Nitrogen diffusion in single crystalline austenitic stainless steel during ion beam nitriding and subsequent thermal annealing

Nitrogen diffusion in single crystalline austenitic stainless steel during ion beam nitriding and subsequent annealing is investigated. Single crystalline [orientations (001), (110) and (111)] and polycrystalline AISI 316L austenitic stainless steel (ASS) has been ion beam nitrided at 400 °C for 60 min using a Kaufman-type ion source with an acceleration voltage of 1 keV and a current density of 0.5 mA cm−2. XRD analysis shows the presence of the phase usually called “expanded” austenite or γN phase. The samples have been subsequently vacuum annealed at 400°C for 30 min. The nitrogen distribution profiles have been determined using nuclear reaction analysis (NRA). The profiles of as-nitrided samples can be depicted by an initial quasi-linear decrease followed by a sharp leading edge. Despite identical nitriding conditions, the nitrogen penetration depth is significantly higher in the single crystal with the (001) orientation than in the samples with the orientations (011) or (111) which cannot be explained by the orientational dependence of the sputtering rate. The surface concentration for the single crystal with the (001) orientation was about 27 at.% with the depth of the quasi-linear part about 1.5 µm. For the orientations (011) and (111) the surface concentration was 25 at.% and the depth of the quasi-linear part about 1 µm and 0.7 µm respectively. Polycrystalline ASS presents an intermediate case. Subsequent annealing results in the decrease of near-surface nitrogen concentration, flattening of the quasi-linear part of the depth profile and nitrogen inward diffusion without any detectable nitrogen loss due to out-diffusion.
The “trapping-detrapping” model, which is able to reproduce the full shape of the nitrogen depth profile in as-nitrided as well as subsequently annealed single crystalline ASS, has been used to fit nitrogen depth profiles and extract the diffusion coefficient values. The nitrogen profile fitting shows that the nitrogen diffusion coefficient is strongly orientation dependent during ion beam nitriding confirming the tendencies deduced directly from the nitrogen distribution profiles. This anisotropy is not present during thermal annealing, while the diffusion coefficients extracted from the fitting exhibit significantly lower values than those obtained from as-nitrided samples.