Influence of carbon and nitrogen on electronic structure and hyperfine interactions in fcc iron-based alloys

Carbon and nitrogen austenites, modeled by Fe8N and Fe8C superstructures are studied by full-potential LAPW method. Structure parameters, electronic and magnetic properties as well as hyperfine interaction parameters are obtained. Calculations prove that Fe-C austenite can be successfully modeled by ordered Fe8C superstructure. The results show that chemical Fe-C bond in Fe8C has higher covalent part than in Fe8N. Detailed analysis of electric field gradient formation for both systems is performed. The calculation of electric field gradient allow us to carry out a good interpretation of Moessbauer spectra for Fe-C and Fe-N systems.Comment: 8 pages, 3 figures, IOP-style LaTeX, submitted to J. Phys. Condens. Matte

ÉTUDE PAR SPECTROMÉTRIE MÖSSBAUER DES SOLUTIONS SOLIDES Fe-N CUBIQUES ET TÉTRAGONALES

Les différents environnements détectés par spectrométrie Mössbauer permettent de préciser les distributions d'interstitiels dans l'austénite γ et les martensites α' et α" ainsi que l'influence des lacunes dans Fe4Nγ'. Les paramètres hyperfins sont comparés aux valeurs dans Fe-C et Fe-N.Mössbauer spectra of Fe-N solid solutions reveal new environments which allow the accurate determination of the interstitial distributions in austenite (γ) and martensites (α') and (α"). The influence of vacancies is detected in γ'(Fe4N). The hyperfine pararneters are compared to other published values for Fe-C and Fe-N

Etude des phénomènes de précipitation dans un acier austénitique à 19% de chrome et 19% de manganèse, et à très forte teneur en azote

This paper describes the precipitation evolution that occurs in a not prestmined 1%N Mn-Cr austenitic steel aged in the [40O°C-900°C] temperature range, and for various times up to 150 hours. Thermal treatments carried out between 700°C and 900°C lead to a discontinuous precipitation of plates of Cr2N at grain boundaries. The nucleation stage of this precipitation reaction arises fiom the migration of the grain boundary along the high-energy interface, in order to reduce the interfacial energy of the primary surface between the plate and the grain boundary. On the contrary, the cell growth is characterized by numerous properties that differ from the hypotheses of usual theories. These non-steady state growth features arise from the continuous modifications of the diffusion-controlled mechanisms during isothermal aging. Beside this cellular precipitation, the sigma phase forms with significant volume fractions in the cells