Effects of Quenching Conditions on Two-Step Aging Behavior of Al-Mg-Si Alloys

Two-step aging behavior of Al-Mg-Si alloys was investigated using hardness and electrical resistivity measurements, adiabatic calorimetry and transmission electron microscopy. In particular, the effects of quenching conditions have been extensively examined in terms of the negative effect of two-step aging. The specimens used were Al-1.4%Mg 2 Siand Al-1.4%Mg 2 Si-0.3%Si alloys (in mass%). Quenching conditions such as the water-quenching (W.Q.), step-quenching (S.Q.) and the subsequent first-aging were found to affect markedly the peak hardness of two-step aged specimens. The step-quenching and the first-aging are extremely effective to reduce the negative effect of the pre-aging at room temperature. These behaviors were interpreted in terms of the competitive nucleation of the two types of clusters and the formation of Si-vacancy clusters during the water-quenching and subsequent pre-aging.