Effect of impurities on structural, cohesive and magnetic properties of grain boundaries in α-Fe

Abstract

The effects of several metalloid (B), metalloid-like (C, P) and nonmetallic impurities (N, O and S) on structure, energetics and mechanical properties of Σ3(1 1 1) and Σ5(2 1 0) grain boundaries (GBs) in ferromagnetic α-Fe have been studied from first principles. For two different concentrations and positions of impurity atoms, the variations in GB properties have been analysed. Most of the impurities enhance the relaxation of the interplanar spacing of the pure grains. Interstitial impurities at both GBs are shown to increase the separation of the grains while substitutional ones in general either do not alter or decrease the grains' separation. It is shown that at the Σ5 GB for all impurity atoms considered the positions in the boundary layer are energetically favoured independently of the interstitial or substitutional site, whereas the enrichment of the Σ3 GB is favoured for the impurities of the interstitial sites as well as for a substitutional P and C. We have found that, in most cases, impurity atoms both in interstitial and substitutional positions at GBs act as embrittlers. Interstitial B, P and C strengthen the Σ3 GB cohesion, and B and C at lower concentrations strengthen the Σ5 GB. All substitutional impurities, except for B, weaken the GBs. The magnetic moments on the impurities are very small and in most cases align antiparallel to the moments on the neighbouring Fe atoms.