Classical thermodynamic approach to void nucleation in irradiated materials
Abstract One of the major problems in building a future fusion power station is the development of suitable structural materials. These materials will be exposed to high energy neutron bombardment, with consequent changes in their mechanical properties – embrittlement, hardening and swelling, for example. A missing link in modelling these effects is an effective treatment of the nucleation of voids under irradiation. These voids are initially stabilised by transmutation helium but, once formed, grow by vacancy accumulation. In this paper, a quasi-chemical model is developed to calculate the entropy of a steel/helium system. Although a substantial contribution from quasi-chemical effects might be expected, the steady state concentrations of dissolved helium are found to be too low for such effects to manifest. The steady state concentration of dissolved helium is low in absolute terms, but the resulting supersaturation is very high, making it reasonable to assume that all available nucleation sites are instantaneously activated.
Helium in metals
Abstract The paper will first give an overview of important technological issues relating to the precipitation of helium gas bubbles in materials subjected to radiation damage in nuclear reactors. These are the phenomena of swelling, blistering, flaking, radiation creep and He embrittlement. A number of currently debated fundamental issues in the physics of He in metals will be presented, in particular processes of He bubble nucleation and growth, blister formation and ordering of bubbles in superlattices.
Formation of helium bubbles and dislocation loops in tritium-charged vanadium
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 w...