Atomic determination of the energetics and configurations of N-Cr/Cr-N-C-O and N-Cr/Ni-vacancy complexes in austenitic Fe alloys

The energetics and configurations of small N-Cr/Cr-N-C-O and N-Cr (Ni)-vacancy complexes in austenitic Fe alloys were investigated using a first-principles method. The interaction between N and Cr/vacancy is attractive (0.10/0.59 eV) while Ni/O repels N (−0.21/−0.46 eV). Energetically, interstitial N and Cr can form stable Cr_mN_n complexes, the binding energy of the clusters increases from 0.24 to 2.07 eV with the size (m = 2–16). The tetrahedral Cr₄N₄ structure acts as a stable unit to form larger size by connecting N (point to point). The presence of Cr strengthens the stability of VN_n clusters energetically while Ni weakens it, the Cr-VN_n and VN_n (n = 1,2) act as main complex defects. Furthermore, we determined the stabilities of serval Cr-N-O/Cr-N-C-O complexes and found that the synergistic interaction of Cr-N-C-O strongly stabilize the clusters and O impurities, then enhancing mechanical properties. Finally, we predicted the effect of Cr/Ni on N migration and effective diffusivity. These results deepen understanding for the synergistic interactions between interstitial N and alloying elements/vacancies in austenitic stainless steels.