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

Abstract

The evolution of irradiation damage cascades in a metal has been simulated by molecular dynamics, using a many-body potential. Over 100 cascades have been produced with random knock-on directions and primary knock-on atom (PKA) energies ranging from 60 to 2 keV. The cascade evolution has been followed for times typically up to about 10ps and in some cases up to about 30 ps. The cascades are characterized 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 a liquid-like structure with cavitation. The annealing phase leaves loosely clustered vacancies at the cascade centre but collapse to a vacancy loop is not observed. The onset of this thermal spike occurs for cascades with energy of 100 eV or higher; below this the cascade is made up of a series of closely coupled short replacement collision sequences. 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. 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 the production of point defects and defect clusters on the subsequent evolution of the microstructure is discussed.