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Age hardening in Al-10 and 12 wt%Zn alloys at room temperature was studied with a careful attention to the position of impression in hardness tests. The specimens 1.0 mm in thickness quenched at temperatures from 300° to 550°C and aged have a softened surface layer. This soft layer is considerably thick near the grain boundary in the specimens quenched at temperatures below 400°C. The specimens 0.2 and 0.4 mm in thickness have nearly the same hardness both in the surface layer and interior, although they have a softened surface layer near the grain boundary when quenched at temperatures below 400°C. Hardening proceeds at a slower rate in the soft region than in the hard region and ceases incompletely under certain conditions. These facts and transmission electron micrographs are discussed on the basis of a decrease in vacancies near the sinks such as surface and grain boundary.

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