Examination of the Effect of Vacancy Detachment Rates on Kinetic Monte Carlo Simulations of bcc Metals
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Examination of the Effect of Vacancy Detachment Rates on Kinetic Monte Carlo Simulations of bcc Metals Richard T Hoffman III1, Alexander P Moore1, Chaitanya S Deo1 1
Nuclear and Radiological Engineering Program, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 ABSTRACT A Kinetic Monte Carlo simulation, using a modified version of the SPPARKS code, of simple defects and complex vacancy clusters was run on a bcc lattice. In this simulation the complexity of void formation was varied by introducing a detachment rate for individual vacancies leaving the void and either treating this value as constant for all size voids or having this value be dependent on the size of the void. Molecular Dynamics simulations were used to determine the binding energies of vacancies for voids of varying size. The simulation was then run over long time periods to determine the number of defects in the simulation under irradiation conditions. It was found that the additional complexity of size dependent void detachment rates had little effect on the defect concentrations and thus a constant barrier should be sufficient for simulations of voids in bcc metals. INTRODUCTION BCC metals are of interest as structural materials in radiation environments because they have low defect accumulation compared to FCC metals under similar irradiation. However, constant irradiation will still cause defects which affect the performance of these metals. In particular the effect of voids on the behavior of the metals has limited experimental evidence and must be examined through computational efforts. The most common method for performing these simulations is through crystal plasticity models that examine the physical properties of the metals by using rate equations to determine the statistical concentrations of relevant defects. In this paper we examine the effect of void structure complexity within models on the resulting defect concentrations. In particular we examine the effect of reversible voids with two types of vacancy emission mechanisms. In the first case the likelihood of vacancy emission is independent of the size of the void and in the second case the rate is size dependent and is calculated using the results of Molecular Dynamics (MD) simulations of varying void sizes. We use the more robust and geometrically dependent method of Kinetic Monte Carlo (KMC) for the simulations in order to ensure that we are not losing any effects by statistically averaging the model values. We present a method for calculating the void binding energy based on power law equations derived from MD simulations using the EAM and MEAM potential. Atomistic simulations of iron voids will be able to obtain some of the base energetic properties of voids, which are not directly or easily obtainable using experimental methods. The energetics of the simulated void system can be coupled with the larger scale Kinetic Monte Carlo simulations in order to bridge the simulation gap from the atomistic scale to the continuum scale.
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