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.
Bibliographical noteFunding Information: We acknowledge the support provided by Dr. Peng Zeng and Dr. Le Ma from the Sorby Centre for Electron Microscopy at the University of Sheffield and by Mr. Paul Stanley from the Centre for Electron Microscopy at the University of Birmingham.
Publisher Copyright: © 2022 Elsevier B.V.
- Expanded austenite
- Ni-based superalloys
- Transmission electron microscopy (TEM)
ASJC Scopus subject areas
- Condensed Matter Physics
- Surfaces and Interfaces
- Surfaces, Coatings and Films
- Materials Chemistry