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-based superalloys, either polycrystals (Haynes®230, Inconel®718, Udimet®720Li, N18) or single crystals (MC2 and MC-NG), presenting from 100 vol.% to 30 vol.% 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 ~ 1 μm to ~ 8 μm 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 ~ 25 at.% 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 3-5 at.%. 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.