Correlation between creep stress exponent and ductility in Al-10 at. pct Zn
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formation that occurred as specimens were cooled made it difficult to adopt standard metallographic techniques. Each specimen was pulled in tension on an Instron testing machine operating at a constant rate of cross-head displacement; the strain rates quoted therefore refer to initial strain rates calculated from the initial gage length. Ductility tests on AI-10 at. pct Zn were performed at a constant temperature of 800 K. Also, ductility tests were performed on AI under the same experimental conditions of temperature and grain size to provide a comparison between the behavior of the alloy and that of the pure metal. Figure 1 gives the percentage elongation of each specimen at fracture, e pct (e pct = AL/Lo, where AL is the increase in length and L0 is the initial length of the specimen), as a function of initial strain rate, k. For A1-10 at. pet Zn, e pct increases from - 1 4 0 at 10 -5 s -1, reaches a maximum value of - 3 0 0 at 10 -2 s -l, and then decreases to - 125 at 10~ s -~. By contrast, the value of e pct for A1 remains essentially constant ( - 1 2 5 ) over the whole range of strain rates except the very high strain rates where a slight decrease in ductility occurs. In addition, Figure 1 shows that at < 10-3 s -~, e pct for AI-10 at. pct Zn decreases very slowly with decreasing strain rate and that in this range of strain rates, the difference in e pct between the alloy and AI is very small. The ductility data of Figure 1, when combined with creep information on AI-10 at. pct Zn ~ and A1, 3 lead to several important findings that are compatible with the wellestablished correlation between stress exponent and ductility, i.e., e pct = Function (l/n). First, maximum ductility in AI-10 at. pct Zn occurs at intermediate strain rates, which, according to the creep data of the alloy, l are associated with the minimum value of the stress exponent; n -~ 3. Second, e pet for A1 is insensitive to strain rate over low and intermediate strain rates, an observation which is in harmony with the finding 3:5 that the stress exponent for AI, under experimental conditions (temperature and strain rate) similar to those used in the present investigation, is constant and is close to a value of 4.6. Additionally, present calculations indicate that the decrease in ductility level at strain rates higher than 3 • 10 -2 s -I is probably the result of the advent of the creep power law breakdown region 3 in which the stress exponent becomes higher than 4.6; according to
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MOSTAFA S. MOSTAFA, Graduate Research Assistant, and FARGHALLI A. MOHAMED, Professor, are with the Department of Mechanical Engineering, University of California, Irvine, CA 92717. Manuscript submitted June 3, 1985.
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Fig. 1--Strain to failure (elongation pct) AI-10 at. pct Zn and AI at 800 K.
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VOLUME 17A, FEBRUARY 1986--365
creep data reported for A1, 3 the breakdown of the
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