[en] Implantation of a low carbon austenitic stainless steel, type 316L, with N₂⁺ ions is expected to improve wear resistance and fatigue behaviour. This stainless steel is known to have good corrosion resistance. The improvement of mechanical properties and fatigue corrosion resistance which results from nitrogen implantation cannot occur in the event of the formation of chromium nitrides, which are known to induce pitting corrosion. The aim of this work was to determine implantation conditions that avoid the formation of CrN and Cr₂N nitrides. Implantation was performed under the following conditions: 100% N₂⁺ ions, 5 μA cm^-2 ion current, 80 keV beam energy and samples cooled by water circulation, mean temperature less than 70 ^oC at the surface. Microstructural changes were determined by conversion electron Mossbauer spectroscopy and grazing X ray diffraction. Both techniques allow the analysis of the shallowest surface layers corresponding to the implanted region (≤200 nm). The results lead to the following conclusions. Nitrogen implantation, at 2 x 10¹⁷ ions cm^-2 fluence and 5 μA cm^-2 current density, on 316L stainless steel leads to two austenitic phases, γ_R and γ_P, in the first 0.2 μm. The nitrogen concentration of γ_R phase is much higher than in thermodynamic equilibrium conditions: 15N/100Me. The reaction 316L + implantation → γ_R + γ_P is made possible by a redistribution of interstitial (N) as well as substitutional atoms. There is no evidence of formation of chromium or iron, nitrides. However, γ_R could act as a precursor of CrN-like nitride because of the similarity of crystallographic structures. (3 figures, 1 table, 12 references) (Author)

[en] Moessbauer Spectra of Fe_1-xNi_xN_y austenite with 0< x<0.4 and 0< y<0.1 display environments for which hyperfine parameters and local Debye temperature vary discretely with interstitial neighbourhood. The role of vibrational contribution are discussed in accordance with the central atom model. (orig.)

[en] This work aims to examine the effects of ion energy and current density during nitrogen implantation of steel. The microstructure and the nitrogen distribution at the surface of austenitic and ferritic steels were compared and discussed. The surface of the austenitic X2CrNiMo17-13-2 and ferritic X1CrNiMoNb28-4-2 steel was nitrided by N⁺ ions implantation at doses of 0.3-10 x 10¹⁷ N⁺cm^-2, with an energy of 60 or 100 keV. The beam current density is 1.5 - 3.5 μAvcm^-2. X ray diffraction (XRD), conversion electron Moessbauer spectroscopy (CEMS) were used to investigate the nitride phases resulting from the nitrogen implantation. The formation of Cr₂N, α'-martensite, γ_R-N enriched austenite, Fe₄N γ'-like structure, h.c.p. ε and metastable f.c.c. phases is also detected depending on steel grade and nitriding conditions. Evolution of the phase formation is associated with chromium and nitrogen migration and segregation during the nitriding process. (orig.)

[en] Different experimental measurements show that transformations of martensite occur from very low temperature, often below 77K. In order to understand the redistribution of interstitial atoms which is studied by Moessbauer spectrosopy near room temperature the previous low temperature changes are briefly discussed. The Moessbauer data related to cubic γ, γ', α' and α'' Fe-N alloys are presented. It is shown that the interstitial atom distribution and redistribution of interstitital atoms is partially inherited from austenite and that γ' Fe₄N nuclei corresponds in γ and α' to N-Fe-N chains oriented along left angle 001 right angle in γ and along left angle 110 right angle in α'. (orig.)