Effect of Nb Content on the Stability and Electronic Properties at bcc-Fe/NbN Interface

First-principles calculations based on density functional theory were employed to systematically investigate the atomic structure, stability, and Nb segregation behavior of the bcc-Fe(100)/NbN(100) interface. Convergence tests of surface energy determined that an interface model consisting of 7 bcc-Fe layers and 5 NbN layers is appropriate. The work of adhesion and interfacial energy were calculated for four interface configurations with different terminations. Interface 2 was identified as the most thermodynamically stable configuration, exhibiting a work of adhesion of 0.569 J/m2 and an interfacial energy of 3.221 J/m2. Nb atoms displayed pronounced site-selective segregation at the interface; the segregation tendency decreases in the order site 1 (−0.74 eV) > site 2 (−0.59 eV) > site 4 (−0.38 eV) > site 3 (−0.24 eV). Electronic structure analysis indicated that strong hybridization between Nb-4d and Fe-3d orbitals near the Fermi level leads to localized charge accumulation, which is the electronic origin of interfacial strengthening. As the Nb concentration increases from 1.6 at.% to 6.3 at.%, the segregation energy continuously drops from −0.74 eV to −1.55 eV, the work of adhesion monotonically increases from 0.569 J/m2 to 0.786 J/m2, and the interfacial energy decreases from 3.221 J/m2 to 2.374 J/m2, demonstrating that Nb segregation significantly enhances the interfacial stability. The calculation results provide a theoretical framework for understanding the experimentally observed evolution of Nb(C,N) precipitates and offer insights for the optimization of Nb microalloying in 442D ferritic stainless steel.