Effect of Carbon Impurity Content on Microstructural Evolution in Neutron-Irradiated Alpha Iron: Cluster Dynamics Modeli
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Effect of Carbon Impurity Content on Microstructural Evolution in Neutron-Irradiated Alpha Iron: Cluster Dynamics Modeling Yosuke Abe1, Tomohito Tsuru1 and Shiro Jitsukawa1, 2 1 Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, 2-4 Shirakatashirane, Tokai-Mura, Ibaraki, 319-1195, Japan 2 Mechanical and Electrical System Engineering, Fukushima National College of Technology, 30 Nagao, Taira-Kamiarakawa, Iwaki, Fukushima, 970-8034, Japan ABSTRACT Cluster dynamics (CD) modeling has been used to estimate the long-term evolution of point defect (PD) clusters. However, previous studies have often simplified the governing equations by assuming the maximum size of mobile self-interstitial atom (SIA) clusters and by ignoring the one-dimensional (1D) reaction kinetics of SIA loops. They have also conducted parameter fittings, such as the clustered fraction and the maximum size of clusters produced by collision cascade, to reproduce experimental data. In this study, in addition to modeling the 1D motion of SIA loops in the framework of the production bias model (PBM), reaction rates associated with carbon impurity atoms present in alpha iron were formulated to consider the trapping effect of one-dimensionally migrating SIA loops by a vacancy-carbon (V-C) complex that was shown to have strong bindings with SIA loops by previous atomistic simulations. Calculations results for neutron-irradiated alpha iron showed that the developed CD model can successfully reproduce the saturation trend of the number density of immobile SIA loops in contrast to the prediction using a model without the trapping effect. INTRODUCTION Recently, in situ observations with a high-voltage electron microscope have revealed the remarkable effects of interstitial impurities on the one-dimensional (1D) migration behavior of self-interstitial atom (SIA) loops [1-7]. They have shown that SIA loops undergo stepwise positional changes at irregular intervals; these results differ from those of molecular dynamics (MD) simulations [8-13]. It should be valuable to note that 1D jump distances monotonically decrease with the lowering of purity in alpha iron [5, 6]. This finding clearly indicates that interstitial impurities would act as invisible obstacles or traps for one-dimensionally migrating SIA loops depending on their content in irradiated materials. Therefore, a rationalization for radiation-induced microstructure changes that would be greatly affected by the 1D motion of SIA loops under the influence of interstitial impurities is an essential component in understanding material property changes. With regard to the trapping effect of impurities for glissile SIA loops on the microstructural evolution driven by electron, ion, and neutron irradiations in pure Fe and Febased alloys, computational studies based on an object kinetic Monte Carlo simulation have been employed [14, 15]. It was concluded that to obtain a reasonable agreement with experiments, generic traps for glissile SIA clusters must be introduced. On the other hand, the assum
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