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This research was supported by the National Science Foundation under Grant No. NSF-DMR 8147961. The M A T R I X beam line, a joint project of midwestern universities, is funded by the Department of Energy, Grant No. DE-FG02-85ER45183. We thank Drs. G. Ice and C.J. Sparks, Jr., of ORNL for the use of their beam line and their help and advice during the measurements. We also thank Professor Hayakawa for the crystal and Mr. Hasan Yalcinkaya for assistance with the data reduction.

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The Effect of Copper, Chromium, and Zirconium on the Microstructure and Mechanical Properties of AI-Zn-Mg-Cu Alloys JOHN A. W A G N E R and R.N. S H E N O Y The effect of systematic variation of copper, chromium, and zirconium contents on the microstructure and mechanical properties of a 7000-type aluminum alloy was evaluated. Ingots with 1.5, 2.0, and 2.5 wt pct copper were cast with either 0.11 pct Zr or 0.20 pct Cr. Fracture toughness and tensile properties were evaluated for each alloy in both the peak aging, T8, and the overaging, T73, conditions. Results showed that dimpled rupture essentially characterized the fracture process in these alloys. In the T8 condition, a significant loss of toughness was observed for alloys containing 2.5 pct Cu due to increase in the quantity of A1-Cu-Mg-rich S-phase particles. When examining T8 alloys at constant Cu levels, it was observed that Zr-bearing alloys exhibited higher strength and toughness than the Cr-bearing alloys. In the T73 condition, Cr-bearing alloys were inherently tougher than Zr-bearing alloys. A void nucleation and growth mechanism accounts for the loss of toughness in these alloys with increasing copper content. Evolution of A1-Zn-Mg-Cu alloys through modifications of the A1-Zn-Mg ternary system has led to the develop