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 Cu-added 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.