Photo 4 Step-aged structures in specimens of 0.1mm thickness (pre-aging time, 15min). Rerfer to Fig. 7. Foil orientation, [001]. The figure shows that the structure is not homogeneous in this case.

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The role of Cu in aging phenomena in AI-Zn-Mg-Cu alloys was investigated by hardness and specific heat measurements and transmission electron microscopy. Various specimens, based on (a+T; type AI-Zn-M ~ alloys, with a nearly fixed amount of 5.5 at % in (Zn-;-Mg) and with different Cu contents were used. Tia specimens were solution-treated at 470 °C for 2 hrs, quenched into iced-water, and finally subjected to aging ai various tempera tures. The results obtained were as follows :(1) As Cu content increased, the initial rate of age hardening and the maximum hardness attained al 125-150°C aging increased. The time required for hardness to reach the maximum also increased. This indicate that the quarternary alloys are superior to the ternary alloys with respect to hardness and heat resistant character lstlcs.(21 Specific heat measurement and observation on transmission electron microscopy showed that twc sorts of precipitation processes would exist in Al -Zn-Mg-Cu alloys when Cu contents exceeded about 0.7wt°~,,One process was that observed in Al-Zn-Mg alloys and the other was that observed in AI-Cu-Mg alloys. aculty of Engineering, University of Tokyo , (Tokyo)