Effects of Isothermal Aging on Clustering and Paint-bake Hardening Behavior in an Al-Mg-Si Alloy

The relationship between the cluster morphology formed during natural or artificial aging and the paint-bake hardening response in an Al-0.62Mg-0.93Si (mass%) alloy have been investigated using atom probe tomography (APT). Increasing the subsequent aging time at 170 degrees C causes a gradual increase in hardness in the artificially aged materials, while the retardation period of the hardness increase appears in the naturally aged materials at the early stage of aging. The statistically-proved records in the APT analysis have shown that the artificially aged materials have some large clusters. It is revealed that the hardening at the early stage of the subsequent aging at 170 degrees C. is not promoted in the long-time naturally aged material although the number density of small clusters increases approximately 1.3 times by prolonged natural aging. Hence, we believe that the small clusters are hard to transform continuously into the beta '' phase during aging at 170 degrees C. As for the naturally aged materials, the long-time aging leads to a significant drop in hardness at the early stage of aging at 170 degrees C. It is speculated that the Mg-Si mixed clusters formed after long-time natural aging can be reversed during the subsequent heat treatment.