The vacancy redistribution mechanism in the depleted zone of a displacement cascade was investigated by means of molecular dynamics simulations. A simplified model of the thermal spike was used in which a cell was cooled down along one axis after vacancies and kinetic energy had been introduced into its center. The effect of electron-phonon coupling upon the sweeping of vacancies in this highly disordered hot core as it cooled was then investigated. The characteristic cooling time, which was required for heat transfer to the electron system by ion-electron interaction, was a variable parameter (1 to 10ps). According to the literature, characteristic cooling times of less than 1ps and of more than 5ps were typical of metals such as Ni and Cu, respectively. In the case of a non-uniform distribution of the thermal spike, the maximum size of the real melt became a function of the characteristic cooling time. The effect of strong coupling upon vacancy redistribution was more pronounced for thermal spikes in which the average atomic kinetic energy in the melt was close to 3kT +L, where T was the melting point and L was the latent heat of fusion. In this case, the time which was required to form the liquid structure by melting was comparable to the time which was required to cool the zone to below the crystallization temperature. It was demonstrated that the coupling also affected the number of vacancies that was trapped by the melt during the advance of the solid/liquid interface. It was shown that, in metals with strong electron-phonon coupling, the highly disordered structure of the melted region could be frozen-in when a depleted zone with a high concentration of vacancies (more than 2 to 5at%) solidified.

V.G.Kapinos, D.J.Bacon: Physical Review B, 1994, 50[18], 13194-203