Transmission electron microscopic observation of precipitates in age-hardened die-cast aluminum alloys

Squeeze die casting with a composition similar to JIS ADC12 aluminum alloy was aged at 453 K with or without prior solution treatment. Aging following solution treatment, which is termed T6-treatment, increased the hardness of this alloy. Using transmission electron microscopy, we investigated precipitates responsible for the age-hardening. Concentration of solute silicon, copper and magnesium during aging led to the formation of several types of precipitates in the matrix face-centered cubic (fcc) Al-phase. Precipitates exhibiting equiaxed shape and possessing copper-enriched faces at the interface with the Al-phase were observed. We suggest that these precipitates have fcc structure containing silicon, copper and magnesium atoms substituting for aluminum. Prolonged aging caused the formation of another type of precipitates with elongated shape, and they were identified to be the S″-phase. Some regions within the precipitates of the S″-phase transformed into the Q-phase with increasing silicon content in the precipitates. Precipitates of the S-phase and θ′-phase also formed by over-aging.