Influence of Sn on the age hardening behavior of Al-Mg-Si alloys at different temperatures

Addition of minute amounts of Sn to Al-Mg-Si alloys is known to have a pronounced effect on their age hardening characteristics. In this study, the influence of Sn addition on the ageing behavior of lean and concentrated alloys at five different temperatures was studied. Hardness, positron annihilation lifetime spectroscopy and transmission electron microscopy measurements complemented by differential scanning calorimetry yield information that allow one to assess the microscopic mechanisms that govern ageing. It is found that Sn slows down the ageing kinetics at 100 degrees C and 140 degrees C but accelerates the kinetics and enhances the hardening response at 210 degrees C and 250 degrees C. At the standard artificial ageing temperature of 180 degrees C, the effect of Sn on ageing varies depending on the alloy composition. The observed different ageing kinetics can be explained by the different vacancy behaviors in the presence of Sn. Moreover, the activation energy analysis reveals that the ageing process in Sn-added alloys is controlled by both the separation of Sn-vacancy complexes and the migration of solute-vacancy complexes.