Vacancy Behavior and Solute Cluster Growth During Natural Aging of an Al-Mg-Si Alloy

The natural aging behavior of an Al-0.46Mg-1.05Si-0.14Fe (wt pct) alloy was studied with positron annihilation lifetime spectroscopy at four temperatures and with 25Mg solid-state nuclear magnetic resonance. The evolution of positron lifetime is shown to consist of three distinct stages. In an effort to understand the physical processes occurring during natural aging, a phenomenological model was developed for the first two stages and is shown to describe the experimental observations well. The description accounts for the decay in positron lifetime due to diffusion of vacancies to sinks and an increase in positron lifetime attributed to growing solute clusters. NMR measurements of 25Mg solute partitioning during natural aging support the model.