Abstract
In this study, we investigated the microstructural evolution, surface hardening and general corrosion properties of a precipitation-strengthened Inconel 718 Ni-superalloy after triode-plasma nitriding (TPN) at low treatment temperatures of 400-450 degrees C (i.e. thermodynamic paraequilibrium conditions) and a high treatment temperature of 700 degrees C. At low treatment temperatures, apart from the formation of nitrogen-expanded austenite (gamma N) from the high-Cr gamma matrix, the pre-existing gamma' and gamma '' intermetallic nano-precipitates appear to exhibit different nitriding responses. The spheroidal N-modified gamma' (or gamma'(N)) precipitates were 'slightly-expanded', leading to slightly shifted XRD peaks, i.e. 2-theta angles of similar to 0.2 degrees from gamma(substrate)(111) and similar to 0.5 from gamma(substrate)(200). In contrast, N-modified gamma '' (or gamma ''(N)) could experience substantial lattice expansion close to that of the gamma(N) matrix. With increasing treatment temperature, nitride formation started as additional nano-sized precipitates (e.g.-3-6 nm diameter as observed at 450 degrees C) and grew into laths (e.g.-5-10 nm thick and-15-30 nm wide as observed at 700 degrees C). Without changing core microstructure/properties, surface nitrogen modification and hardening were obtained on alloy 718 after TPN (e.g. from-486 HV0.025 to-1212 HV0.025 after TPN at 400 degrees C). No degradation of corrosion performance was observed for the nitrogen-supersaturated surface after TPN at 400 degrees C. However, the 450 degrees C TPN-treated surface showed a slightly increased current density in the anodic region, which can be associated with early-stage nitride formation. The significantly deteriorated corrosion performance after TPN treatment at 700 degrees C is due to pronounced nitride formation and segregation of substitutional alloying elements.
NSTL
Elsevier