Effect of Si concentration on formation of vacancy complexes in electron-irradiated Fe16Cr-Si alloys studied by positron annihilation

The effect of Si concentration (0.2, 0.75 and 1.5 at.%) on properties of vacancy type defects formed under 5 MeV electron irradiation near 300 K in Fe-16 at.% Cr matrix has been studied by means of angular correlation of annihilation radiation technique. It is found that vacancy clusters are accumulated in Fe16Cr and Fe16Cr-0.2Si. Accumulation of monovacancies and their decoration with Si atoms are observed in Fe16Cr-1.5Si. Accumulation in Fe16Cr-0.75Si demonstrates transient features. Post-irradiation annealing kinetics in Fe16Cr and Fe16Cr-0.2Si differs from that in Fe16Cr-0.75Si and Fe16Cr-1.5Si. Annealing of vacancy clusters takes place in Fe16Cr and Fe16Cr-0.2Si, while vacancy clusters form and start annealing in Fe16Cr-0.75Si and Fe16Cr-1.5Si above 425 K. It is concluded that Si atoms aggregates are formed in Fe16Cr- 0.75Si and Fe16Cr-1.5Si under irradiation, which capture monovacancies and hold them up to 425 K. Mechanism of Si aggregate formation is discussed based on published data on interaction between vacancies and atoms of Si in Fe-Si alloys.