Graduation date: 1968The base Al-Zn superplastic alloy was investigated at 250°C to\ud determine the effect of small amounts of magnesium on mechanical\ud properties. Six alloys of nominal composition 0.00, 0.10, 0.25,\ud 0.50, 0.75, and 1.00 weight percent magnesium, constant 78 weight\ud percent zinc, and variable 21 to 22 weight percent aluminum were\ud each tested in tension at strain rates of 0.02, 0.20, and 2.00 in/min\ud to determine the flow stress and elongation at each strain rate for\ud each composition.\ud Superplastic elongation occurred at all three strain rates in\ud the specimens containing no magnesium. The addition of any magnesium\ud content investigated destroyed any significant superplastic deformation\ud and led to intercrystalline fracture of all other test specimens.\ud The intermetallic compound Mg₂Zn₁₁ was uniformly distributed throughout the microstructure, limiting plastic deformation.\ud In general, the flow stress was found to increase to a maximum\ud as the magnesium content was increased up to about 0.75 percent,\ud then to start dropping off. At the same time, after the initial drop in\ud elongation due to the change from the superplastic to the intercrystalline\ud fracture mode, the trend in elongation was to decrease slowly\ud with increasing magnesium content. For a given magnesium content,\ud a higher strain rate resulted in a higher value for the flow stress
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