Irradiation Induced Vacancy-Cu Aggregations in Fe-Cu Model Alloys of Reactor Pressure Vessel Steels Studied by Positron Annihilation

Abstract

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.

Current understanding of radiation-induced degradation in light water reactor structural materials

“…Vacancy components such as vacancies and microvoids can be examined by using PA techniques. Several studies using positron lifetime measurements showed that microvoids containing more than 10 vacancies were formed in binary or ternary model alloys [105,106]. A recent study showed that only vacancies and divacancies existed in irradiated Fe-Mn-Ni and Fe-Mn-Ni-Cu model alloys, while vacancy clusters containing more than 10 vacancies were formed in Fe-Cu alloys [104].…”

“…However, it is necessary to carefully consider interactions between point defects and solute atoms. It is evident that Cu-rich clusters in high-Cu model alloys are formed in a manner that Cu atoms cover the inner surface of microvoids [84,105]. Some fraction of vacancies must be assumed for explaining SANS data (mass density of the clusters) of the clusters formed in commercial steels [3].…”