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Modeling the Diffusion of Interstitial Impurities and their Impact on the Ageing of Ferritic Steels

Abstract : The diffusion of interstitial atoms (C and O) in bcc solid solutions (Fe-Cr and Nb-V) is modelled and compared to experimental data. A set of binding energies and migration barriers for the direct interstitial diffusion mechanism in different local chemical environments are first calculated using Density Functional Theory. Two different pair interaction models are developed in order to reproduce these data and predict the migration barriers in all possible environments. The diffusion models are then implemented in a kinetic Monte Carlo method to simulate tracer diffusion experiments, using a standard procedure, and internal friction experiments, using a novel method. In the Fe-Cr-C systems our internal friction simulations show a unique Snoek peak in the whole concentration range, between pure iron and pure chromium. The average migration enthalpy for C diffusion in Fe-Cr alloys is found to increase progressively with the Cr concentration, with a small rate below 6 %Cr. In Cr-rich alloys, the effective migration barrier for C diffusion is found to be larger in tracer diffusion than in the internal friction simulations. We conclude that the effective migration barrier extracted from tracer diffusion is closely related to trapping effects of C atoms in Fe-rich local environments, whereas the migration barrier associated with internal friction is mainly controlled by the spectrum of migration barriers of the most frequent configurations, as it is clearly shown in the Cr-rich domain. In the dilute Nb-V-O alloys, we find a high temperature Snoek peak when the concentration of oxygen is lower than the vanadium content. But when the oxygen concentration is higher, we see the appearance of a second peak but at a lower temperature and a shift of the first peak to lower temperatures. We conclude that the high temperature peaks correspond to the oxygen-vanadium pairs, and the low temperature peak corresponds to the oxygen Snoek peak in pure Nb. We also use our model in order to validate the Koiwa model in infinitely dilute ternary alloys. Both approaches are in good agreement and we observe that the high temperature peak cannot be directly related to a single jump frequency but to a complex function of them. We also show preliminary results on the effect of C in the precipitation of the Fe-Cr phase separation under irradiation. We observee that a strong attraction between carbon atoms and point defects (vacancies and self-interstitials) might be able to slow down the acceleration of the α’ precipitation.
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Submitted on : Wednesday, February 20, 2019 - 9:43:08 AM
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Rafael Herschberg Basualdo. Modeling the Diffusion of Interstitial Impurities and their Impact on the Ageing of Ferritic Steels. Materials Science [cond-mat.mtrl-sci]. Université Paris-Saclay, 2018. English. ⟨NNT : 2018SACLS443⟩. ⟨tel-02026020⟩



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