Electronic effects of alloying elements Nb and V on body-centred-cubic Fe grain boundary cohesion

The segregation effects of Nb and V on bcc FeΣ3[11̄0](111) grain boundary cohesion are studied by the first-principles DMol method within the framework of density functional theory. The calculated segregation energy difference between the grain boundary and the corresponding free surface is -0.51 eV (-0.58 eV) for solute Nb (V), which indicates that both Nb and V could enhance the grain boundary cohesion in body-centred-cubic Fe. We found that the chemical effect and the geometry effect of Nb (V) play crucial but opposite roles in determining whether a material is brittle or ductile. The chemical effect is dominant and advantageous for grain boundary cohesion. Also, Nb and V show very different behaviours: in chemical effect, Nb is more conducive to ductility than V; while in the geometry effect, Nb is less conducive to ductility and more conducive to embrittlement than V.