The irradiation of metals by energetic particles causes significant degradation of the mechanical properties, most notably an increased yield stress and decreased ductility, often accompanied by plastic flow localization. Such effects limit the lifetime of pressure vessels in nuclear power plants, and constrain the choice of materials for fusion-based alternative energy sources. Although these phenomena have been known for many years, the underlying fundamental mechanisms and their relation to the irradiation field have not been clearly demonstrated. Here we use three-dimensional multiscale simulations of irradiated metals to reveal the mechanisms underlying plastic flow localization in defect-free channels. We observe dislocation pinning by irradiation-induced clusters of defects, subsequent unpinning as defects are absorbed by the dislocations, and cross-slip of the latter as the stress is increased. The width of the plastic flow channels is limited by the interaction among opposing dislocation dipole segments and the remaining defect clusters.
Large-scale molecular dynamics of cascade production of the primary damage state are performed in fcc nanocrystalline Ni of average grain diameters of 5 and 12 nm. Primary knock-on atom kinetic energies of 5-30 keV are simulated. During the thermal spike phase, significant atomic motion towards the surrounding grain boundary structure is observed, characterized by many replacement-collision sequences. Upon resolidification, the excess volume condenses to form vacancy dominated defects with a complex partial dislocation network forming at higher energies.
In order to preserve the conditions for an environmentally safe machine, at present the selection of materials for the structural components of fusion reactors is made not only on the basis of adequate mechanical properties, behaviour under irradiation, and compatibility with other materials and cooling media, but also on their radiological properties, i.e. radioactivity, decay heat and radiotoxicity. These conditions strongly limit the number of suitable materials to a few families of alloys, generically known as low activation materials. The criteria for making decisions about such materials, the alloys resulting from the application of these ideas and the main issues and problems with their use in a fusion environment are discussed.
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