Plasma assisted nitriding of Ni-based superalloys with various microstructures

Ni-based superalloys are composed of a matrix with an austenitic face-centered cubic (FCC) crystal structure (γ phase) which is generally strengthened by the presence of a high fraction of ordered FCC precipitates γ′ (Ni3(Ti,Al) type) and/or γ″ (Ni3Nb type). Plasma assisted nitriding of various Ni-b...

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Bibliographic Details
Published in:Surface & coatings technology 2013-11, Vol.235, p.318-325
Main Authors: Chollet, S., Pichon, L., Cormier, J., Dubois, J.B., Villechaise, P., Drouet, M., Declemy, A., Templier, C.
Format: Article
Language:English
Subjects:
Applied sciences
Austenitic stainless steels
Cross-disciplinary physics: materials science
rheology
Exact sciences and technology
Expanded phase γN
Face centered cubic lattice
Heat treatment
Intermetallic compounds
Magnesium base alloys
Materials science
Metals. Metallurgy
Nickel base alloys
Nickel-based superalloys
Nitrides
Nitriding
Physics
Precipitates
Precipitation
Production techniques
Superalloys
Surface treatments
Swelling
Thermochemical treatment and diffusion treatment
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Summary:Ni-based superalloys are composed of a matrix with an austenitic face-centered cubic (FCC) crystal structure (γ phase) which is generally strengthened by the presence of a high fraction of ordered FCC precipitates γ′ (Ni3(Ti,Al) type) and/or γ″ (Ni3Nb type). Plasma assisted nitriding of various Ni-based superalloys, either polycrystals (Haynes®230, Inconel®718, Udimet®720Li, N18) or single crystals (MC2 and MC-NG), presenting from 100vol.% to 30vol.% of γ phase, has been performed at moderate temperature (400°C) to shed light on the specific responses of the different (δ, γ′, γ″) precipitates under nitriding. Characterizations by glow discharge optical emission spectroscopy and scanning electron microscopy have evidenced the formation of nitrided layers with thicknesses varying from ~1μm to ~8μm depending on alloys and treatment duration. In all alloys, X-ray diffraction on nitrided samples confirms that the γ phase is nitrided, as in austenitic stainless steels, to form the expanded austenitic FCC phase γN, a nitrogen insertion solid solution, with a mean concentration of ~25at.% N. CrN nitrides were also identified. The incorporation of nitrogen within the precipitates appeared to depend on the alloy microstructure: for polycrystals, regardless of their proportion, the δ, γ′ or γ″ precipitates seem nitrided in similar proportion than the matrix γ although the exact nature (nitrides or expanded δ, γ′, γ″ phases) of the obtained nitrided phases was not confirmed. On the contrary, in the single crystal MC2, the γ′ precipitates accommodate a much lower nitrogen amount, in the range 3–5at.%. Whatever the materials, the surface swelling induced by the nitrogen uptake is only relative to the total incorporated nitrogen quantity and is roughly independent on the nature of the nitrided phases. •Polycrystalline and single crystalline Ni-based superalloys were plasma nitrided at 400°C.•The FCC γ phase is nitrided over few microns with about 20–25at.% N after few hours.•In polycrystals, the δ, γ′, and γ″ precipitates accommodate similar amount of nitrogen as in the γ phase.•In single crystals, the γ′ precipitates are almost not nitrided.•Nitrogen incorporation induces cell expansion and surface swelling.
ISSN:0257-8972
1879-3347
DOI:10.1016/j.surfcoat.2013.07.060