First-principles calculation of influence of alloying elements on NbC heterogeneous nucleation in steel

The NbC precipitated in steel is in favor of the heterogeneous nucleation of ferrite, which is affected by the alloying elements at the ferrite/NbC interface. However, it is difficult to clearly understand the effect of alloying elements on the ferrite/NbC interface behavior experimentally. Therefore, the first-principles calculation is employed to address this problem in this paper. First of all, the segregation behaviors of alloying element X (= Cr, Mn, Mo, W, Zr, V, Ti, Cu and Ni) on the ferrite(100)/NbC(100) interface are systematically explored. And then, we investigate the influences of these alloying elements on the property of the ferrite/NbC interface. The work of adhesion (W-ad), interfacial energy (gamma(int)) and electronic structure of ferrite/NbC interface alloyed by these elements are also analyzed. The results show that the (Cr, V, Ti)-doped interfaces have negative segregation energies, which indicates that Cr, V and Ti are easily segregated at the ferrite/NbC interface. Conversely, the Mn, W, Mo, Zr, Cu and Ni are difficult to segregate at the interface. When Mn, Zr, Cu and Ni replace the Fe atoms in the ferrite/NbC interface, the adhesive strength of the interface will decrease, thus weakening the heterogeneous nucleation of ferrite on NbC surface. However, the introduction of Cr, W, Mo, V and Ti will improve the stability of the ferrite/NbC interface due to the larger W-ad and lower gamma(int). Therefore, the Cr, W, Mo, V and Ti on the ferrite side of the interface can effectively promote ferrite heterogeneous nucleation on NbC surface to form fine ferrite grain. The analysis of difference charge density indicates that after the introduction of Zr and Cu in ferrite/NbC interface, the interactions among interfacial Zr, Cu and C atoms was weaken. However, when Cr and W are introduced into the clean interface, the strong Cr-C and W-C non-polar covalent bonds are formed, which enhances the adhesion strength of the ferrite/NbC interface. In addition, the minimum Cr-C bonding length at the Cr-doped interface suggests that the interface has the highest interface strength. The Mulliken population analysis shows that for the (Cr, W, Mo, V, Ti)-doped interfaces, the transfer charges of Cr, W, Mo, V and Ti are 1.12, 0.84, 0.54, 0.33 and 0.28, respectively. Nevertheless, for the clean interface, the transfer charge of Fe is only 0.05. Therefore, the interactions among interfacial Cr, W, Mo, V, Ti and C atoms are stronger than that between interfacial Fe and C atoms, which is in good accordance with the above analysis.