Modulated substitutional-interstitial solute–atom clustering in nitrided austenitic Fe–34Ni–V–alloys

The microstructure of Fe–34–Ni–V alloys internally nitrided in NH3/H2 at 500–900°C is investigated by electron microscopy and X-ray methods. At low solute supersaturations and high temperatures (above 780°C) homogeneous precipitation of the equilibrium face-centred cubic nitride VN occurs in austenite. However, above a critical supersaturation determined by the nitrogen activity (aN), the vanadium activity (av), and the temperature, a very fine modulated microstructure consisting of meta stable substitutional-interstitial solute–atom clusters, periodically distributed along <100>, is developed. On overageing, the modulated state transforms either directly to spheroidal VN precipitates in nitrogen-saturated austenite or via a grain boundary discontinuous precipitate reaction to thin laths of VN in austenite. The metastable modulated state is characterized by (i) periodic electron microscopy images, (ii) sidebands in both electron and X-ray diffraction patterns and, (iii) unit-cell dimensions increased with respect to those of the equilibrium Fe–34Ni–N austenite. The effects of the thermodynamic factors aN and av and of temperature on the development and overageing behaviour of the modulated structures are described and discussed. It is concluded that the mechanism by which the periodic substitutional-interstitial solute–atom clusters develop in nitrided austenite is consistent with spinodal decomposition.