First-Principles Study of Nb Segregation Behavior at α-Fe Coherent Interfaces

The segregation behavior of Nb at the α-Fe (100) coherent interface, along with the effects of alloying elements and vacancies on Nb segregation, was investigated using first-principles calculations. Results indicated that when the Nb doping concentration was between 0.29% and 0.89%, the segregation energy was negative, suggesting a tendency for interfacial segregation. However, when the Nb content exceeded a threshold of 0.89% to 1.13%, the segregation energy turned positive, indicating that Nb atoms preferentially remained within the bulk matrix. Analysis of charge density difference plots revealed that d-orbital hybridization near the Fermi level between alloying elements and Fe was the key factor influencing Nb segregation, as reflected in the interfacial density of states. The potency of alloying elements and vacancies in promoting Nb segregation followed this descending order: Ni > Cu > Cr > Mo > Mn > Ti > Vacancy. As electron acceptors, Ni and Cu exhibited strong electronic interactions with Nb/Fe, thereby reducing the energy barrier for Nb segregation and enhancing it. Cr and Mo had a moderate effect, while Mn and Ti, acting as electron donors, exhibited weaker interactions and had a limited promoting effect. This study elucidates the mechanism of Nb interfacial segregation from atomic and electronic perspectives, providing a theoretical basis for the composition design and interface control of high-performance steels.