Electronic effects of nitrogen and phosphorus on iron grain boundary cohesion

The segregation effects of N and P on the Fe Σ3[1 1̄ 0](1 1 1) grain boundary cohesion are studied by an electronic structure calculation using the discrete variational method within the framework of density functional theory. The difference of segregation energies of an adatom between the grain boundary and the corresponding Fe(1 1 1)-free surface is adopted to determine the relative embrittling or cohesion-enhancing effect of the impurity on the grain boundary. As a result, a negative difference (−0.99 eV/adatom) is obtained for N and a positive difference (1.03 eV/adatom) for P, indicating that theoretically N is a cohesion enhancer, while P is an embrittler of the grain boundary. Further analyses show that the adatom-induced local distortion of lattice, or the adatomic size, plays an important role in this mechanism. In the whole computation, the necessity of structural relaxation is emphasized.