Evolution of 3He bubble microstructure in TiT2 films during aging

TEM observation was performed on titanium tritide films at He:Ti ratios of 0.036-0.356. Most helium atoms remained in the TiT2 lattice until at a He/Ti ratio of 0.06, almost all helium atoms entered the bubbles. The density of helium bubbles in Ti tritide increased with the increase in helium content, while their size distribution remained almost unchanged until the bubbles in the samples begin to interconnect at a He/Ti ratio of 0.247. Helium bubbles preferred to distribute in Ti {111} planes for all the samples. The morphology of helium bubble distribution changes not only with the density of bubbles but also with the strain in the films. Helium tended to migrate to the grain boundaries, and plenty of cracks were developed along the grain boundaries. The extension of cracks to the film surface is the cause for the large bursts of 3He into the atmosphere. The observed microstructural evolution and its dependence on helium concentration, density of bubble, and stress distribution are discussed in terms of diffusion of interstitial He atoms and their clustering.