Effects of Cu and transition metals on the precipitation behaviors of metastable phases at 523 K in Al-Mg-Si alloys

Morphology and crystal lattice of precipitates formed in the 6000 series Al-Mg-Si alloys containing Cu and transition metals of Cr and Fe (TM) after aging at 523 K were investigated by high-resolution transmission microscopy (HRTEM). The precipitates in the Cu-bearing alloys are more finely and densely distributed than those in the Cu-free alloys, whereas those in the TM-bearing alloys were coarser and formed inhomogeneously at the interfaces between the dispersoids and the matrix. Three kinds of metastable phases were detected by HRTEM as follows: β'-phase in the balanced alloy, the TYPE-B precipitate in the excess-Si alloy, and Q'-phase in the Cu-bearing alloys. No significant difference in between the precipitates in the TM-free alloys and in the TM-bearing alloys was observed. However, the Type-B precipitate in the excess-Si alloy was replaced with the β'-phase by the addition of TM. This suggests that the chemical composition of the excess-Si alloy changes to the balanced composition, because the excess Si in the matrix has been consumed by the formation of the AlSi(Fe, Cr) and AlSiFe dispersoids.