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

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 γN phase and CrN precipitates. TEM image of the nitrided layer exhibited the Moiré patterns resulting from overlapping of the γN 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 γN corresponded to Baker-Nutting relationship: (100)CrN // (100)γ; [001]CrN // [001]γ. The small lattice expansion and distortion occurred on the γN 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 γN phase and precipitation of CrN in the γN 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 γN phase as the matrix of case-layer.