The role of helium in the development of damage structure and swelling in neutron-irradiated aluminium

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Abstract
Helium and its state of agglomeration in pure aluminium prior to neutron irradiation are investigated with respect to its influence on the microstructural features developed during irradiation to doses of 0·005 to 2·6d.p.a. at about 328 K (0·35 Tm). A level of 4 a.p.p.m. helium, in either submicroscopic form or as small bubbles at the resolvable limit of 2 nm diameter, induces large increases in both the cavity and dislocation components of the damage structure. It also causes swelling to begin much earlier in the neutron dose and to exhibit two different stages of evolution, one early in the dose where the dose exponent of swelling is high, the other at later stages with lower exponent. When the helium is coarsened into 7-nm bubbles by annealing at 0·96 Tm before irradiation it has relatively small effects on damage structure and swelling. The increases in concentrations of cavities and dislocation loops in the presence of helium are attributed to trapping of radiation-produced vacancits by the helium. and a consequent retention of more interstitials in loops. Cavity nucleation data are shown to be consistent with a predicted critical cavity size of 1–2 nm diameter stabilized by helium. The swelling data are analysed by cavity-growth theory. The rapid swelling at low dosage involves an unusually high ratio of vacancy retention to vacancy production and is considered in terms of gas-affected cavity growth and in terms of higher dislocation bias when loops predominate. At later stages, the swelling rate is reduced and agrees quantitatively with the theory of steady-state bias-driven cavity growth. Pre-irradiation annealing treatments diminish the effects of the helium in a manner consistent with estimates of the coarsening of helium bubbles and with the reduced effectiveness of such coarse bubbles on vacancy gettering during irradiation.