BORON SOLUTION AND DISTRIBUTION IN a-FE: applicaion to boron steel

Abstract

Formation-energy formalism for a general interface is developed and applied to alpha-Fe host with boron (B) impurities. In bulk alpha-Fe, B impurities prefer to be located at substitutional position rather than at interstitial. The estimated formation energy of substitutional B is lower than that of interstitial B by 0.10 eV in the dilute impurity limit. At the surface, however, the interstitial site is found to be preferred, and B impurities on top of the surface are the most stable as compared to those in the subsurface positions. The stability of B impurities increases as they get close to the free surface, indicating that B impurities tend to segregate toward the free surface. This surface segregation of B impurities is found to be the direct reflection of the surface-energy minimization of Fe-B system. Based on the first-principles band calculations, it is deduced that the dominant B diffusion behavior near the free surface is described by the interstitial-to-interstitial diffusion mechanism.