Evidence for the coalescence of cavities in nickel irradiated with 500 keV 4He+ ions

Transition electron microscopy has been used to study the effect of dose on the depth distribution of cavities (voids or bubbles) and dislocation damage in nickel irradiated at 5000C, with 500 keV 4He+ ions. The diameter, number density and volume fraction of cavities were measured as a function of depth from micrographs taken from samples sectioned parallel to the direction of the incident beam. The results for the dose range 2 x 1019 to 1 x 1021 ions/m2 show an increase in the average cavity diameter, number density, and volume fraction with increasing dose. A further increase in dose from 5 x 1021 to 1 x 1022 ions/m2 shows a significant decrease in the number density (a factor of approx. 40) and a corresponding increase in the average diameter (a factor of approx. 8) of the cavities in the peak volume-swelling region. This observation is interpreted as evidence for the coalescence of cavities. The peak in the volume-swelling distribution occurs at depths approx. 10% deeper than the calculated average projected-range of 500 keV 4He+ ions in nickel. (orig.)