Molecular dynamics simulation of irradiation damage cascades in copper using a many-body potential

The evolution of irradiation damage cascades in copper has been simulated by molecular dynamics, using a many-body potential. Nearly 200 cascades have been produced with random knock-on directions and primary knock-on atom (PKA) energies in the range from 60 eV to 10 keV. The starting temperature for these simulations was 100 K and 600 K, this report will confine itself to the data obtained at 100 K. The cascade evolution has been followed for times typically up to ∼10 psec and in some cases up to ∼30 psec. The cascades are characterised by the sudden emission of replacement collision sequences and with shape variations due to local channelling events. At the higher energies the core has been shown to be liquid-like structure with cavitation. The annealing phase leaves loosely clustered vacancies at the cascade centre but collapse to a vacancy loop is not generally observed. A feature of the more energetic cascades is the production by a ballistic mechanism of interstitial atom clusters at the periphery of the cascades. The large number of simulations have enabled an analysis of the efficiency of point defect production to be followed as a function of PKA energy. The value obtained falls sharply from the classical value of 0.8 to 0.37 at 250 eV, followed by a steady decline to 0.28 at 2 keV and 0.15 at 10 keV. Thus the creation of point defects in metals during irradiation is likely to be markedly less than the standard value of 0.8, used in many irradiation dose assessments. The effect of pre-existing features on the cascade evolution has also been investigated. Here we insert into the block of atoms a range of features such as vacancy and interstitial loops and small voids. These structures are in equilibrium at 100 K prior to initiation of a 1 keV cascade. In all of these simulations a significant reduction in final defect production is observed.