ON THE FORMATION OF VACANCY TYPE DISLOCATION LOOPS FROM DISPLACEMENT CASCADES IN NICKEL

The damage structures produced in Ni by heavy-ion irradiations with 50 keV Kr+ and 50 and 100 keV Ni+ ions have been studied as a function of irradiation temperature (30 and 300 K) and ion dose (10(15)-10(17) ions m-2). Both irradiations and electron microscopy were performed in the HVEM-Accelerator facility at Argonne National Laboratory. The effects of the irradiations were characterized in terms of the loop densities, loop sizes and the Burgers vector distribution. Dislocation loops were formed from isolated displacement cascades both at room temperature and 30 K, although the loop formation probability was significantly lower at 30 K. Additional loops appeared when specimens irradiated at 30 K were warmed to room temperature. At both irradiation temperatures the defect production rate decreased with increasing ion dose. During continued irradiation it was observed that new loops formed from isolated displacement cascades, and some pre-existing loops disappeared, shifted position of coalesced with a neighbouring loop. These effects can be explained in terms of the molecular dynamic computer simulation model that displays 'local melting' within the displacement cascade. The decreasing defect production rate with increasing ion dose is attributed to the coalescence of existing loops and the annihilation/reformation of loops that occurs when a fresh cascade spatially overlaps with the existing loops.