Recent studies have indicated that ion beam annealing of amorphous crystalline interfaces occurs by an ion-beam-triggered but thermally-activated process near the interface. This observation is used to develop a simple model for the accumulation of amorphousness under direct impact amorphisation conditions with competing ion-beam-triggered and additional thermally-activated annealing processes relevant to heavy-ion implantation of semiconductors. The effects of variations of ion flux density, ion fluence and substrate temperature are explored.