A model for the formation mechanism of depleted zones with a high concentration of vacancies in displacement cascades in metals

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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 previously introducing into it vacancies and kinetic energy. The time evolution of the temperature profile and the vacancy concentration distribution was calculated, and it was found that in a nonuniform temperature field, vacancies are swept to the centre of the thermal spike as a result of the advance of the solid-liquid interface. This effect leads to the development of a zone where the average concentration of vacancies is several times larger than that produced before the onset of the thermal spike phase. The redistribution of density of the surrounding matrix which is realized in this way is analogous to ‘collapse’ of a vacancy platelet and the consequent formation of a vacancy loop, a feature which is characteristic of high-energy-density cascades in metals. A model for interpretation of the computer simulation results has been proposed, built on the assumption, which is consistent with simulation data, that the total pressure in the melted region is constant and does not depend on position. This pressure equalization mechanism results in the redistribution of the material density within the melted region. According to this model, the vacancy-sweeping mechanism is a consequence of the formation of compressed liquid layers at the solid-liquid interface under the influence of a large temperature gradient.