Diffusion of yttrium in bcc-iron studied by kinetic Monte Carlo simulations

The formation of oxide nanoclusters in oxide dispersion strengthened steels is controlled by the diffusion of yttrium. Yttrium atoms and other oversized solutes show a high binding energy to vacancies and a considerable relaxation from their lattice site towards a neighboring vacancy. In the case of yttrium the relaxation is so prominent, that the resulting situation may also be considered as an interstitial atom sitting in between two vacancies. We calculated the yttrium-vacancy binding energy and the migration barriers of vacancy jumps in the vicinity of a yttrium atom by means of nudged-elastic band calculations using density functional theory calculations. These barriers were used in a kinetic Monte Carlo code to calculate the diffusivity of yttrium and investigate the diffusion mechanism of yttrium in bcc iron with focus on correlation effects. The results reveal that the diffusion of yttrium is due to a sequence of vacancy jumps between the nearest and third nearest neighbor shell of the yttrium atom.