Kenji Matsuda1, Tomoo Yoshida1, Tomomi Wada1, Megumi Yoshida1,Yasuhiro Uetani2, Tatsuo Sato3, Akihiko Kamio3 and Susumu Ikeno1
1Department of System Engineering for Materials and Life Science, Faculty of Engineering, Toyama University, 3190, Gofuku, Toyama 930-8555
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 (HVmax). In the over-aged condition, many β′ phase appeared and, especially, the parallelogram type precipitates having interior angles near 60° also were observed.
The HVmax 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. HVmax 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.
(Received March 19, 1998)
aluminum-magnesium-silicon alloy, precipitation sequence, two-step aging, excess magnesium, high resolution transmission electron microscopy
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