Ni specimens with two different purities were irradiated at 290°C by D-T neutrons using RTNS-II to a fluence of 6.4 × 1022 n/m2 and their damage structure was examined by means of electron microscopy. Voids, stacking fault tetrahedra (SFT), aggregates of vacancies and interstitial loops were observed but the density and size depended not only on fluence but also on purity. At relatively low fluence, dense SFT, which may be nucleated with the assistance of impurity atoms, are formed in the lower purity Ni, while voids are dominant vacancy clusters in the higher purity Ni. The present observation showed that the interstitial loops grow abruptly when vacancy clusters formed inside. In such cases, formation of voids is enhanced due to the strong bias effects of the growing interstitial loops.
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