Displacement cascades are the primary damage mechanism in fast-neutron irradiated metals, and the clustering of point defects, particularly to vacancy loops, during the cascade event can have a potent influence on microstructural development. Little is known about this process in hexagonal close packed (hep) metals and alloys despite their current and potential importance. Attention is focused in this paper on cascade collapse to vacancy dislocation loops in alpha-titanium (α-Ti). Two purities were employed: a high-purity iodide material and a commercial purity sample.
A particularly convenient method of studying cascade damage is to employ energetic heavy ions that simulate the energetic recoils created in collisions between fast neutrons and lattice atoms. The collapse in isolated cascades produced by such irradiation may be studied by transmission electron microscopy, which enables loop geometries, number densities, and sizes to be analyzed in detail. In the present studies, molecular antimony ions have been employed with energies between 50 and 150 keV. Molecular ions were employed because the probability of clustering in titanium was found to be very low for atomic ion irradiations. In molecular ion irradiation, faulted and perfect vacancy loops on the prism planes were observed, and the efficiency of collapse increased with increasing molecular ion energy and mass. In commercial titanium, collapse was slightly more efficient than in the high-purity metal, and from the change in the relative numbers of perfect and faulted loops, it is shown that the prism-plane stacking fault energy is lower in the former material. A further notable feature of the results is that a small population of basal-plane loops with a c-component to their Burgers vector was observed under certain irradiation conditions. The trends observed are discussed in terms of current models of the processes involved in cascade formation in radiation damage.