Mechanism and kinetics of clustering in naturally-aged Al-Mg-Si alloys

Al-Mg-Si alloys are attractive for automotive applications due to their light weight and reasonable strength. It is well known that during natural ageing, small clusters of Mg and Si atoms form. These are detrimental to subsequent high temperature ageing as they reduce the achievable hardening during the paint-bake cycle. In this contribution, hardness, electrical resistivity and tensile tests are used to follow various aspects of the clusters evolution at -20, 0, RT and 50°C. In order to investigate the effect of vacancies during clustering, two different solution treatment temperatures were used. The results indicated that the higher solution treatment temperature will result in faster kinetics and higher degree of clustering. It was found that the major part of the cluster formation process happens during the first 30 minutes of ageing. A mechanical model for interaction of clusters with dislocations was also used to estimate the kinetics of clustering. It was shown that formation of clusters with time follows the JMAK model. Copyright © 2007 MS & T07®.