Role of Interstitial and Interstitial-Impurity Interaction on Irradiation-Induced Segregation in Austenitic Steels
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*CEA-Saclay, DECM/SRMP, 91191 Gif sur Yvette, France **Illinois Univ., Dept of Materials Science and Engineering, Urbana, IL ***EDF, DER/EMA, Moret-sur-Loing, France ABSTRACT Segregation under irradiation in austenitic steels is due to a permanent flux of vacancies and interstitials produced by irradiation towards sinks like surfaces and interfaces. A model based on a mean field lattice rate theory is proposed where kinetics and thermodynamics are treated in a mutually consistent way. For a Fe-Ni-Cr ternary alloy, the 15 parameters defining the jump frequencies of vacancies were fitted on equilibrium properties including ordering energies and tracer diffusion experiments with no use of segregation data. Measurements of RIS by Auger Electron Spectroscopy (AES) were used in the last step of the fitting procedure in order to choose the best set of the 27 interstitial jump frequencies. This fitting procedure strongly supports the idea that the interstitials are contributing to RIS in Fe-Cr-Ni alloys. We also simulate the trapping of interstitials by an impurity model and reproduce the total inhibition of RIS by this impurity as observed experimentally [1]. INTRODUCTION Radiation-induced segregation (RIS) in austenitic Fe-Cr-Ni leads to a chromium depletion at grain boundaries, and this depletion is suspected to favor stress corrosion craking in pressurized water reactors' core components [2]. An important concern is therefore the understanding of the relevant parameters which govern segregation, as well as the ability to design alloys with controlled segregation while in service. RIS is due to the permanent flux of vacancies and interstitials produced by irradiation towards sinks such as surfaces and interfaces, more specifically to the differences in atom-vacancy jump rates (the inverse Kirkendall effect) and in interstitial jump rates. Since the developement of the most commonly used model of RIS [3], a main improvement has been to realize that mobility of vacancy depends on the local composition of the alloy, and therefore is affected by thermodynamic parameters, such as enthalpy of mixing and surface energies [4, 5]. Allen et al.[5] applying Grandjean's method for calculating migration energies improved their RIS predictions compared to those obtained with the Perks model for Fe-Ni-Cr alloys [3]. Their conclusion is that segregation data can be reproduced by only involving vacancy fluxes. This conclusion is however arguable since they use ordering energies as fitting parameters to reproduce segregation data. We propose here a model based on the lattice rate equations of Grandjean [4] which treats both vacancy and interstitial migration. The 15 parameters defining the vacancy jump frequencies are fitted on equilibrium properties including ordering energies and tracer diffusion experiments with no use of segregation data. The 27 jump frequencies of dumbbells are fitted using the effective migration energies of interstitials measured by Benkaddour et al. [6]. Several sets of parameters give a good agreement with Benka
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