Microstructure of helium-implanted α-Al2O3 after annealing

Polycrystalline alumina (α-Al2O3) was homogeneously implanted to depths of 80μm with helium to a concentration of 1000appm at room temperature, and subsequently annealed at temperatures from 800 to 1400°C for 0.1–10h. 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 (0001), and both were identified to be of interstitial type. The loops on (0001) had a round shape while those on {101¯0} were rectangular with the short edges in [0001]. On annealing at 1400°C, the dislocation density decreased dramatically. Bubbles became visible at T⩾1000°C (1h). 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 0.1h. 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 2400appm with subsequent annealing for 1h 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.