POSTIRRADIATION RECOVERY OF A REACTOR PRESSURE-VESSEL STEEL INVESTIGATED BY POSITRON-ANNIHILATION AND MICROHARDNESS MEASUREMENTS

Positron lifetime and microhardness measurements have been performed o n untreated, thermal-aged, neutron-irradiated, and postirradiation-ann ealed samples of reactor pressure vessel steels with the purpose of in vestigating the mechanisms of irradiation-induced hardening and recove ry of the mechanical properties in these materials. The positron lifet ime experiments have not revealed any evidence of the formation of a s ignificant concentration of voids or vacancy clusters in samples irrad iated at approximately 290-degrees-C with fluences less-than-or-equal- to 2.71 x 10(23) n/m2 (E > 1 MeV), but they suggest a dislocation anne aling induced by the irradiation. Isochronal annealing experiments wit h neutron-irradiated samples show a simultaneous recovery in their pos itron lifetime and microhardness at approximately 340-degrees-C. From the microhardness measurements, the yield strength of the irradiated m aterial has been estimated. The results appear to be consistent with a model of hardening due to irradiation-induced dissolution of precipit ates with formation of small metastable precipitates after postirradia tion aging and recovery induced by the disappearance of these metastab le precipitates.