Materials Transactions Online

Materials Transactions, Vol.43 No.6 (2002) pp.1371-1380
© 2002 The Japan Institute of Metals

Effects of Excess Mg and Si on the Isothermal Ageing Behaviours in the Al-Mg2Si Alloys

Long Chau Doan*, Kiyomichi Nakai, Yoshitsugu Matsuura*, Sengo Kobayashi and Yasuya Ohmori

Department of Materials Science and Engineering, Ehime University, Matsuyama 790-8577, Japan

The effects of excess Mg and Si contents on the isothermal ageing processes of Al-Mg2Si alloys have been investigated by means of transmission electron microscopy and the following results were obtained. After the formation of β'' needles inducing large age hardening, cuboid β particles precipitate in both the excess Mg and the quasi-binary alloys, but fine Si particles nucleate in the excess Si alloys. In the quasi-binary alloy, the following reaction is the precipitation of β' rods and then β plates form. In the excess Si alloys, various rodlike precipitates form after the precipitation of β'' needles and Si particles in the sequence: Type-A rods→Type-B rods→β' rods. This precipitation sequence can be understood by considering the chemical compositions of them determined by Matsuda et al..

(Received February 6, 2002; Accepted April 24, 2002)

Keywords: aluminum-magnesium-silicon alloys, isothermal ageing, β'' needles, cuboid β particles, β' rods, Type-A rods, Type-B rods, β-Mg2Si plates

*Graduate Student, Ehime University.

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