Short-range ordered structure and phase stability of supersaturated nitrided layer on austenitic stainless steel

The structures of short-range ordered Fe-Cr-N clusters in face-centered cubic (f.c.c.) iron have been systematically studied by first-principles calculation to understand the atomic structure and phase stability of supersaturated nitrided layer on austenitic stainless steel. The clusters are formed by aggregation of Cr and N atoms due to their strong interaction. The unit of the clusters can be considered as a Fe6-nCr (n) N octahedral cluster, in which the N atom occupies an octahedral interstitial site of the f.c.c. lattice and the metal atoms are at the first nearest neighbor sites to the N atom. Moreover, the Cr atoms prefer to distribute in pairs around the N atom. When the N concentration increases, a larger cluster may be formed by combination of the octahedral clusters through edge-shared mode, and Cr atoms prefer the shared sites. The cluster structure is affected by both the local lattice distortion around the cluster and Coulombic interaction in the cluster. Then, the atomic structure of supersaturated nitrided layer can be described as the f.c.c. iron dispersively embedded with the short-range ordered clusters. Furthermore, the stabilization mechanism of the metastable phase was examined based on the structure model. It was suggested that the stabilization of the metastable phase is mainly a chemically-driven mechanism by Cr and N atoms forming short-range ordered clusters. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.