An investigation of precipitation strengthened Inconel 718 superalloy after triode plasma nitriding

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 °C (i.e. thermodynamic paraequilibrium conditions) and a high treatment temperature of 700 °C. At low treatment temperatures, apart from the formation of nitrogen-expanded austenite (γ _N) from the high-Cr γ matrix, the pre-existing γ′ and γ″ intermetallic nano-precipitates appear to exhibit different nitriding responses. The spheroidal N-modified γ′ (or γ′ _N) precipitates were ‘slightly-expanded’, leading to slightly shifted XRD peaks, i.e. 2-theta angles of ~0.2° from γ _substrate(111) and ~0.5° from γ _substrate(200). In contrast, N-modified γ″ (or γ″ _N) could experience substantial lattice expansion close to that of the γ _N matrix. With increasing treatment temperature, nitride formation started as additional nano-sized precipitates (e.g. ~3–6 nm diameter as observed at 450 °C) and grew into laths (e.g. ~5–10 nm thick and ~15–30 nm wide as observed at 700 °C). Without changing core microstructure/properties, surface nitrogen modification and hardening were obtained on alloy 718 after TPN (e.g. from ~486 HV _0.025 to ~1212 HV _0.025 after TPN at 400 °C). No degradation of corrosion performance was observed for the nitrogen-supersaturated surface after TPN at 400 °C. However, the 450 °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 °C is due to pronounced nitride formation and segregation of substitutional alloying elements.