High-throughput investigation of Cr-N cluster formation in Fe-35Ni-Cr system during low-temperature nitriding

The expanded austenite (γN) produced by low-temperature nitriding of austenitic stainless steels with a Cr content of 18–20 at% is conventionally regarded as a nitrogen-supersaturated fcc solid solution with Cr-N short-range ordering, while obvious clustering between Cr and N in γN was recently reported for a Fe-35Ni-10Cr (at%) alloy. To investigate the dependence of Cr-N cluster formation on Cr concentration in γN, a high-throughput approach is proposed in the present work where a diffusion couple of Fe-35Ni and Fe-35Ni-30Cr alloys was plasma-nitrided at 673 K for 30 h. Systematic nanostructure characterization of the γN conducted by transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) revealed the variations in nanosized Cr-N clustering as modulated structures with different Cr content. Considering a simultaneous concentration fluctuation of Cr and N, computational thermodynamics of chemical driving force, strain energy, and modulation wavelength for coherent spinodal were performed, and the results were consistent with observed nanostructure evolution. The nanostructures of γN in austenitic stainless steels were also understood as further increments of Cr and N from a moderate composition lowered the driving force. The spinodal decomposition is also promoted by increasing Ni content in the alloys.