Thermal Stability and Transition Behavior of Nanoclusters during Two-Step Aging at 250°C in Al–Mg–Si(–Cu) Alloys

Abstract

Two types of nanoclusters, Cluster (1) and Cluster (2), play an important role in the age-hardening behavior in Al–Mg–Si alloys. Both Cluster (1) formed during natural aging at room temperature and Cluster (2) formed during pre-aging at 100°C cause the higher hardness and higher number density of the β″ phase during the two-step aging at 250°C than those of the single-aged specimen. Both of Cluster (1) and Cluster (2) result in the positive effect of two-step aging even though the mechanism is different. The positive effect of two-step aging for the naturally-aged specimen is caused by the atomic rearrangement from Cluster (1) to the Pre-β″ phase at the early stage of aging at 250°C. On the other hand, Cluster (2) is stable up to 250°C and directly transforms into the β″ phase. The thermal stability of nanoclusters and their transition behavior are discussed based on the age-hardening phenomena.