Fragmentation of displacement cascades into subcascades: A molecular dynamics study

•The subcascades formation in copper and iron is studied by molecular dynamics method.•A two-point density correlation function is a tool for detecting subcascade formation.•A fractal analysis served to determine the subcascade formation threshold energy.•The temporal evolution of displacement cascades is investigated. The fragmentation of displacement cascades into subcascades in copper and iron has been investigated through the molecular dynamics technique. A two-point density correlation function has been used to analyze the cascades as a function of the primary knock-on (PKA) energy. This approach is used as a tool for detecting subcascade formation. The fragmentation can already be identified at the end of the ballistic phase. Its resulting evolution in the peak damage state discriminates between unconnected and connected subcascades. The damage zone at the end of the ballistic phase is the precursor of the extended regions that contain the surviving defects. A fractal analysis of the cascade exhibits a dependence on both the stage of the cascade development and the PKA energy. This type of analysis enables the minimum and maximum displacement spike energies together with the subcascade formation threshold energy to be determined.