The behavior of carbon and nitrogen atoms in iron based solid solution is studied by ab initio density-functional-theory calculations. The interaction of a C or a N atom in $\alpha$-Fe with a vacancy, other C or N interstitials as well as self-interstitial atoms is discussed and compared to known experimental results. The migration of these two foreign interstitial atoms is determined in pure Fe or when a vacancy is present in the supercell. According to our results, there is a strong binding energy of C or N with vacancies, whereas a repulsion is observed with self-interstitial atoms. Furthermore, a vacancy can trap up to two C, and a covalent bonding forms between the two C atoms. The situation is not as clear for N atoms, and a competition between the formation of N-V pairs and NN-V triplets is very probable.

• Received 12 December 2003

DOI:https://doi.org/10.1103/PhysRevB.69.144112