A positron study of early clustering in Al Mg Si alloys

Early stages of clustering in quenched Al Mg Si alloys during natural ageing were studied by positron annihilation lifetime spectroscopy utilizing its unique sensitivity to electron density differences in various atomic defects. Two different positron trapping sites could be identified, one related to a vacancy type defect, the other to solute clusters. The first trap is deep, i.e. traps positrons irreversibly, the second shallow, from which positrons can escape, which creates the signature of a temperature dependent positron lifetime. During the first 80 min of natural ageing, the vacancy related contribution decreases, while the solute clusters increasingly trap positrons, thus reflecting their continuous growth and power to trap positrons. Coincidence Doppler broadening spectroscopy of the annihilation radiation shows that the annihilation sites are Si rich after quenching but contain more Mg after 70 mi