Evolution of the second phase particles during the heating-up process of solution treatment of Al-Zn-Mg-Cu alloy

The evolution of the second phase particles during the heating-up process of solution treatment of the Al-Zn-Mg-Cu alloy has been studied by OM, SEM, TEM observations together with XRD, hardness, electrical conductivity and tensile tests. The evolution of the second phase particles consisted of three stages: (i) the precipitation of MgZn2 at the low temperature range (from room temperature to 350 °C); (ii) the dissolution of MgZn2 and coarsening of Al2CuMg at the medium temperature range (from 350 °C to 450 °C); and (iii) the dissolution of Al2CuMg at the high temperature range (above 450 °C). The coarsening of the Al2CuMg at the range of medium temperature is believed to be the main reason for the decrease of the mechanical properties of the alloy because the coarsened Al2CuMg has considerable difficulty to dissolve in the following solution treatment.