Effects of Quenching Conditions on Nanostructure and Age-Hardening Behavior of an Al-Zn-Mg Alloy

In this study, nanostructure formation and age-hardening behavior of an Al-6Zn-0.75Mg alloy were investigated by varying cooling conditions during solution heat treatment. For the sample furnace-cooled from 723 K to around 303 K, the hardness after cooling was higher than the hardness after quenching of the WQ sample, and the maximum hardness was equivalent to that of the WQ sample. On the other hand, samples taken out of the furnace at around 373 K during furnace cooling showed a slower hardening rate and a relatively lower maximum hardness. Microstructure observations at as quenched (A.Q.) and as cooled (A.C.) state showed nanostructures in the FC-303 K sample that were considered to have formed during furnace cooling, which is consistent with the structure reported as GP(I) with TCO structure. Clusters and GP zones formed during furnace cooling tend to form at temperatures below 373 K, suggesting that they contribute to age hardening.