To study the formation of microvoids and Cu precipitates in Fe-Cu mode alloys (0–1.0 wt. % Cu) and A533B reactor pressure vessel (RPV) steel, positron lifetime and Vickers microhardness have been measured after fast neutron irradiation to a fluence of 8.3×1018n/cm2 below 150°C. Long positron lifetimes for the Fe-Cu alloys ranging from 300 to 420 ps and 270 to 350 ps for A533B RPV steels have been observed, which show microvoid formation. The long lifetimes in the Fe-Cu alloys depend on the Cu content and thermal aging at 550°C before irradiation. This fact suggests incorporation of irradiation-induced vacancies at Cu atoms and their precipitates leading to suppression of the microvoid formation. The long lifetime component for the Fe-Cu alloys recovers around 350°C, while that for A533B steel recovers around 300°C. To estimate the microvoid sizes, superimposed-atom model calculations of positron lifetimes in microvoids consisting of 1 (V1) to 66 (V66) vacancies have been performed, respectively. Based on these calculations, the formation and post-irradiation annealing behaviour of microvoids are examined.
In addition, irradiation-induced hardening monitored by Vickers microhardness is discussed with reference to positron annihilation experiments.