Positron annihilation study of vacancy-solute complex evolution in Fe-based alloys

Irradiation-induced vacancy-type defects in Fe-based dilute binary alloys (Fe-C, Fe-Si, Fe-P, Fe-Mn, Fe-Ni, and Fe-Cu), model alloys of nuclear reactor pressure vessel steels, are studied by positron annihilation methods, positron lifetime, and coincidence Doppler broadening (CDB) of positron annihilation radiation. The vacancy-type defects were induced by 3 MeV electron irradiation at room temperature. The defect concentrations are much higher than that in pure Fe irradiated in the same condition, indicating strong interactions between the vacancies and the solute atoms and the formation of vacancy-solute complexes. The vacancy-solute complexes in the Fe-Cu, Fe-Ni, and Fe-C alloys are definitely identified by the CDB technique. In particular, the single vacancies in Fe-Cu are surrounded by Cu atoms, resulting in vacancy–multi-Cu-atom complexes of V − Cu_n (n > 6). The vacancy clusters are formed in Fe-Ni and Fe-P, while they are not observed in the other alloys. The isochronal annealing behavior is also studied. The ultrafine Cu precipitates coherent with the Fe matrix are formed in Fe-Cu after annealing at 350 °C, while the other alloys show complete recovery without forming any precipitate that traps the positrons.