A comparative study on microstructure of the plasma-nitrided layers on austenitic stainless steel and pure Fe

The microstructure of the plasma-nitrided layer on austenitic stainless steel and pure Fe was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the nitrided layer on pure Fe is composed of equilibrium phases ε-Fe2–3N and γ′-Fe4N, which have lamellar microstructure. The superlattice reflections appear in XRD pattern and electron diffraction pattern, which correspond to the ordered γ′-Fe4N. The nitrided layer on austenitic stainless steel is composed of a metastable supersaturated nitrogen solid solution, whose XRD pattern and electron diffraction patterns show that it has an expanded fcc structure and its lattice parameter is much larger than that of the γ-austenite, even larger than that of γ′-Fe4N. The superlattice reflections similar to ones of γ′-Fe4N resulting from the nitrogen atoms at octahedral holes of cubic center can be observed in its electron diffraction pattern although no superlattice reflections were detected in its XRD pattern. However, the diffracted intensity of the superlattice reflections is weaker than that of the γ′-Fe4N formed on pure Fe. Calculation of the structure factor of the supersaturated solid solution suggests that weaker diffracted intensity of the superlattice reflections is attributed to the nitrogen atoms at octahedral holes of the edges in fcc lattice. Their fundamental electron diffraction reflections show intense diffuse scattering effect resulting from the clustered Cr–N. The results indicate that the supersaturated nitrogen solid solution is based on γ′-Fe4N and nitrogen atoms supersaturated in γ′-Fe4N are in form of the clustered Cr–N. A lot of slip bands are formed on the layer of austenitic steel due to the strain resulting from supersaturating nitrogen.