Cavity formation in single crystal Al 2O 3 irradiated with triple beams of O, H and He ions

Abstract

A cross-sectional electron microscopy technique was used to examine the microstructural evolution in single crystalline pure alumina (Al 2O 3) irradiated at 923 K with triple beams of 0.25 MeV H +, 2.4 MeV O 2+ and 1.l MeV He + to fluences of 6.3, 2.8 and 6.7 × 10 20/m 2. Irregularly shaped cavities are observed in two regions of depths of 0.2–1.8 and 2.2–2.5 μm. The cavity densities peaked at about 1.0 and 2.3 μm (7 × 10 22/m 3 and 4 × 10 22/m 3, respectively) with average sizes of 15 and 8 nm, respectively. The cavities formed in the He ion damaged region are aligned and mostly interconnected in the 〈0 0 0 1〉 direction to form irregularly shaped cavity tunnels. Tiny cavities, smaller than 5 nm, are formed inhomogeneously along irradiation-produced dislocations in the region 0.4–1.0 μm from the sample surface, where neither H nor O atoms are predicted to be stopped. This formation of cavities in the near-surface region is in marked contrast to that observed in the Al 2O 3 sample irradiated with simultaneous beams of 0.25 MeV H + and 2.4 MeV O 2+ ions to a fluence 2.5 times higher than that in the triple beam case. In the triple beam irradiation, cavities with comparable sizes are observed only near the end of ion ranges.