Aging behavior of step castings at different cooling rates in Al-10%Si-0.3%Mg alloy were investigated by micro-vickers hardness measurement, optical, scanning electron and transmission electron microscopy. The hardness of as-cast states increased with increasing cooling rate. Despite the difference in hardness before artificial aging in as-cast states, the hardness in artificial aging curves was varied with aging time. The difference in hardness between fat and thick portions was due to refining eutectic Si phase with increasing cooling rate. There was no precipitation in any wall thickness in the as-cast castings. On the other hand, in peak-aged specimens obtained from castings with 15mm thick walls, which consists of two regions, one is the domain of coarse precipitation and the other one is fine precipitation. The rod-like and granular precipitates were observed in peak-aged specimens obtained from castings with 1mm thick walls.
