The formation process of defect clusters in copper and nickel irradiated with He+ ions has been studied to understand the effects of the small fraction of damage cascades and He atoms on it. Wedge-shaped specimens of copper and nickel were irradiated with 20-100 keV He+ ions at 300-570 K in an ion accelerator or an HVEM on line with an ion accelerator. The results on microstructural evolution were analyzed in terms of the energy of He+ ions, the irradiation temperature and the specimen thickness as well as the difference between copper and nickel. It is concluded that the small fraction of damage cascades induced by He+ ions provides the direct nucleation of vacancy- and interstitial-clusters and that the threshold energy for forming vacancy clusters in nickel is slightly larger than in copper.
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