Low temperature nitriding behaviour of Ni3Al-like γ′ precipitates in nickel-based superalloys
Résumé
Several nickel-based superalloys were nitrided at 400 °C for 4 h in a plasma-assisted low pressure gaseous nitriding set-up. Composed of an austenitic γ phase matrix with Ni3(Al,Ti,Nb)-type γ′ precipitates, the studied superalloys include the polycrystalline Udimet®720Li (U720), the directionally solidified MarM200-DS and the single crystal René N4 and MC2 superalloys. The nitrided samples were characterized by scanning electronic microscopy, energy dispersive spectroscopy and glow discharge optical emission spectroscopy to determine the nitriding behaviour of the γ′ precipitates. The nitrogen incorporation is confirmed in the γ phase, up to 25–30 at.%, corresponding to the expanded γN phase, similar to the one obtained in low temperature nitriding of alloys with a pure austenitic structure. However various behaviours of the γ′ precipitates were observed in the different alloys. Various complementary experiments have shown that the γ′ nitriding is not dependent on the size of the γ′ precipitates, on the γ′ crystalline orientation nor on the γ/γ′ interface coherency. The main influent parameter is the intrinsic γ′ composition, that is the Al substitution by solute elements like Ti or Nb: the closer to pure Ni3Al is the γ′ composition, the lower is the nitrogen incorporation. This influence of the composition on the nitriding efficiency is discussed through thermodynamic and diffusion kinetic considerations. However the γ′ nitriding is thought to be favoured by the propagation of dislocations in the γ′ precipitates thanks to the high compressive stress induced by the surrounding γN phase expansion. So, the γ′ chemical composition controls the γ′ nitriding ability as it influences the shear strength of the γ′ precipitates.