen to segregate to the twin boundary region. Using the site and transition energies and an exact solution to the master equation, we then compute the diffusivity of oxygen in the presence of the twin boundary and find enhanced diffusivity around the boundary in all directions. Enhanced diffusivity towards the boundary determines the feasibility of oxygen segregation to favorable sites at the boundary, while increased diffusivity in the boundary plane provides a path for fast diffusion of oxygen. This result reveals the underlying mechanism governing the slow growth of ( 10 1 ¯ 2 ) twin by pinning at the segregated oxygen interstitials.