The Nature and Origin of “Double Expanded Austenite” in Ni-Based Ni-Ti Alloys Developing Upon Low Temperature Gaseous Nitriding

Gaseous nitriding of Ni-4 wt pct Ti alloy plates led to the development of double expanded austenite (γ _N1 and γ _N2) at the surface of the nitride plates. Grazing-incidence X-ray diffraction analysis demonstrated that the component γ _N1 is located close to the surface and the component γ _N2 is located at a certain depth below the specimen surface, in correspondence with a layered character of the nitrided zone beneath the surface as revealed by optical microscopy. Electron probe microanalysis, atom probe tomography, and Laue microdiffraction analysis did not reveal a significant difference in nitrogen content of the γ _N1 and γ _N2 sublayers. By X-ray diffraction stress analysis it was shown that the only significant differences of the two expanded austenite layers is a pronounced difference in compressive stress parallel to the surface: the γ _N1 layer is subjected to a huge compressive stress, as large as a few GPa, whereas a relatively modest stress prevails in the γ _N2 layer.