Influence of sub-grain boundaries on quenching process of an Al-Zn-Mg-Cu alloy

The effects of sub-grain boundaries on the quenching sensitivity and the precipitation behavior in Al-7.01Zn-1.26Mg-1.43Cu alloy were investigated by an end-quenching test. Specimens were solution treated at 440 °C and 480 °C to get different recrystallization fractions, respectively. The results show that the maximum hardness value of the Al-Zn-Mg-Cu alloy can be improved by the sub-grain boundaries, but the depth of age-hardening layer decreases significantly. The precipitation temperature and the activation energy are reduced by the changes of surface energy, which is induced by sub-grain boundaries. So, the precipitation process from η′ phase to η phase becomes much easier. In this way, an increase in the number of sub-grain boundaries promotes the precipitation of MgZn2 particles, especially η′-MgZn2.