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

Xiao Tao, John Kavanagh, Xiaoying Li, Hanshan Dong, Allan Matthews, Adrian Leyland

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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 (γN) 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 γsubstrate(111) and ~0.5° from γsubstrate(200). In contrast, N-modified γ″ (or γ″N) could experience substantial lattice expansion close to that of the γN 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 HV0.025 to ~1212 HV0.025 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.

Original languageEnglish
Article number128401
JournalSurface and Coatings Technology
Publication statusPublished - 25 Jul 2022


  • Expanded austenite
  • Ni-based superalloys
  • Nitriding
  • Precipitation
  • Transmission electron microscopy (TEM)


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