Based on a systematic first-principles study, the lowest-energy migration mechanisms and barriers for small vacancy-solute clusters $\left(V_n{X}_m\right)$ are determined in $\alpha$-Fe for carbon, nitrogen, and oxygen, which are the most frequent interstitial solutes in several transition metals. We show that the dominant clusters present at thermal equilibrium ($VX$ and $V{X}_2$) have very reduced mobility compared to isolated solutes, while clusters composed of a solute bound to a small vacancy cluster may be significantly more mobile. In particular, $V_{3}X$ is found to be the fastest cluster for all three solutes. This result relies on the large diffusivity of the most compact trivacancy in a bcc lattice. Therefore, it may also be expected for interstitial solutes in other bcc metals. In the case of iron, we find that $V_{3}X$ may be as fast as or even more mobile than an interstitial solute. At variance with common assumptions, the trapping of interstitial solutes by vacancies does not necessarily decrease the mobility of the solute. Additionally, cluster dynamics simulations are performed considering a simple iron system with supersaturation of vacancies, in order to investigate the impacts of small mobile vacancy-solute clusters on properties such as the transport of solute and the cluster size distributions.

• Received 28 April 2015
• Revised 22 July 2015

DOI:https://doi.org/10.1103/PhysRevB.92.104102