Microstructure of helium-implanted α-Al2O3 after annealing

Polycrystalline alumina (α-Al2O3) was homogeneously implanted to depths of 80 μm with helium to a concentration of 1000 appm at room temperature, and subsequently annealed at temperatures from 800 to 1400 °C for 0.1–10 h. The microstructure was investigated using transmission electron microscopy. Dense populations of faulted dislocation loops and of bubbles were observed from 1000 to 1200 °C. Most loops had habit planes of {101¯0} and some of (0 0 0 1), and both were identified to be of interstitial type. The loops on (0 0 0 1) had a round shape while those on {101¯0} were rectangular with the short edges in [0 0 0 1]. On annealing at 1400 °C, the dislocation density decreased dramatically. Bubbles became visible at T ⩾ 1000 °C (1 h). They were associated with loops and were arranged in disc-shaped arrays. Disc-shaped features of vacancy type (platelets) were observed in a narrow window of annealing temperatures and times around 1200 °C and 0.1 h. Bubbles were observed on low-angle grain boundaries but not on normal grain boundaries. Platelets were been observed in magnesia (MgO), implanted from 1000 to 2400 appm with subsequent annealing for 1 h at temperatures from 800 to 1000 °C. The temperature regime where helium clustering occurs is in accord with that of previously observed lattice expansion in both materials.