Low-Temperature Nitridation of 2205 Duplex Stainless Steel

Low-temperature gas-phase nitridation has been studied in -ferrite in 2205 duplex stainless steel. High-resolution spatially-resolved compositional and structural analysis revealed two competitive responses to nitridation. Some regions revealed a nitrogen atom fraction approaching 25 at. pct—greater than 10^6 times the equilibrium solubility limit at room temperature. Remarkably, there is no expansion or distortion of the body-centered cubic lattice. This is similar to the response of -ferrite in this alloy to low-temperature carburization. In conventional transmission electron microscopy bright-field images, the supersaturated ferrite grains show no diffraction contrast—resembling the appearance of amorphous structures—suggesting an unusually high defect density. These grains exhibit spinodal decomposition of the ferrite to nanometer-scale Fe-rich and Cr-rich ferrite domains. High-resolution imaging reveals pristine Fe-rich nanocrystals, whereas the Cr-rich domains are apparently amorphous. Elsewhere in the nitrogen-rich case, an isothermal ferrite-to-austenite phase transformation occurred. The austenite transformation product formed martensitically with a high-aspect-ratio plate-like morphology in the Nishiyama–Wassermann orientation relationship to the ferrite matrix.