Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The nature of the S phase in austenitic stainless steel (ASS) produced by incorporation of large amounts of nitrogen is investigated. The S phase has been produced by plasma nitriding of AISI 304L ASS at 400 °C for 30 h. The S phase structure was studied using a combination of global (X-ray diffraction (XRD), nuclear reaction analysis, glow discharge optical emission spectroscopy) and local (Conversion Electron Mössbauer, X-ray absorption near edge structure, extended X-ray absorption fine structure (EXAFS) spectroscopies) probe techniques. This combination of analytical methods allows direct comparison of the data obtained from atomic up to macroscopic levels. The results show that Fe, Cr and Ni have different local chemical environments: Cr is very similar to CrN, Fe is in a phase similar to Fe4N, while Ni stays metallic. The N coordination is by a factor of 3 higher around than around Fe while negligible around Ni. Three lattice parameters for Fe, Cr and Ni, and a large disorder have been detected by EXAFS analysis. None of these phases could be identified by XRD, except a set of broad peaks which are commonly attributed to the supersaturated nitrogen solid solution in the literature. It is suggested that the incorporation of interstitial nitrogen destabilizes homogeneous distribution of ASS constituents and leads to the segregation of elements with the zones rich in Cr and Ni being small and coherent with the Fe-rich matrix. The data provide strong evidence that the decomposition of the nitrogen saturated ASS is of spinodal nature. The findings of this work on S phase microstructure are correlated to mechanical, magnetic and electrochemical properties.