The discrete variational (DV) and DMol methods within the framework of density functional theory are used to study the effect of alloying element Nb on Fe γ phase.The impurity formation energy in bulk and segregation energies at grain boundary and free surface are calculated.The results show that Nb prefers to segregate at grain boundary.The difference in segregation energies between the grain boundary and the corresponding free surface is -0.39 eV for solute Nb.According to Rice-Wang model,it can be predicted that Nb can enhance grain boundary cohesion.The calculated results of interatomic energy and charge density show that charge would be redistributed,and the bonds across grain boundary are strengthened by the substitution of Nb for Fe.As a result,it is difficult for the grain boundary to move.Thus the dragging effect of Nb is explained electronically.
