Analysis of in situ XRD measurements for low energy ion beam nitriding of austenitic stainless steel

• In situ XRD investigations can help to understand the nitriding mechanism.
• Transition from supply to limited layer growth has been observed.
• Deduction of the lifetime of expanded austenite
• Significant stress or stress gradients inside expanded austenite layers

The formation of expanded austenite after nitrogen insertion into austenitic stainless steel, CoCr or Ni base alloys is still not completely understood. Ex situ X-ray diffraction (XRD) investigations, which can be very sophisticated nowadays, lack the exact time evolution while additional annealing processes may occur during cooling phases. Furthermore a time resolution of a few minutes is required, necessitating modern detectors. After mastering the challenge of building such equipment, in situ experiments can lead to a multitude of information which can be used to reassess current models. Here, a critical discussion about the topics where these in situ XRD investigations during low energy ion implantation (LEII) can help for further understanding and where they cannot help is presented. Especially, the following subjects are addressed: (i) influence of ion current density and grain orientation on diffusion and phase formation, (ii) decay of expanded austenite, (iii) correlation between lattice expansion and nitrogen content, and (iv) mechanical deformation of expanded austenite. While results on the first two points can be readily obtained, additional, external information on the nitrogen content and the orientation distribution function of grains is necessary for the latter two points, respectively.