Microstructures of the commercial 7075 Al alloy in the T651 and T7 tempers
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I.
INTRODUCTION
THE aging characteristics of pure ternary Al-Zn-Mg alloys have been studied extensively due to the technical importance of this system. Much of the effort has concentrated on elucidating the nature of the precipitate microstructures (i.e., their morphology, distribution, and crystallography) formed during aging, with the ultimate aim of understanding the effect of microstructure on mechanical behavior. The results of early X-ray studies ~-6 provided evidence for the existence of solute-rich clusters, spherical G. P. zones, and the .1' transition phase, from which it was deduced that the aging sequence at temperatures below 190 °C 7'8 is a ~ G. P. zones ~ .1' ~ .1 The equilibrium r/phase, MgZn2, has the hexagonal crystal structure (C14) with a = 0.521 nm and c = 0.860 nm. 9 Nine different orientation relationships with the matrix have been reported, the most frequently observed being 1°,11in the classification scheme of GjCnnes and Simensen, 1°
Type 2:
( 0.0).1,11 (O01)h,; (00-1).1,11 (ll0)A~ (00" 1).1211(1]-I)Ai; (10.0)*1~ll(ll0)Al
Type 4:
(TR'0)r/411 (1]-I)AL;
Type 1:
(00" 1).1/411 (ll0)At
In addition, the T phase, (A1Zn)49Mg32, which has a cubic crystal structure, can form above - 1 9 0 °C. 6'~2 Spherical G. P. zones are usually the first decomposition product at low temperatures (usually -
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