In situ investigation of phase formation during low energy ion nitriding of Ni80Cr20 alloy

The formation of expanded austenite after nitrogen insertion into austenitic stainless steel, CoCr or Ni base alloys is characterised by an unusually high nitrogen content in solid solution representing a diffusion layer. However, the process is still not completely understood. Using in situ X-ray diffraction (XRD) experiments and additional data from SEM, the phase formation and diffusion after low energy nitrogen ion implantation into commercial Ni80Cr20 (Nichrome) are investigated in the temperature range from 350 to 500 °C. A compound layer consisting of a phase similar to fct Ni4N is formed on the surface, most likely with a composition of (Ni,Cr)4N, followed by a diffusion zone extending about 1 μm into the material. For increasing temperature, the total nitrogen uptake is decreasing. The time evolution shows that a saturation is reached earlier for higher temperatures whereas no saturation has been found for 375 °C and 90 min. A competition exists between nitrogen loss at the surface and nitrogen supply by the ion beam. This loss process is thermally activated and depends on the surface orientation. Additionally, at 500 °C some CrN precipitates are found.