Nitrogen-related short-range order in Fe-Ni-Cr austenitic stainless steels: First principles and cluster expansion study

Nitrogen (N) is a key alloying element that enhances the performance of Fe-Ni-Cr austenitic stainless steels, improving austenite stability, corrosion resistance, and yield strength. In experiments, N-rich alloys often exhibit planar slip, suggesting microstructural changes associated with local chemical order. However, theoretical analysis of the N effects on chemical ordering, particularly short-range order (SRO) and long-range order (LRO), is complicated by the multisublattice structure and magnetic interactions in these alloys. In this work, we combine first-principles calculations with the spin cluster expansion (spin CE) method to systematically investigate the effects of N on chemical ordering in Fe-Ni-Cr alloys. Our atomistic models confirm a strong affinity between N and Cr, which drives the formation of N-Cr SRO and, at higher N concentrations, stabilizes M 4 N-type ordered phases (M = metal). Monte Carlo simulations reveal that low N concentrations promote local N-Cr or N-N SRO, while increasing N content leads to the emergence of Cr- and N-rich LRO structures. We also show that the presence of N suppresses intrinsic Fe-Cr and Ni-Cr SRO by competing with these interactions, particularly at high concentrations. The impact of Cr content on ordering diminishes as N approaches its solubility limit. These findings are consistent with experimental observations in high-N austenitic steels. Finally, we discuss the influence of kinetic and magnetic effects on SRO evolution in high-N alloys. This study provides a comprehensive framework for understanding N-driven chemical ordering and offers insights into microstructural changes during nitriding processes.