Atomic collision cascades initiated from high energy PKAs (primary knock-on atoms) produce defect clusters in metals and alloys irradiated with fast neutrons or energetic ions. They will affect not only the early stage of microstructural evolution but that at high fluence. We have performed heavy ion irradiation experiments with simultaneous electron microscopy using the 400 kV Cockcroft-Walton type accelerator/200 kV electron microscope facility of the University of Tokyo. In the present study, 99.99% pure gold is selected as a reference material for face centered cubic (fCC) metals. Thin foil specimens are irradiated with 400 keV Xe⁺ ions at elevated temperatures up to 770 K in the electron microscope, and cascade cluster formation and interaction processes are investigated from the video recording of TEM images.

Instantaneous formation of vacancy loops and SFTs (stacking fault tetrahedra) is observed during ion irradiation. Most of the defect clusters shrink and annihilate with very short lifetimes under irradiation compared with those during isothermal annealing without irradiation. Lifetimes of the clusters are strongly influenced by impinging ion flux, indicating that interstitials coming from other cascades annihilate the vacancy clusters. At the irradiation temperature of 470 K, we also observed that some of the vacancy loops showed sudden growth in size.

The number of subclusters at the moment of cluster formation decreases at higher temperature. The initial size distribution of observed cascade clusters is also measured between 470 and 770 K. Higher ion flux at 670 K promotes formation of larger vacancy clusters, which can be attributed to cascade overlapping taking place in a limited time scale.