Effect of Post-Irradiation Annealing on Defects in Irradiated Model Alloys by Means of Positron Annihilation Coincidence Doppler Broadening Spectroscopy

The phenomena of ductility recovery by thermal annealing can be studied by isochronal heating of irradiated reactor pressure vessel steels. A start was given in this thesis. Two irradiated binary model alloys Fe 0.3% Cu; 0.1 dpa and Fe 0.3% Cu; 0.2 dpa were annealed over a temperature trajectory from 300°C to 700°C and analyzed by means of coincidence Doppler broadening (CDB) spectroscopy. It is known that positrons are attracted, trapped and annihilated at both vacancy and nano-size copper precipitate sites. Therefore, CDB spectroscopy is an interesting technique for studying the evolution of matrix damage and copper precipitates with temperature. The main finding is the confirmation of two clear recovery stages during the annealing process, each with its rather well defined temperature. At around 450°C, the vacancies which are believed -if surviving- to be trapped in the copper precipitates, dissociate from these precipitates to diffuse in the bulk and finally disappear at sinks. At around 650°C, the copper precipitates dissolve and diffuse into the bulk where they probably homogeneously disperse. The irradiation dose did not seem to have a particular influence on the phenomena apart from the obvious larger initial amount of vacancies in the second sample.