Co-existence of γ'N phase and γN phase in nitrided austenitic Fe-Cr-Ni alloys - II: A pragmatic modeling approach

A recent investigation of the nitriding response of austenitic Fe-Cr-Ni alloys with various compositions showed that the surface adjacent zone generally identified as expanded austenite consists of a duplex structure. In the near-surface part, with the highest N content, long-range ordering (LRO) among the nitrogen atoms occurs in addition to short-range ordering (SRO) of N and Cr atoms; and in the near-matrix part, with a lower N content, only SRO of N and Cr atoms is encountered. These zones are dubbed γ'N and γN, respectively. In the present contribution, a pragmatic model is presented to effectively describe the nitrogen arrangements in γN and γ'N and the transition of one into the other by changing the Cr and N contents. The model considers the occupancy of octahedral interstices surrounded by more than one Cr atom and, depending on the number of Cr atoms constituting the octahedron, distinguishes between strong and weak interactions between the octahedral sites and N atoms. In order to account for the competition between Cr-N SRO and Fe4N-like LRO, the occupancy of neighboring interstices with N surrounding a central metal atom is taken into consideration. The distribution of N atoms over differently coordinated interstices is obtained by pragmatically fitting the model to experimental data. The model is validated by comparison of the predicted distributions of occupied interstices to literature data for Mössbauer spectroscopy and Extended X-ray Absorption Fine Structure, respectively. Furthermore, the model provides a consistent interpretation of thermodynamics and diffusion data for nitrogen-stabilized expanded austenite.