Kinetic Monte Carlo simulations of cascades in Fe alloys
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Kinetic Monte Carlo simulations of cascades in Fe alloys C. Domain1, C.S. Becquart2 and J.C. Van Duysen1 1
EDF-R&D, Département EMA, F-77250 Moret sur Loing, France Laboratoire de Métallurgie Physique et Génie des Matériaux, UMR 8517, Université de Lille I, F-59655 Villeneuve d’Ascq Cédex, France 2
ABSTRACT The Pressurized Water Reactor vessel steels are embrittled by neutron irradiation. Among the solute atoms, copper play an important role in the embrittlement and different Cu-rich defects have been experimentally observed to form. We have investigated by Kinetic Monte Carlo (KMC) on rigid lattices the evolution of the primary damage. Since the point defects created by the displacement cascades have very different kinetics, their evolution is tracked in two steps. In a first step, we have studied their recombination in the cascade region and the formation of interstitial clusters using “object diffusion”. The parameters of this model are based on MD simulations, or on first principles calculations. In a second part, we have investigated the subsequent evolution of the primary damage with a model based on a vacancy jump mechanism. These simulations which rely on an adapted EAM potential show the formation of copper rich defects. Some of the potential’s predictions that played a key role in the model were checked by ab initio calculations. The defects obtained from these simulations, subsequent to the primary damage created by displacement cascades, exhibit similarities with the ones observed by atom probe. The influence of temperature and Cu content on the final damage was investigated. INTRODUCTION The pressure vessels of light water reactors are made with low alloy steels. Under irradiations, and depending upon temperature, dose rate and flux conditions, many defects are observed experimentally: interstitials loops or microvoids as well as dilute solute complexes and solute atmospheres; the latter's origin, structure and stability being still under investigation. It is well known that neutron interactions with matter lead to the formation of displacement cascades which produce the so called primary damage: a confined area with point defects. These defects are much too small to be resolved experimentally and Molecular Dynamics (MD) simulations [1-3] have proven very useful to characterise them. However because of the short span covered by MD simulations (10 to 15 ps), other tools must be utilised to investigate the evolution of these defects towards experimentally resolvable entities. This paper presents the approach we used to study the medium term evolution in time and space of the vacancies and interstitials left by MD displacement cascades [3,4] in Fe and FeCu. The kinetics of the evolution of interstitials are commonly regarded to be orders of magnitude faster than that of vacancy diffusion and thus vacancies and interstitial processes can be modeled separately, at least when considering a single cascade. Thus two different lattice Kinetic Monte Carlo (KMC) methods were coupled / linked which constitute the
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