Aging characteristics of AM60 magnesium alloy cast into sand mold were investigated by means of hardness measurement and microstructure observation with optical microscopy. The microstructure of this alloy in the as cast condition with the cooling rate of 5.2 K/s was composed with a primary crystallized alpha magnesium phase and eutectic phase consisting of alpha magnesium and beta Mg17Al12 intermetallic compound. The eutectic phase was divided to none-equilibrium crystallized phase and precipitated phase. The hardness of cellular precipitation area in aged samples decreased with an increasing of the aging time. On the other hand, the hardness of intragranular precipitation area was constant against the aging time. Variation of hardness with aging was found to be caused mainly by the discontinuous precipitation along cell boundaries from the rule of mixture in hardness.
