Chromium-vacancy clusters in dilute bcc Fe-Cr alloys: an ab initio study

TL;DR

This study uses ab initio calculations to analyze the stability and magnetic properties of vacancy-chromium clusters in dilute Fe-Cr alloys, explaining experimental Cr segregation phenomena.

Contribution

It reveals the stable configurations and magnetic origins of vacancy-Cr clusters, extending understanding of Cr behavior below solubility limits in Fe-Cr alloys.

Findings

Single vacancies attract up to four Cr atoms.

Binding energy correlates with Cr atom position and magnetic moments.

Di-vacancy clusters show increased binding energy ratios.

Abstract

Using an ab initio approach, we explore the stability of small vacancy and vacancy-chromium clusters in dilute body-centred cubic Fe-Cr alloys. To explain experimental observations described in C.D. Hardie et al., J. Nucl. Mater. 439, 33 (2013) and showing the occurrence of Cr segregation in low-Cr alloys, we investigate if chromium can form stable bound configurations with vacancies in alloys with chromium concentration below the low-temperature chromium solubility limit of 10-11 at. %. We find that a single vacancy can attract up to four Cr atoms in the most energetically favourable cluster configuration. The binding energy of a cluster containing a single vacancy and from one to eight Cr atoms can be well described by a linear function of the number of chromium atoms in the second, third and fifth nearest neighbour coordination. The magnetic origin of the binding energy trend is confirmed by a correlation between the average value of the magnetic moment of a Cr atom and the binding energy. Similar trends are also found for di-vacancy-Cr clusters, confirming that they likely also characterise larger systems not yet accessible to ab initio calculations. The ratio of the binding energy to the number of Cr atoms increased more than twice in the di-vacancy case in comparison with a single vacancy case.