A high voltage electron microscope study of the effects of preinjected helium on electron damage in aluminium

Abstract

Defect clusters were formed and observed simultaneously in electron irradiated foils of aluminum, both uninjected and preinjected with about 8 atomic ppm helium, in a high voltage electron microscope operated at 650V. The nominal irradiation temperatures of the specimens were room temperature, 150°C and 200°C. The defect clusters in the form of dislocation loops formed in greater concentrations and persisted to longer times in the preinjected specimens than they did in the uninjected metal. At 200°C, no beam-induced loops were observed in the uninjected aluminum foil of usual thickness, but they were seen in the helium injected material. Faulted dislocation loops were more prevalent in the preinjected specimens. Under certain conditions the irradiated microstructure evolved to one resembling an unirradiated structure. It is concluded that the preinjected helium itself (rather than the associated alpha particle damage) causes the enhanced dislocation loop formation in the electron bombarded foils.