Low energy ion accelerated diffusion of interstitial nitrogen in austenitic stainless steel

N diffusion in austenitic stainless steel (ASS) under low energy Ar ion bombardment is investigated. ASS samples were ion beam nitrided at 400°C with the N ion energy of 1 keV and flux of 3.11015 s-1 cm-2. The nitriding time was adjusted to obtain a thickness of the nitrided layer of ~1m. The samples were subsequently annealed at 400°C under simultaneous 700 eV Ar+ bombardment with different fluxes (1.3-3.8×1015 s-1 cm-2). N depth profiles were obtained using nuclear reaction analysis, and the phase structure was measured using X-ray diffraction (XRD). XRD reveals the modified layer to be composed of only γN phase in both as-nitrided and Ar+ irradiated samples, with no evidence of nitride formation. It is observed that Ar+ bombardment increases the N mobility in the depths far beyond the ion penetration depth, resulting in an increased broadening of the N depth profile as a function of Ar+ flux. This effect cannot be explained by any established mechanism of radiation enhanced diffusion. Another set of samples was annealed at 400°C under low flux (4×1011 s-1 cm-2) 700 eV Ar ion bombardment of variable charge state (from 1+ to 8+). These charge states correspond to the potential energies of 16-578 eV. The latter is dissipated by electronic excitations in metals. It was found that low flux Ar ion bombardment enhances the N mobility; however, the ion potential energy does not play any role on deep N diffusion. The latter indicates that N diffusion enhancement over large distances (~1 m) is due to nuclear collisional processes during ion stopping in the matter. Tentatively, the observed phenomenon is discussed in terms of collisionally induced quasi-particles which propagate large distances from the layers directly affected by ion bombardment enhancing interstitial N diffusion.