Role of quenched-in excess vacancies in two-step aging behavior of Al-Mg-Si alloys

Abstract

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.