Electronic effects in radiation damage simulations in metals.

Radiation damage has traditionally been modelled using classical molecular dynamics, in which the role of the electrons is confined to describing bonding via the interatomic potential. This is generally sufficient for low radiation energies. However high energy atoms lose a significant proportion of their energy to electronic excitations, therefore a simulation of the relaxation of a metallic lattice after a high energy event requires a description of the energetic interaction between atoms and electrons. The mechanisms of inelastic collisions between electrons and ions, coupling between electrons and phonons and the diffusion of energy through the electronic system to the rest of the lattice become signficant. We have coupled large scale MD simulations of the lattice to a continuum model for the electronic temperature evolution. Energy lost by the atoms due to elastic and inelastic electronic collisions is gained by the electronic system and evolves according to a heat diffusion equation. The electronic energy is coupled to the lattice via a modified Langevin thermostat, representing electron-phonon coupling. Results of the simulation of both displacement cascades and ion tracks, representing the low and high extremes of incident ion energy respectively, are presented. The effect of annealing of pre-existing damage by electronic excitation is studied and the behaviour under swift heavy ion irradiation in iron and tungsten is compared. In simulations of displacement cascades, the strength of coupling between the atoms and electrons emerges as the main parameter determining residual damage. Our new methodology gives rise to reduced damage compared to traditional methods in all cases. Ion track simulations demonstrated that the relaxation dynamics, and hence the residual damage, was dependent on the magnitude and temperature dependence of the electronic thermal parameters.