Positron lifetime study of the formation of vacancy clusters and dislocations in quenched Al, Al–Mg and Al–Si alloys

The clustering kinetics in quenched pure Al, binary Al–Mg and binary Al–Si alloys were studied by positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC) during natural ageing (NA). Shortly after quenching, positrons annihilate either in the bulk material or in vacancy-type defects such as mono-vacancies (in Al) and vacancy–solute complexes (in Al–Mg and Al–Si alloys). Upon NA, vacancy clusters of various sizes and number densities are formed. In Al, such clusters contain typically 3 vacancies. In Al–Mg and Al–Si alloys, complexes containing various vacancies and also solute atoms are formed. The presence of shallow positron traps was detected in temperature-dependent positron lifetime experiments. They were identified as quenched-in dislocations rather than Mg or Si clusters as no solute clustering signal during NA was observed in DSC runs of the binary Al–Mg and Al–Si alloys.