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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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Commercially available A7075 and A7050 alloys showed environmental degradation in ductility accompanied by intergranular fracture when tested at a slow strain rate of 10 Ϫ7 /s, compared to the ductility obtained by testing at a conventional strain rate of 10 Ϫ4 /s in a laboratory air. The degradation or embrittlement became severe with the increase in relative humidity of the testing environment. In every humidity, the embrittlement of the A7050 alloy was more serious than that of the A7075 alloy. Laboratory-made X7075 alloy which had the same amount of major solutes with the A7075 alloy but containing Zr in place of Cr revealed that additive Zr brought about more serious environmental degradation compared to the Cr-bearing alloy. The reasons for these phenomena were discussed in detail.