Microstructure and properties of plasma nitrided layers on Ni-based superalloy Ni-20Cr

Highlights: • Ni-based superalloy Ni20Cr was plasma-nitrided at low temperature of 450 °C. • The nitrided layer exhibited "featureless" appearance in OPM and SEM. • The phases present in the layer was revealed by XRD to be γ_N and CrN. • Moiré pattern was revealed by TEM due to coexisting of γ_N and CrN in the layer. • Tribological behavior of nitrided Ni-based superalloy was improved significantly. -- Abstract: Ni-base superalloy Ni20Cr was plasma-nitrided at low temperature of 450 °C. Microstructural characteristics of the case layer were investigated by X-ray diffraction (XRD), optical microscopy (OPM) and scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A thin nitrided layer of about 4 μm was produced on the nitrided Ni-based superalloy which exhibited "featureless" appearance in OPM and SEM observations without strong etched regions, usually representative of nitride precipitation. Nevertheless, XRD revealed the nitrided layer contained an expanded austenite γ_N phase and CrN precipitates. TEM image of the nitrided layer exhibited the Moiré patterns resulting from overlapping of the γ_N phase and CrN lattices. It was suggested that the nature of the nitrided layer was a N-saturated matrix in which nanometric CrN precipitates dispersed. The orientation relationship between CrN and γ_N corresponded to Baker-Nutting relationship: (100)_CrN // (100)_γ; [001]_CrN // [001]_γ. The small lattice expansion and distortion occurred on the γ_N phase due to lower solubility of nitrogen in the austenite containing high Ni content, compared to the Fe-rich alloy. Lower saturation of nitrogen in the γ_N phase and precipitation of CrN in the γ_N phase are attributed to the repulsion of excessive nitrogen by nickel and the trapping effect of nitrogen played by chromium. The nitrided surface was found to be a three-time increase in hardness. Tribological behavior of nitrided Ni-based superalloy was improved significantly due to increased surface hardness. The corrosion resistance of nitrided Ni-based superalloy was not deteriorated but a little improvement in spite of the precipitation of nanometric CrN in γ_N phase as the matrix of case-layer.