Radiation damage in pure and helium-doped α-Al₂O₃ in the HVEM Qualitative aspects of void and aluminium colloid formation

Thick foils of α-Al₂O₃ single crystals of both high purity and doped with 1000 at. p.p.m. helium have been irradiated with 1 MV electrons at temperatures of 900–1130 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 helium-doped and undoped samples exhibited three radiation damaged 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.