First-principles appraisal of solute ultra-fast diffusion in hcp Zr and Ti

We revisit the ultra-fast diffusion characteristics of Fe, Co, Ni, and Cu solutes, in the hcp hosts Ti and Zr, by using Density Functional Theory. The energetics of several point defect configurations, deemed relevant for solute diffusion, is evaluated. The results support the long standing beliefs that the diffusing species is interstitial in nature, and that some kind of complexing is involved at low temperatures. Though quantitative agreement with experiment is difficult to assess, we show that a rather simple dissociative model is able to rationalize the observed trends, in particular, why the Arrhenius graphs are straight for Ti whereas, generally, they are curved downwards for Zr.