Clusters/Precipitates Evolution and Strengthening Mechanisms of an Al–Mg–Si Alloy during Ageing

The mechanical properties and microstructure evolution of an Al–Mg–Si alloy in the early stages of artificial ageing were analyzed by hardness tests, high-resolution transmission electron microscopy (HRTEM), and differential scanning calorimetry (DSC). The following precipitation sequence was found: supersaturated solid solution (SSSS) → MgSi spherical clusters → MgSi needle-shaped Guinier–Preston (GP) zones → pre-β" phase precipitates. The orientation of the needle-shaped GP zones changed from 〈111〉Al to 〈100〉Al during this sequence. The needle-shaped GP zones with the two orientations had relatively complete lattice parameters, but these parameters deviated slightly from those of the Al matrix. The significant increase in the hardness of the alloy at the beginning of ageing was attributed to the high number density of clusters/precipitates. The β″ phase played the dominant role in strengthening the Al–Mg–Si alloy. The main strengthening mechanisms of the alloy during different ageing stages, as well as the major factors affecting the alloy’s electrical conductivity were also studied.