Post-β″ phases and their influence on microstructure and hardness in 6xxx Al-Mg-Si alloys

Starting from solid solution (T4) or a condition with β″ precipitates (T6), three Al-Mg-Si alloys with similar total solute content (1.3 at%), but different Si/Mg ratios (2, 1.25 and 0.8) were isothermally heat-treated at 250 or 260°C and investigated by transmission electron microscopy. The result microstructure for all alloys and conditions consisted of metastable, needle-shaped precipitates growing along 〈100〉 directions in aluminium. Each of the phases β″-Mg5Si6, β′-Mg1.8Si, U1-MgAl2Si2 and U2-MgAlSi could be identified as main precipitate in the alloy with its solute Si/Mg ratio closest to the same ratio in the composition of that particular phase: The highest Si content alloy produced coarse needles of the trigonal U1-phase coexisting with finer precipitates of hexagonal B′-phase. The most common phase in the Mg-rich alloy is coarse needles of hexagonal β′-type. The Si/Mg ratio of 1.25 in one alloy is similar to the Si/Mg ratio in β″. Here the microstructure changes from that of fine β″ needles to fine needles of the orthorhombic U2-phase. This material remains strongest during heat-treatment. Nucleation on dislocations, mainly by the B′-phase, was observed to be significant in the case of Si-rich alloys heat-treated from T4-condition.