Surveillance specimens of reactor pressure vessel steel SA 508 irradiated to a fluence of $1.03×10^{19}$ n/㎠ (E > 1MeV) were given isothermal annealing to study recovery of irradiation hardening.
After isothermal annealing at three different temperatures in the range of 405-454℃ for a time interval up to 2353 minutes, microhardness tests were conducted. Instead of the fractional recovery method based on a single activated process, the superposition principle that is based on the sum of unirradiated microhardness and microhardness changes due to presence of various defect types was employed to analyze hardening recovery mechanisms and activation energies associated with their annihilations.
The present analysis showed that irradiation hardening recovery may be divided into two mechanisms. Recovery mechanism 1 with activation energy of 1.89eV is accounted for by the coalescence due to vacancy evaporation of depleted zones that exist so closely to interact with one another. Recovery mechanism 2 with activation energy of 2.19eV give rise to for 75% of total microhardness recovery. This mechanism is accounted for by both coarsening of Cu precipitates/annihilation of Cu-vacancy aggregates and vacancy evaporation from microvoids that act independently from one another or solely by the former process involving Cu-bearing entity.
The activation energy determined by the fractional recovery is 1.92eV. This energy appears to be biased with respect to ratio of defects responsible for recovery mechanism 1 and 2, namely, 25% and 75% respectly.
