The characterization of nanoscale clusters (nanoclusters) was performed using differential scanning calorimetry (DSC) and a three-dimensional atom probe (3DAP), to clarify the complicated aging behavior of an Al-Mg-Si alloy. The DSC results conducted over the temperature range 223 to 473 K revealed that two types of nanoclusters, i.e., Cluster(1) and Cluster(2), were formed near room temperature (RT) and 373 K, respectively. In the present work, the quantitative estimation of atom maps of the 3DAP analysis revealed the difference in the growth mechanism and the composition distribution of the two types of nanoclusters. The distribution of both the size and Mg/Si ratio of Cluster(1) does not change during prolonged natural aging. On the other hand, Cluster(2) grows gradually with preaging time. The Mg/Si ratio of the larger-sized Cluster(2) approaches a constant value that is equal to that of the b¢¢ phase. The difference in the two-step aging behavior can be explained by the different growth mechanisms and chemical compositions of the two nanoclusters. This means that only Cluster(2) can easily transform continuously into the b¢¢ phase during the BH treatment, due to its size and compositional similarity.

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To correlate quantitatively the mechanical properties of precipitate free zones (PFZ) with the corresponding microstructural and compositional characteristics, TEM observation, EDX analysis and nanoindentation measurement have been performed in the vicinity of grain boundaries in Al -4.9 mass%Zn -1.8 mass%Mg (2 0.28 mass%Ag) alloys. The remarkable decreases in hardness and solute concentrations were observed towards grain boundaries even in the regions just outside PFZ. With increasing aging time, it is firstly revealed that the hardness inside PFZ monotonously decreases although the hardness inside grains increases in the earlier stage of aging. Three distinct regions of "PFZ", "Transition-area" and "Grain-region" were therefore proposed to explain the origins of such age-hardening behavior observed in this work. In the Ag-added alloy, on the other hand, the hardness could be maintained up to closer regions to grain boundaries at the same level as that inside grains. q

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The formation of nano-scale clusters (nanoclusters) prior to the precipitation of the strengthening β" phase significantly influences the two-step aging behavior of Al-Mg-Si alloys. In this work, two types of nanoclusters are found to be formed at different temperatures. The characterization of these two nanoclusters has been performed from the viewpoints of composition and thermal stability using a three-dimensional atom probe (3DAP) and differential scanning calorimetry (DSC). Mg-Si co-clusters formed at room temperature (RT), Cluster(1), play a deleterious role in the subsequent formation of the β" phase because of the high thermal stability even at the bake-hardening (BH) temperature of 443K. In contrast, the nanoclusters formed by pre-aging at 373K, Cluster(2), are effective in the formation of the refined β", suggesting that Cluster(2) transforms more easily into the β" phase than Cluster(1). The quantitative estimation of the chemical compositions of the two nanoclusters suggests that the Mg/Si ratio is one of the key factors in addition to the internal structures consisting of Si, Mg and probably vacancies. The detailed two-step aging mechanism in Al-Mg-Si alloys is proposed based on the characteristics of the two types of nanoclusters.