A MODEL FOR THE FORMATION MECHANISM OF DEPLETED ZONES WITH A HIGH-CONCENTRATION OF VACANCIES IN DISPLACEMENT CASCADES IN METALS

The mechanism of redistribution of vacancies in the depleted zone of a displacement cascade in copper has been investigated by molecular dyn amics simulations. A simplified model of the thermal spike was used, f or which the calculation cell was cooled down along one axis after pre viously introducing into it vacancies and kinetic energy. The time evo lution of the temperature profile and the vacancy concentration distri bution was calculated, and it was found that in a nonuniform temperatu re 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 vacan cies is several times larger than that produced before the onset of th e thermal spike phase. The redistribution of density of the surroundin g matrix which is realized in this way is analogous to 'collapse' of a vacancy platelet and the consequent formation of a vacancy loop, a fe ature which is characteristic of high-energy-density cascades in metal s. A model for interpretation of the computer simulation results has b een proposed, built on the assumption, which is consistent with simula tion data, that the total pressure in the melted region is constant an d does not depend on position. This pressure equalization mechanism re sults in the redistribution of the material density within the melted region. According to this model, the vacancy-sweeping mechanism is a c onsequence of the formation of compressed liquid layers at the solid-l iquid interface under the influence of a large temperature gradient.