Abstract
The nature of the near-surface γ N phase produced by low-temperature (∼400 °C) plasma-assisted nitriding of an austenitic stainless steel 304L is studied. A combination of global probes (X-ray diffraction, nuclear reaction analysis, glow discharge optical emission spectroscopy) and local probes (field ion microscopy, conversion electron Mössbauer, X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies) is employed to reveal the morphology, phase structure, atomic ordering and chemical state of the obtained γ N phase. The results consistently reveal the heterogeneous nature of the nitrided layer consisting of nanometric CrN precipitates embedded in a Fe 4N-like matrix. The size of the precipitates is found to be larger at the surface than at the nitrided layer-steel interface. The precipitates have irregular, sphere-like shapes. Moreover, X-ray spectroscopic investigation revealed three different intermetallic distances and different chemical environments for Fe, Cr and Ni, accompanied by a large static disorder. These findings suggest that the presence of the interstitial N destabilizes the homogeneous element distribution in 304L even at such low temperatures. This leads to the segregation into Cr-rich zones that are coherent with the Fe 4N matrix. Possible atomistic decomposition mechanisms are discussed. Based on the heterogeneous nature of the γ N phase revealed in 304L, an alternative view of its remarkable combination of properties such as large hardness, induced ferromagnetism and preserved corrosion resistance is considered.
Original language | English (US) |
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Pages (from-to) | 4065-4076 |
Number of pages | 12 |
Journal | Acta Materialia |
Volume | 60 |
Issue number | 10 |
DOIs | |
State | Published - Jun 2012 |
Externally published | Yes |
Keywords
- Decomposition
- Interstitial diffusion
- Precipitation
- Stainless steel
- Surface alloying
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys