Kentarou Yamada and Tatsuo Sato
Department of Metallurgy and Ceramics Science,Tokyo Institute of Technology, Tokyo 152-8552
The effects of various quenching conditions on both the pre-aging at room temperature(R.T.) and subsequent two-step aging behavior of an Al-Mg-Si alloy were investigated mainly by electrical resistivity and hardness measurements. Three types of quenching methods ; i.e. water-quenching(W.Q.) , step-quenching (S.Q.) and the first pre-aging treatment at 373 K were used before the subsequent two-step aging treatment. The results were discussed in terms of the role of quenched-in excess vacancies in the early stage of precipitation (i.e. Si-rich clusters and GP zones). The results clearly indicate that the quenched-in excess vacancies are trapped inside GP zones during the first pre-aging at 373 K. This effect suppresses the Si-rich cluster formation in the second pre-aging at R.T. and resultantly the negative effect in the final aging at 453 K. It is, therefore, important to control the heat treatment sequence for Al-Mg-Si alloys so that the quenched-in excess vacancies are more associated with the GP zones, the effective nucleation sites for the β′′ phase, before the formation of the Si-rich clusters which are non-effective nucleation sites for the β′′ phase.
(Received June 19, 2001)
aluminum-magnesium-silicon alloy, two-step aging, quenched-in excess vacancy, step-quenching, Guinier-Preston zone, silicon-rich cluster
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