The nature of expanded austenite

This paper attempts to reduce some of the confusion that exists over the na ture of the nitrogen-rich layer produced by nitriding austenitic stainless steel at temperatures below 500 degreesC. Cross-sectional transmission elec tron microscopy shows that the modified layer is dominated by a cubic phase with considerable expansion of the austenite lattice. In some cases, a thi n (< 0.1 <mu>m) layer of CrN and alpha -Fe precipitates occurs right at the surface. There is an increase in dislocation density near the interface wi th the underlying material. These sublayers are not observed in X-ray diffr action, which confirms the dominant phase as an expansion of the austenite lattice and indicates a slight triclinic distortion, although a tetragonal lattice is not completely ruled out. Magnetic-force microscopy shows that t he expanded austenite is ferromagnetic over most of the layer but becomes p aramagnetic as the nitrogen concentration drops towards the interface with the underlying material. These features are common to austenitic stainless steel nitrided by low-pressure rf plasmas, plasma-immersion ion implantatio n and pursed dc-glow discharges. (C) 2000 Elsevier Science B.V. All rights reserved.