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Fe-Cu model alloys (0, 0.05, 0.15, 0.30 and 1.00 wt.%Cu) of Nuclear Reactor Pressure Vessel Steels irradiated by fast neutrons (8.3×1018 n/cm2) at about 100°C are studied by positron annihilation spectroscopy, positron lifetime and coincidence Doppler roadening, in order to analyze the irradiation-induced vacancy-Cu aggregation. We have observed nanovoids (vacancy clusters) whose inner surfaces are covered by Cu atoms in not only high and medium Cu (0.15–1.0 wt.%) alloys but also low Cu (0.05 wt.%) alloys. The typical size of the nanovoids and the fraction of the covering with Cu on the inner surfaces depend on the Cu content. The dissociation of vacancies from the nanovoids by the post-irradiation isochronal annealing at about 400°C leads to the formation of the ultrafine Cu precipitates even for the Fe-0.05wt.%Cu alloy.

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