Effects of Cu Addition on Behavior of Nanoclusters during Multi-Step Aging in Al-Mg-Si Alloys

Abstract

Two types of nanoclusters, i.e. Cluster (1) and Cluster (2), play a strongly important role in the bake-hardening (BH) response in Al-Mg-Si alloys. Different formation behaviors of two types of nanoclusters were studied by means of hardness, differential scanning calorimetry (DSC), electrical resistivity measurement, transmission electron microscopy (TEM) and high resolution TEM observation in the both Cu-free and Cuadded Al-Mg-Si alloys. As the results, Cluster (1) formed during natural aging at room temperature causes a deleterious effect, whereas Cluster (2) formed by the pre-aging at 100 C is effective for the suppression of the negative effect on the two-step aging behavior of the both Cu-free and Cu-added alloys. On the other hand, the microalloying of Cu strongly affects the nanocluster formation due to the strong interaction with Mg, Si atoms and vacancies. The first-principle calculation for the two-body interaction energies provides quite useful information to understand the early stage of the phase decomposition. The effects of nanoclusters and the Cu addition on the age-hardening behaviors of Al-Mg-Si alloys are discussed based on the multi-step age-hardening phenomena.

Influence of Natural Aging Time on Two-Step Aging Behavior of Al-Mg-Si(-Cu) Alloys

“…The higher hardness in the Cu-added alloy is obtained than that in the Cu-free alloy during over-aging regardless of the NA time. The over-aging behavior is confirmed at the previous study 24,25) since the copper atoms segregated at the interface between the precipitates and the aluminum matrix suppress the growth of the precipitates. 19,33) Figure 7 shows the changes of the electrical resistivity during aging at 170°C with the different heat-treatment conditions in the Cu-free alloy.…”

“…25,30,3437) However, the increase in the hardness during aging at 170°C after NA is newly found during the middle stage. Namely, three stages of the two-step aging behavior with the NA time are first found in this study.…”

“…The similar results are also confirmed elsewhere. 25,3032) Namely, once NA is performed, Cluster (1) which does not directly transform into the ¢AA phase is formed resulting in the little change of the hardness during aging at 170°C up to 0.3 or 0.6 ks as shown in Figs. 3 and 4.…”

“…The electrical resistivity is affected by the nanocluster formation during NA and the change of the amount of the solute atoms such as the magnesium and silicon atoms and vacancies in the matrix. 25,29) The electrical resistivity is increased during NA, where the nanocluster formation and the decrease in the amount of the solute atoms and vacancies in the matrix simultaneously occur. The electrical resistivity change is more strongly affected by the nanocluster formation during the low temperature aging.…”

Formation Behavior of Nanoclusters in Al–Mg–Si Alloys with Different Mg and Si Concentration

“…The control of nanoclusters by the microalloying elements as well as the heat-treatment histories becomes extremely important in terms of the nanocluster formation at the early stage of the phase decomposition. 8,9) Furthermore, Otsuka et al 10) reported the effect of alloy composition on the formation of nanoclusters and nanoprecipitates. They concluded that the formation of nanoclusters markedly depends on the Mg, Si concentration and the Mg/Si ratio.…”