Aging behavior of direct-quenched (D. Q.) and water-quenched (W. Q.) after solution treatment in Al-10%Si-0.3~1.2%Mg alloy castings were investigated by micro-vickers hardness measurement, optical microscopy and transmission electron microscopy. The hardness of D. Q. specimens were the same as W. Q. specimens, when aged temperature lowered at 373K in Al-10%Si-0.3Mg, Al-10%Si-0.6Mg alloy and at 398K in Al-10%Si-1.2Mg alloy, respectively. When aging temperature was over 398K, the peak-hardness of W. Q. specimens was higher than that of D. Q. specimens in the Al-10%Si-0.3Mg and Al-10%Si-0.6Mg alloys. On the other hand, when the aging temperature was over 423K, the peak-hardness of W. Q. specimens was higher than that of D. Q. specimens in the Al-10%Si-1.2Mg alloy. The reason of those tendency contributes to the difference between clusters and G. P zone in the α-matrix.
