Temperature dependence of precipitation behavior of Al-6Mg-9Si-10Cu-10Zn-3Ni natural composite and its impact on mechanical properties

The effects of aging temperature (25-220 °C) on precipitation behavior and the resulting alloy strength were characterized for the Al-6Mg-9Si-10Cu-10Zn-3Ni (wt%) natural composite. At an aging temperature of 25 °C, spherical and ellipsoidal Guinier-Preston (GP) zones (Zn clusters) are formed within the Al matrix. The fastest kinetics of Zn clustering is exhibited at 70 °C, which is caused by the combined effects of accelerated Zn diffusion and decreased phase thermal stability with increasing aging temperature. Thus, the Zn clusters and precipitates are formed prior to those of Cu (GPI and θ'') at 70 °C. As the precipitation kinetics of θ accelerates and Zn decelerates at above 70 °C, the precipitation sequence changes at 170 °C; clusters and precipitates of Cu are formed prior to those of Zn at this temperature. At an aging temperature of 220 °C (above the solvus of hcp Zn), θ'-Al2Cu and Q′-Al4CuMg6Si6 phases are mainly precipitated instead of Zn clusters and precipitates. The matrix strengthening induced by the fine clusters and precipitates that form below 70 °C results in a significant improvement in the strength of the composite. However, high-temperature aging has only a small strengthening effect due to the lower matrix hardening caused by precipitate coarsening and the softening caused by the decreased volume fraction and spheroidization of the secondary phases.