Effect of Nb Segregation on the Stability of the FeΣ3(111) Grain Boundary by First Principles Investigation

The segregation of niobium (Nb) at the FeΣ3(111) grain boundary and its influence on interfacial cohesion were investigated via spin-polarized density functional theory calculations. Nb atoms exhibit strong site-selective segregation, with Site 1 being the most thermodynamically favorable one (segregation energy of –2.47 eV), owing to its largest local Voronoi volume. Electronic structure analyses reveal pronounced Nb-4d/Fe-3d orbital hybridization and localized charge accumulation between Nb and neighboring Fe atoms, enhancing covalent bonding at the boundary. First-principles tensile simulations show that single-Nb segregation increases the critical strain from 13.58% to 15.76% and the theoretical tensile strength from 16.32 GPa to 19.64 GPa. However, double-Nb segregation reduces the work of separation to 3.26–4.24 J/m2, revealing a competition between segregation strengthening and solute-induced weakening that implies an optimal Nb concentration window for grain boundary engineering.