Decohesion of iron grain boundaries by sulfur or phosphorous segregation: First-principles calculations

We performed first-principles calculations to simulate the grain boundary decohesion in ferromagnetic bcc iron (Fe) Σ3(111) symmetrical tilt grain boundaries by progressively adding solute atoms [sulfur (S) or phosphorous (P)] to the boundaries. We show that there are two mechanisms of decohesion: (i) fracture surface stabilization with reference to the grain boundary by the segregated solute atoms without interaction between them, and (ii) grain boundary destabilization by a repulsive interaction among the segregated and neighboring solute atoms. It is found that the dominant mechanism for the S-induced decohesion is the former (i), while that for P is the latter (ii). This difference makes P a much weaker embrittling element comparing with S because the mechanism (ii) simultaneously brings about the reduction of the grain boundary segregation energy.