Molecular Dynamics simulations of displacement cascades: role of the interatomic potentials and of the potential hardeni
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Molecular Dynamics simulations of displacement cascades: role of the interatomic potentials and of the potential hardening. C.S. Becquart1, C. Domain2 , A. Legris1 and J.C. van Duysen2 1 Laboratoire de Métallurgie Physique et Génie des Matériaux, UMR 8517, Université de Lille I, 59655 Villeneuve d’Ascq Cédex, France 2 EDF – R&D Département EMA, Les renardières, F-77818 Moret sur Loing Cédex, France
ABSTRACT The role of the interatomic potentials on the primary damage has been investigated by Molecular Dynamics (MD) simulations of displacement cascades with three different interatomic potentials dedicated to α-Fe. The primary damage, caused by the neutron interaction with the matter, has been found to be potential sensitive. We have investigated the equilibrium parts of the potential as well as the “short distance interactions” which appear to have a strong influence on the cascade morphology and defects distribution at the end of the cascade. The static properties as well as dynamical (thermal) characteristics of the potentials have been considered; the kinetic and potential energy transfers during the collisions have also been studied. INTRODUCTION Displacement cascades have been studied for more than 30 years using numerical simulations. The potentials impact on the results has been emphasised before and an ample discussion of this question is already given in [1]. Both the equilibrium part and the short distance interactions part (hardening) of the potentials influence the final results. In this work, we examine this last point in more detail, and bring forward new evidence of the equilibrium part influence on the tendency to form clusters. COMPUTIONAL PROCEDURE DYMOKA [2], is a user oriented code devoted to MD as well as Monte Carlo simulations. The Newtonian equations of motion are integrated using a fifth order Gear predictor-corrector algorithm. The neighbour search is done through a link cell method combined with a Verlet list so that the code is fully linear with the number of atoms. The interatomic potentials are tabulated and interpolation of the potentials is made through a 5th order Lagrange polynomial. To simulate displacement cascades the following commonly used approximations were made: the effect of electron-phonon coupling has been ignored, the boundary atoms were not damped to extract heat or attenuate the out-going pressure wave. Periodic boundary conditions (PBC) were used with a choice of the simulation box size depending upon the energy of the PKA. At the beginning of the simulation, the system of particles is allowed to equilibrate, for 5 ps, at the chosen temperature ; most of the time this is 600K, which is close to the vessel irradiation temperature. When the lattice is at thermal equilibrium, one atom, the Primary Knock-on Atom (PKA) is given a momentum corresponding to energies varying from 1 to 30 keV. The actual timestep is adapted to the PKA velocity, and it can be as low as 10-17 s. As soon as the extreme collisions are over, a much longer timestep is adequate, i.e. 10-16 to 10-15 s
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