Effect of impurities on grain boundary cohesion in bcc iron

The effect of B, N and O impurities placed in interstitial and substitutional positions at symmetric Σ5(2 1 0) Fe grain boundary is studied by means of first principles calculations. Full relaxation of supercell shape and volume is applied which results in stable asymmetric grain boundaries, depending on the impurity and its position. In all cases the big shifts (0.47-2.33 Å) of grains with respect to each other are observed. The equilibrium distance between the grains is decreased for impurities in substitutional positions, and increased for atoms in interstitial sites, compared to the relaxed clean GB. We found that nitrogen both in interstitial and substitutional positions, and boron in substitutional position enhance cohesion while oxygen in both positions, and interstitial boron, are embrittlers. The magnetic moments of Fe atoms at clean grain boundary are remarkably increased compared to the bulk. They tend to the bulk value in the middle of the grain in an oscillatory way.