PRECIPITATION SEQUENCE IN AL-MG-SI ALLOYS WITH EXCESS MAGNESIUM

The precipitation sequence and the behavior of two-step aging in the A l-1.0 mass%Mg2Si-0.4 mass%Mg (excess Mg alloy) alloy was investigated by hardness and electrical resistivity measurements, and high resoluti on transmission electron microscopy (HRTEM). In the excess Mg alloy at the early stage of aging at 473 K, two types of precipitates were obs erved as reported previously. First, the random type precipitates whic h indicate the non-periodic arrangement of bright dots in its HRTEM im age; Second, the parallelogram-type precipitates which indicate a para llelogram arrangement of bright dots in its HRTEM image. A number of t he parallelogram-type precipitates appeared near the maximum value of the hardness (HVmax). In the over-aged condition, many beta' phase app eared and, especially, the parallelogram type precipitates having inte rior angles near 60 degrees also were observed. The HVmax in the exces s Mg alloy aged at 473 K after pre-aging at 293 K for 6 ks was lower t han that aged at only 473 K and this was a typical negative effect. Th e microstructure in this aging condition was coarse and there were a n umber of the random type precipitates with coarse beta' phase and cube -shaped phase. The HVmax in the excess Mg alloy aged at 473 K after pr e-aging at 343 and 423 K for 6 ks were higher than that only aged at 4 73 K and this was a typical positive effect. The microstructure in thi s 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 temperatures higher than 343 K.