Effect of Ni Doping on the Embrittlement of Liquid Zinc at Σ5 Fe Austenite Grain Boundary

In this study, first-principles computational tensile tests have been performed for the Σ5 symmetrically tilted grain boundaries of the face-centered cubic (fcc) Fe to investigate the effects of Zn and Zn-Ni doping on the boundary energy and electronic structure. The obtained results indicate that the mismatch between the sizes of Zn and Fe atoms at the Zn-doped grain boundary causes its expansion, which increases the lengths of Fe-Fe bonds, leading to their weakening, and reduces the overall boundary strength. After the Zn doping of the Fe grain boundary, Zn atoms form covalent bonds with Fe atoms, that decreases the charge density of Fe-Fe bonds and their strength. Meanwhile, the strength of the newly formed Fe-Zn covalent bonds oriented at a certain angle with respect to the grain boundary direction is very low. The breakage of Fe-Fe bonds that occurs under tensile loading rapidly decreases the boundary strength. Finally, after the Zn-Ni co-doping of the Fe grain boundary, Ni atoms form metallic bonds with Fe atoms, thus increasing both the charge density of Fe-Fe bonds (as compared with that of the Fe-Fe bonds at the Zn-doped grain boundary).