Si containing a band of nanocavities has been irradiated with Si+ ions at elevated temperatures to study interactions of irradiation-induced defects with open volume defects. For irradiation at 100 °C, nanocavities are shown to be preferential nucleation sites for amorphization. It is proposed that this behavior occurs to minimize the local free energy, whereby less dense amorphous Si is free to expand into the cavity open volume. Furthermore, for irradiation at 300 °C, cavities are very efficient sinks for Si interstitials during irradiation, leaving a region denuded of interstitial-based clusters surrounding each nanocavity. © 2000 American Institute of Physics.