Shigetomo Nunomura1 and Kenichi Tsunakawa2
1Research Laboratory of Precision Machinery and Electronics, Tokyo Institute of Technology, Yokohama
The toughness of a 7075 precipitation hardening aluminum alloy was investigated in various heat treated condition. The behaviour of 10 Å radius GP zones was made clear from the variation in electric resistance during isothermal aging at the temperatures of 50, 80 and 120°C. 50 Å radius particls were detected from an early stage of 120°C isothermal aging by the X-ray small angle scattering observation. It was assumed that the maximum hardness observed during 24 h aging at this temperature was due to 50 Å radius GP zones and η' intermediate precipitates. No zone was observed in the specimen aged above 160°C. In the specimens aged for 24 h, the elongation in the tensile test decreased with increasing aging temperature up to 180°C, and recovered a little at 230°C. An instrumented Charpy impact bending test using the fatigue cracked specimen was made for the plane strain fracture toughness measurement. The value for the fracture characteristic, KQ, calculated from the energy to make an unit fracture surface was used instead of the effective KIC. This value was minimum at an isothermal aging temperature of 140°C in contrast to the elongation in the tensile test which was minimum in the case of 180°C aging. The fractography of tensile test specimens revealed that there were no changes corresponding to the variation of the elongation during the tensile test. The fractured surface of the test specimen accounted for the feature of the change in KQ. With respect to the toughness, prolonged aging at 80∼100°C appears to be more preferable to T6 treatment.
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