Positron annihilation studies on neutron irradiated pressure vessel steels

It has been demonstrated on a binary model alloy Fe-0.8%Cu heat treated to different states of Cu-distributions in the iron matrix that positron annihilation applied to such 'simpler' systems can give hints for a detailed understanding of neutron irradiation induced defect structures in complex industrial reactor pressure vessel steels (RPV). This study was concerned primarily with a high Cu-content weldment (0.3%) irradiated in a commercial nuclear power plant to a modest fluence of 5.4x1017 n/cm2 (E>1 MeV) and a medium Cu-content RPV forging (0.17%) irradiated in a materials test reactor up to 1.9x1019 n/cm2. Various positron lifetime and angular correlation-peak height measurements, the latter as a function of annealing temperature up to 750 degrees C, have revealed recovery stages which correlated quantitatively with the microhardness behaviour in the cases of the binary alloy and the forging. The interpretations of the peak height results have led to a detailed picture of precipitation, Ostwald-ripening and dissolution of Cu-rich clusters in the steels. Lifetime measurements in these systems can be understood in terms of trapping effects at the misfit structure of the interfaces between the Cu-precipitates and the iron matrix. They also showed that under the irradiation conditions applied here voids are not produced in the weldment and probably also not in the forging. This finding has been decisive also in the choice of possible model descriptions when interpreting small angle neutron scattering results. (author)