Quench rate sensitivity of age-hardenable Al-Zn-Mg-Cu alloys with respect to the Zn/Mg ratio: An in situ SAXS and HEXRD study

Al-Zn-Mg-Cu alloys with a low Zn/Mg ratio have attracted considerable attention in recent years as a result of an attractive property portfolio based on their ability of age hardening via the precipitation of the T-Mg32(Al,Zn)49 phase and its precursors. In this study, the quench rate sensitivity of an Al-Mg-Zn-Cu alloy with low Zn/Mg ratio is investigated and compared to a commercial reference Al-Zn-Mg-Cu alloy (AA7075) showing a high Zn/Mg ratio. The impact of five different cooling rates in the range of 0.25–100 K/s on the precipitation of quench-induced particles was studied by means of in situ small-angle X-ray scattering and high-energy X-ray diffraction. Subsequent continuous annealing experiments showed the influence of the cooling rate on the precipitation of fine hardening phases. Selected heat treatment conditions were further studied via scanning electron microscopy and atom probe tomography in order to reveal the microstructure and the chemical composition of the T-Mg32(Al,Zn)49 precipitates. The results showed that the Al-Mg-Zn-Cu alloy with low Zn/Mg ratio exhibits a lower quench rate sensitivity than the commercial AA7075 alloy. The lowest cooling rates, at which no quench-induced precipitation occurs, are in the range of 1 K/s for the investigated Al-Mg-Zn-Cu alloy and 100 K/s for the AA7075 alloy. The robust precipitation behavior of the Al-Mg-Zn-Cu alloy with a low Zn/Mg ratio is expected to be beneficial for advanced manufacturing processes.