High dose, high temperature radiation damage of helium-doped alumina in the hvem

Abstract

Single crystal α-Al 2O 3 foils, 1.5–2.0 μm thick, of both high purity and doped with 1000 appm helium have been irradiated with 1 MV electrons at temperatures of 880–1130 K in a high voltage electron microscope (HVEM). Following irradiation the foils were thinned further and examined at room temperature in the HVEM by both bright field and dark field techniques. Both heliumdoped and undoped samples exhibited three radiation damage products which could be observed in bright field. These were dislocation tangles and small features which exhibited white or dark contrast. The white features showed the contrast behaviour of voids, whilst the dark features were best observed using diffraction contrast. Additional diffraction patterns were indexed as being from aluminium metal which, viewed in dark field, showed images corresponding to the dark features observed in bright field. Confirmation that the dark features were aluminium metal precipitates was obtained from loss of diffraction contrast when the specimen was heated above the melting point of aluminium. The voids and aluminium precipitates were found to form after electron doses of ∼ 17 MC/m 2 and ∼ 40 MC/m 2 respectively. The number and size distribution of both voids and aluminium precipitates were determined and the volume fraction of both kinds of defect were found to follow a power law of dose with an exponent of 0.5–0.6. An activation energy for void formation of 0.96 ± 0.1 eV was obtained and void swelling was found to be only slightly increased by helium doping. The activation energy for aluminium precipitate formation was found to be 1.35 ± 0.15 eV.