Microstructure evolution and precipitation behavior of Al-Mg-Si alloy during initial aging

The microstructure evolution and precipitation behavior of Al-Mg-Si alloy during initial aging were studied using hardness testing, conductivity testing, differential scanning calorimetry (DSC), and high resolution transmission electron microscopy (HRTEM). The results show that the precipitation sequence of the Al-Mg-Si alloy during initial aging can be represented as: supersaturated solid solution -> spherical Mg/Si clusters -> needle-like Guinier Preston (GP) zone -> beta ''. Clusters are completely coherent with the Al matrix. The GP zone with relatively complete independent lattice parameters that differ slightly from the Al matrix parameters, is oriented along the direction of < 111 > Al and lying on {111}(Al) plane. The strength of the Al-Mg-Si alloy is greatly enhanced by the threedimensional strain field that exists between the beta '' phase and the two {200}(Al) planes. After aging at 170 degrees C for 6 h, the hardness reaches the peak of 127 HV and remains for a long time. At this stage, the electrical conductivity keeps relatively stable due to the formation of coherent precipitates (Mg/Si clusters/GP zones) and the reduction in solute atom concentration in the Al matrix. The severe coarsening and decreased number density of the beta '' phase during the over-aging stage result in a significant decrease in the hardness.