On the electronic basis of the phosphorus intergranular embrittlement of iron

Using the all-electron full potential linearized augmented plane wave (FLAPW) total energy method, the influence of P impurity atoms on the cohesion of the Fe Σ3[110] (111) grain boundary is studied through direct comparison of phosphorus/iron interactions in the grain boundary and free surface environments. The calculated nearest P-Fe distance in P/Fe(111) is 2.14 A—amounting to a 5% contraction compared to that (2.26 A) measured for the Fe3P compound and assumed for the P-Fe grain boundary. The polar-covalent P-Fe chemical bonding, which is a strong function of the P-Fe interatomic distance, is thus stronger on the Fe(111) surface, while P reduces the spin polarization of the surrounding Fe atoms more efficiently in the grain boundary environment. These effects are examined in terms of the relative segregation energies affecting the work of boundary fracture.