Two-step aging behavior of an Al-10%Si-0.3%Mg alloy cast into permanent mold and then demolded at 473 K was investigated by a micro-vickers hardness measurement and scanning electron microscope observation. The peak hardness indicated almost the same value when the pre-aging temperature was R. T. and then final-aged at 423-523 K. On the other hands, positive effect of the final-aging was occurred after pre-aging at 348-423 K for a relatively long time with significantly increasing hardness in the underaging region. These tendencies were similar to the previously reported result of demolded at 773 K. In the case of the specimens were pre-aged at 373 K and 423 K for a short time without increasing hardness, negative effect of the final-aging was occurred. The behavior was different from previously reported result of demolded at 773 K. The reason of occuring the negative effect was not only difference of solute concentration with the demolding temperature, but also differences of amount of G. P. zones and clusters generated on the early stage of pre-aging at various temperatures.
