First principles study of normal and fast diffusing metallic impurities in hcp titanium

The diffusivities of metallic impurities in hexagonal close packed (hcp) Ti vary widely, with some species showing normal diffusion and others diffusing anomalously fast. Based on a transition-state theory model for impurity diffusion mediated by vacancies, self, and impurity interstitials, we find that the diffusion rate will be limited primarily by the formation energies of the various defects. We use density functional theory to calculate these defect energies, including the formation energies of vacancies, self interstitials, and metallic impurities in substitutional and interstitial positions in hcp Ti. We find that for normal diffusers the vacancy mediated diffusion mechanism dominates, while for fast diffusers the dominant mechanism is hopping directly between interstitial sites. The activation energy for the latter mechanism is significantly lower than that for vacancy mediated diffusion, explaining the large diffusivities and low activation energies observed experimentally for these fast diffusing impurities. (C) 2015 Elsevier B. V. All rights reserved.