Reactive magnetron sputtering as a way to improve the knowledge of metastable f.c.c. nitrogen solid solutions formed during plasma assisted nitriding of Inconel 690

Low temperature plasma assisted nitriding (PAN) treatments of Inconel 690 (a nickel base alloy with a 30 wt.% chromium content) produce a complex layer constituted by two different metastable f.c.c. solid solutions denoted (γN1 and γN2). In addition, different layer thicknesses are observed for differently oriented grains. Reactive magnetron sputtering (RMS) of Inconel 690 in argon–nitrogen containing mixtures is used to produce homogeneous films constituted by a well defined γN phase. In reactive conditions, the as-deposited coatings (T<100 °C) containing up to 30 at.% of nitrogen are nanocrystalline supersaturated f.c.c. solid solution (γN) with a grain size lying between 3 and 7.5 nm. A comparison between the products synthesized by these two techniques (PAN and RMS) is presented in this study. The formation of γN phases in low temperature PAN is discussed in the light of electron backscatter diffraction measurements and by comparison with X-ray diffraction patterns of RMS coatings. By using the results obtained in situ by RMS on heated substrates and by tempering of as-deposited films, the decomposition products of the γN phase at high temperature (⩾450 °C) or long PAN treatments at 400 °C are identified to be f.c.c. CrN and γ(Ni,Fe) depleted in nitrogen.