Effect of Copper Addition on the Cluster Formation Behavior of Al-Mg-Si, Al-Zn-Mg, and Al-Mg-Ge in the Natural Aging

The time-dependent resistivity of Al-Mg-Si(-Cu), Al-Zn-Mg(-Cu), and Al-Mg-Ge(-Cu) alloys are studied over a range of constant temperatures between 255 K and 320 K. The resistivity vs time curves for the samples show three temperature stages associated with solute element–vacancy clustering. Cu addition was found to make the stage transition time longer for the studied samples. Arrhenius plots of the transition time vs temperature provide the activation energy (Q) of clustering from stage I to II and II to III. While the Cu addition increased the Q(I to II) values of Al-1.0 pct Mg₂Si-0.20 pct Cu and Al-2.68 pct Zn-3.20 pct Mg-0.20 pct Cu, it was found that the added Cu decreased the Q(I to II) value of Al-0.44 pct Mg-0.19Ge-0.18 pct Cu. The Q(II to III) values of Al-1.0 pct Mg₂Si and Al-2.68 pct Zn-3.20 pct Mg were slightly decreased by the Cu addition. The different effect of added Cu on the Q values is discussed in terms of diffusivity and binding energy between vacancies and solute elements.