Early-stage Precipitation Phenomena and Composition-dependent Hardening in Al-Mg-Si-(Cu) Alloys

Al-Mg-Si-(Cu), i.e. AA6xxx, alloys are widely used light alloys which can be effectively strengthened through precipitation hardening. The final microstructure, and thus properties, of these alloys after common artificial aging treatments are largely determined by the composition-dependent nano-scale clustering and precipitation that occur during the earliest stage of aging. Therefore, multi-length scale analysis of the earliest-stage of precipitation can provide critical knowledge in understanding the basis for the microstructural evolution during aging and attaining the desired microstructures and properties. Here, we investigate the effect of alloy composition on the evolution of early-stage clusters and precipitates during aging at 180 degrees C using high resolution transmission electron microscopy. The results map a sequential evolution of clusters with an FCC structure but different morphology/orientation characteristics. GP-zones with structures other than FCC, also form in the early stages of aging. The composition-dependent kinetics of beta" phase precipitation and hardening behavior are discussed in light of the results from differential scanning calorimetry experiments, microhardness measurements, and conventional transmission electron microscopy.