Influence of ion-electron interaction on the formation mechanism of depleted zones in displacement cascades in metals.

Abstract

The mechanism of redistribution of vacancies in the depleted zone of a displacement cascade in copper has been investigated by molecular-dynamics simulations. A simplified model of the thermal spike was used, for which the calculation cell was cooled down along one axis after vacancies and kinetic energy had been introduced in the center of it. The influence of electron-phonon coupling on the sweeping of vacancies in this highly disordered hot core as it cools was then investigated. The characteristic cooling time, \ensuremath{\tau}, for transfer of heat to the electron system by ion-electron interaction was a variable parameter and was varied from 1 to 10 ps. According to Finnis et al. [Phys. Rev. B 44, 291 (1991)] the values of \ensuremath{\tau}\ensuremath{\le}1 and \ensuremath{\tau}\ensuremath{\ge}5 ps are typical of metals such as Ni and Cu, respectively. In the conditions of the nonuniform distribution of the thermal spike, the maximum size of the real melt becomes a function of \ensuremath{\tau}. The influence of the strong coupling on the vacancy redistribution is more strongly pronounced for thermal spikes in which the average atomic kinetic energy in the melt is close to the value (3${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{m}}$+L), where ${\mathit{T}}_{\mathit{m}}$ is the melting temperature and L is the latent heat of fusion. In this case the time to form the liquid structure by melting is comparable with the time to cool the zone to below the temperature of crystallization. It has been demonstrated that the coupling also influences the number of vacancies trapped by the melt during the advance of the solid/liquid interface. It is shown that in metals with strong electron-phonon coupling, the highly disordered structure of the melted region can be frozen in when a depleted zone with a high concentration of vacancies (\ensuremath{\ge}2--5 at. %) solidifies.