The influence of helium doping on the damage microstructure of heavy-ion irradiated α-Al₂O₃

Abstract

The results of a transmission electron microscopy study of heavy-ion radiation damage in α-Al₂O₃ are described. Single-crystal specimens were irradiated with 4 MeV Ne⁺ and Ar⁺ ions to doses of 2 × 10²¹ Ne⁺/m² and 1 × 10²¹ Ar⁺/m² over a range of temperatures from 500 to 900°C. Two specimens were irradiated at each temperature and dose; one undoped, and one doped with 1000 a.p.p.m. helium. The various components of the damage microstructure were analysed in order to investigate the microstructural evolution and the effects of helium doping.

Two sets of dislocation loops were observed in both doped and undoped specimens; one set lying on basal and the other on {1010} planes. With increasing dose these grew and intersected to produce a dense dislocation network at temperatures T ≥ 700°C. The high density of basal loops, compared to those on {l010} planes, and the basal nature of the dislocation network, explain the anisotropic swelling under irradiation observed in α-Al₂O₃. The effect of helium on the development of the dislocation microstructure was to produce generally higher loop and network densities.

Helium-doped specimens contained populations of ‘large voids’ (diameter greater than 6 nm) in addition to the ‘small voids’ (diameter less than 6 nm) observed in undoped specimens. The ‘small voids’ were observed to form into an array which became increasingly three dimensional with dose. Helium doping increased total swelling by up to 100%.