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Abstract

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.

Effect of an Addition of Silver on the Age Hardening and the Formation of a Soft Surface Layer in an Al-12mass%Zn Alloy

“…When no surface layer is removed (as aged), hardness number decreases with decreasing load less than 9.8N. If the specimen was homogeneous in hardness from the surface inward, hardness number would not show the dependence on the load, which has been confirmed by measuring a reference specimen [1]. Decrease in hardness with decrease of the load, therefore, means existence of soft surface layer.…”

“…The aging of Al-Zn alloys has been vastly studied for decades. In the previous paper, Ohta et al studied carefully the hardness of the alloy during aging and revealed the existence of softer regions near the surface and the grain boundary than the interior of the grain even after aging for a long time [1]. Such soft regions were also found in the aged Al-Ag alloys [2].…”

Soft Surface Layer of Age-Hardened Al-Zn Alloys and Its Effect on the Fatigue Strength

“…Lately, Ohta et al measured carefully the dependence of hardness on the position of the grain during aging of an Al-Zn alloy containing about 10mass% of Zn and found that the regions near the surface and near the grain boundaries remained a little softer than the regions distant from them, even after aging for a long time [2]. This effect was profound near the surface.…”

“…Pahl et al [5] studied by X-ray small-angle scattering the effect of the repeated tensile loading on the growth of GP zones in Al-Zn alloy and found that small zones with Guinier radius less than about 2nm could grow during the loading but larger zones could hardly grow. The results of the transmission electron microscopy [2] showed that the radius of the GP zones near the surface of the aged specimen was larger than 3nm in the Al-Zn alloy. The hardening near the surface observed after the fatigue test of the specimen with soft surface layer is, therefore, considered to be caused by the work hardening rather than the growth of GP zones.…”