First-principle investigation of electronic structures and interactions of foreign interstitial atoms (C, N, B, O) and intrinsic point defects in body- and face-centered cubic iron lattice: A comparative analysis

Carbon, Nitrogen, Boron and Oxygen are common alloying elements in ferritic as well as in austenitic steels and their small concentration of only few parts per million significantly influence the materials mechanical properties. In this report, a detailed comparative analyses of electronic structures and interactions of foreign solute atoms (C, N, B, and O) with point defects (vacancy/interstitial) in bcc- and fcc-iron lattices as obtained from density functional theory calculations are presented. It is found that the energetically favourable configurational site for B atom is the substitutional site in bcc-Fe whereas it is the octahedral interstitial site in fcc-Fe. For all other solute (C, N, O) atoms octahedral interstitial position is energetically favourable in both Fe lattices. The stability of all solute atoms is discussed from view point of their chemical nature as well as geometrical factor. In addition, the interaction of B atom with the point defects in the two Fe-lattices is found to be quite different as compared to any other solute atoms. We found that B atom strongly binds the point defects in bcc-Fe, in contrast, repels in fcc-Fe lattice. Furthermore, strong resemblance is observed between B atom and vacancy regarding their nature of interaction with other point defects in bcc-Fe lattice. Although vacancy always exhibit strong attraction with solute atoms in both Fe-lattices, the trapping ability of the vacancy in bcc-Fe is found to be stronger than that in fcc-Fe. The interaction among V-SA(m) cluster is also calculated for m upto 5 in order to investigate the trapping ability of a vacancy and covalent bond formation is observed in case of C atoms only in bcc-Fe lattice. Further insights into electronic structure and interaction of the point defects in the two Fe lattices have been obtained from density of states and differential charge density calculations.