Formation of helium bubbles and dislocation loops in tritium-charged vanadium

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Abstract
Dislocation loops and small He bubbles were observed by transmission electron microscopy in V samples which were tritium-charged to produce single α-phase and (α+ β) two-phase regions and subsequently aged up to 14 months at room temperature and at — 196°C. In the β-phase areas, large densities of He bubbles, 1–2 nm in diameter and dislocation loops were observed. Preliminary observations after 14 months of ageing showed that the loop pattern had coarsened into a dislocation network. In the α-phase areas, bubbles and tube-shaped cavities were found along dislocations and in dislocation tangles. Also, isolated bubbles which had grown by prismatic loop punching were observed. In low-angle grain boundaries bubbles were formed at dislocations. These results show that microstructures similar to those observed after energetic He implantation can form in the absence of irradiation-induced defects. It is concluded that He must be mobile at room temperature in both phases and precipitates by a mechanism which does not require He trapping in vacancies as a first step.