Nucleation and Growth of Defects in irradiated Fe using Rate Theory and Kinetic Monte Carlo Methods
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0908-OO05-22.1
Modeling Nucleation and Growth of Defects in Irradiated Fe in the Presence of He using Rate Theory and Kinetic Monte Carlo Methods C. J. Ortiz1, M. J. Caturla1, C. C. Fu2 and F. Willaime2 1 2
Departamento de Física Aplicada, Universidad de Alicante, Alicante E-03690, Spain Service de Recherches de Métallurgie Physique, CEA/Saclay, 91191 Gif-sur-Yvette, France
ABSTRACT He desorption in Fe was studied using rate theory and kinetic Monte Carlo models. A good fit to experimental observations was obtained with a migration energy of 1.1 eV and a formation energy of 1.6 eV for the single vacancy. These results could be explained by the presence of traps/impurities in experiments. Simulations show that the release of He during annealing is mainly due to the dissociative mechanism (Frank-Turnbull) and the release of Hei from small helium-vacancy clusters. INTRODUCTION Development of materials that can withstand high radiation levels as those in fission or fusion reactors requires an understanding of the type of defects produced, the evolution of these defects, and their effects on mechanical properties. Ion beams have been widely used to obtain basic information on defect production and accumulation. In particular, the effects of He in metals have been studied through He implantation and desorption techniques [1,2]. Due to the long time and length scales involved in defect production and evolution, it is necessary to couple different simulation methods to study ion implantation and annealing. The link between ab initio calculations, to obtain basic parameters such as migration and binding energies of defects, with other models such as kinetic Monte Carlo or rate theory to study time evolution has been successful both in the area of metals [3] and in semiconductors [4]. In this paper we address the evolution of He in Fe using a rate theory model with parameters obtained by ab initio calculations as well as with kinetic Monte Carlo simulations, and results are compared to experimental data on He desorption. RATE THEORY SIMULATIONS Rate theory models use a mean-field approximation and are based on coupled diffusion equations and the so-called Master equation. In this simulation sheme, the evolution of each defect, which has a discrete and finite number of atoms is governed by a continuity equation. In order to describe He kinetics we assume that interactions with point defects and agglomeration into clusters occur via binary reactions. We consider that the following atomistic processes govern He evolution in He-irradiated iron: Hes + I U Hei .
(1)
In this reaction known as the kick-out mechanism, Hes and Hei denote He atoms in substitutional and interstitial sites, respectively. I represents a self-interstitial atom. Only the Hei and self-interstitial species are assumed to be mobile. Hei + V U Hes .
(2)
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In reaction (2), also called the Frank-Turnbull mechanism, V, which stands for a vacancy defect, recombines with an interstitial helium to form a substitutional He. He n Vp + V U He n Vp+1
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