Computer simulation study of cascade overlap effects in alpha-iron

The effects on defect production of the overlap of collision cascades in a-iron have been investigated by molecular dynamics computer simulations. A large number of cascades was simulated and this has provided clear evidence that fewer defects result when two cascades produced at different times overlap. A means of defining the spacing of cascades for the purposes of categorising overlap effects has been found. When the two cascade energies are very different, viz. 400 eV and 5 keV, the defects produced by the smaller cascade are effectively lost at complete overlap, regardless of which cascade occurs first. Even when the energies are similar, substantial defect loss arises when overlap occurs. Overlap also changes the distribution of interstitial clusters that results from the primary damage process of cascades, with a higher proportion of the defects occurring in larger clusters. The mechanisms that give rise to these effects have been found from computer visualisations.