Precipitation sequence in Al-Mg-Si alloys with excess magnesium

The precipitation sequence and the behavior of two-step aging in the Al-1.0 mass%Mg₂Si-0.4 mass%Mg (excess Mg alloy) alloy was investigated by hardness and electrical resistivity measurements, and high resolution transmission electron microscopy (HRTEM). In the excess Mg alloy at the early stage of aging at 473 K, two types of precipitates were observed as reported previously. First, the random type precipitates which indicate the non-periodic arrangement of bright dots in its HRTEM image; Second, the parallelogram-type precipitates which indicate a parallelogram arrangement of bright dots in its HRTEM image. A number of the parallelogram-type precipitates appeared near the maximum value of the hardness (HV_max). In the over-aged condition, many β′ phase appeared and, especially, the parallelogram type precipitates having interior angles near 60° also were observed. The HV_max in the excess Mg alloy aged at 473 K after pre-aging at 293 K for 6 ks was lower than that aged at only 473 K and this was a typical negative effect. The microstructure in this aging condition was coarse and there were a number of the random type precipitates with coarse β′ phase and cube-shaped phase. The HV_max in the excess Mg alloy aged at 473 K after pre-aging at 343 and 423 K for 6 ks were higher than that only aged at 473 K and this was a typical positive effect. The microstructure in this aging condition was fine and there were a lot of the multilayer G.P. zones and the random type precipitates. These precipitates existed in the specimen just after pre aging at temperaturos higher than 343 K.