Kinetics of the Nucleation and Growth of Helium Bubbles in bcc Iron
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0929-II01-08
Kinetics of the Nucleation and Growth of Helium Bubbles in bcc Iron Chaitanya Suresh Deo1, Srinivasan G. Srivilliputhur1, Michael Baskes1, Stuart Maloy1, Michael James1, Maria Okuniewski2, and James Stubbins2 1 Los Alamos National Laboratory, Los Alamos, 87545 2 University of Illinois, Urbana, 61801
Abstract Microstructural defects are introduced in materials upon irradiation with energetic particles. These defects can cause degradation of mechanical properties and contribute to material failure. Transmuted helium in irradiated stainless steels exerts deleterious effects on material properties. We have performed kinetic Monte Carlo (kMC) simulations of point defect diffusion and clustering in bcc alpha iron. The model includes helium and vacancy diffusion and spontaneous clustering and dissociation of the point defects from the clusters. We employ the kMC simulations to investigate the time evolution of the point defect configuration leading to defect clustering and bubble formation. The concentration of embryonic point defect clusters is determined as a function of the simulation time.
Introduction During high energy proton irradiation, high-energy neutrons collide with the atoms in the surrounding materials and induce (n, α)-reactions resulting in the formation of helium atoms. Consequently, the first-wall materials in the fusion reactor (typically ferritic steels) contain a high concentration of helium atoms during and after irradiation [1, 2]. These helium atoms have a strong tendency to precipitate into helium-vacancy clusters and bubbles, which are detrimental to the properties of metals and alloys. Studies have shown that helium atoms assist the nucleation and growth of cavities in irradiated materials leading to volumetric swelling. Mechanical properties such as tensile strength and fracture toughness [3, 4] are influenced by the presence of helium atoms. Helium migration and clustering at grain boundaries results in high temperature embrittlement [5]. Thus, understanding helium behavior in metals is key to developing structural materials capable of operation in a high energy proton irradiation environment. The helium- vacancy cluster evolution under irradiation is governed by several mechanisms responsible for transport of He atoms and vacancies in the crystal, such as the migrating He interstitial, migrating vacancy, thermally activated dissociation of helium from a vacancy and the jump of a He atom from one to another vacancy as a basic step in the vacancy mechanism [6]. The effect of irradiation on materials microstructure and properties is an inherently multiscale phenomenon. Radiation damage processes including helium behavior occur over multiple length and time scales, from the collision stage of 10-13 s and10-9 m to the long-term diffusion stage of 103 s and106 m. Primary defect production occurs at nanometer and picosecond scales where helium is produced via the (n,α) reaction and vacancies and selfinterstitials via the elastic collisions of primary knock on atoms. The long-term diff
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