Two types of nanoclusters, termed Cluster (1) and Cluster (2) here, both play an important role in the age-hardening behavior in Al-Mg-Si alloys. Small amounts of additions of Cu and Ag affect the formation of nanoclusters. Two exothermic peaks were clearly detected in differential scanning calorimetry(DSC) curves by means of peak separation by the Gaussian method in the base, Cu-added, Ag-added and Cu-Ag-added Al-Mg-Si alloys. The formation of nanoclusters in the initial stage of natural aging was suppressed in the Ag-added and Cu-Ag-added alloys, while the formation of nanoclusters was enhanced at an aging time longer than 259.2 ks(3 days) of natural aging with the addition Cu and Ag. The formation of nanoclusters while aging at 100˚C was accelerated in the Cu-added, Ag-added and Cu-Ag-added alloys due to the attractive interaction between the Cu and Ag atoms and the Mg atoms. The influence of additions of Cu and Ag on the clustering behavior during low-temperature aging was well characterized based on the interaction energies among solute atoms and on vacancies derived from the first-principle calculation of the full-potential Korrinaga-Kohn-Rostoker(FPKKR)-Green function method. The effects of low Cu and Ag additions on the formation of nanoclusters were also discussed based on the age-hardening phenomena.

The effect of Al addition on the precipitation behavior of a binary Mg-Zn alloy was investigated based on thechanges in the morphology, distribution and element concentration of precipitates formed during aging treatment. The as-castMg-6.0 mass%Zn (Mg-6Zn) and Mg-6.0 mass%Zn-3.0 mass%Al (Al-added) were homogenized at 613K for 48h and at 673Kfor 12h; they were then solid solution treated at 673K for 0.5 h and 1 h, respectively. The Mg-6Zn and Al-added alloys wereaged at 403 K and 433K. The peak hardness of the Al-added alloy was higher than that of the Mg-6Zn alloy at each agingtemperature. Rod-like, plate-like, blocky, and lath-like precipitates were observed in the Al-added alloy aged at 433K for230.4ks, although the rod-like and plate-like precipitates were observed in the TEM microstructure of the Mg-6Zn alloy agedat 433K for 360 ks. Moreover, the precipitates in the Al-added alloy were refined and densely distributed compared with thosein the Mg-6Zn alloy. The Cliff-Lorimer plots obtained by the EDS analysis of the rod-like and plate-like phases in theAl-added alloy peak aged at 433K for 230.4ks were examined. It was confirmed that the phases had higher concentrationof solute Al atom than was present in the phases, indicating that the properties of precipitates can be changed by Al addition.