Cr interactions with He and vacancies in dilute Fe-Cr alloys from first principles

Abstract

We have performed density functional theory calculations to study the effect of Cr in the formation of He-vacancy (He${}_{n}$V${}_{m}$) clusters in a dilute Fe-Cr system. We find a significant repulsive interaction between Cr and He at neighboring tetrahedral sites. The octahedral He close to Cr becomes unstable falling to a nearby tetrahedral site, unlike in pure systems. We note a slight electronic interaction between an interstitial He and the metal atoms, stronger with Cr and responsible of their repulsion. The effect of a Cr atom in the formation of He${}_{n}$V${}_{m}$ clusters is small except for cases with high He-to-vacancy ($n/m$) ratio. In systems with multiple Cr atoms we find that, while interstitial He always repels Cr, a vacancy or a substitutional He may screen the ferromagnetic Cr coupling and promote their clustering around them. Two different behavior regimes may therefore be suggested. When the $n/m$ ratio is close to 1 or lower the Cr atoms seem to be attracted to the He${}_{n}$V${}_{m}$ cluster while when the $n/m$ ratio is greater than 1 the Cr atoms are in general repelled from it. Concerning diffusion, the energy barrier for a tetrahedral He to hop toward a neighboring position of Cr is found to be twice as large as its migration barrier in pure Fe (0.12 vs 0.06 eV). On the other hand, the diffusion of a substitutional He via the dissociative mechanism has a slightly higher dissociation barrier for first nearest neighbor sites of a Cr atom. For the vacancy mechanism we find similar diffusion properties in dilute Fe-Cr systems and in pure Fe except a slightly lower barrier close to Cr that may induce small trapping effects at low temperatures.