Room-temperature superplasticity in a Zn-0.3 Wt Pct Al alloy
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TAE KWON HA, Staff Scientist, Center for Advanced Aerospace Materials (CAAM), and WON BEOM LEE, Graduate Student, and CHAN GYUNG PARK and YOUNG WON CHANG, Professors, Department of Materials Science and Engineering, are with the Pohang University of Science and Technology, Kyungbuk 790-784, South Korea. Manuscript submitted February 10, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—TEM micrograph showing the typical grain morphology of thermomechanically treated Zn-0.3 wt pct Al alloy used in this study.
Fig. 2—Tensile test results of Zn-0.3 wt pct Al under the various initial strain rates at room temperature.
at room temperature. A typical microstructure is given in Figure 1, and the details of microstructural evolution during the thermomechanical treatment process employed in this study will be published elsewhere. Tensile test results obtained at room temperature by varying initial strain rates are given in Figure 2. As shown in this figure, tensile elongation at room temperature is very sensitive to the initial strain rate, and maximum elongation was obtained as about 1400 pct at the strain rate of 2 3 1024/s. The elongation is the largest ever reported in the open literature for dilute Zn-Al alloys, even after considering that a small part of the elongation came from the deformation around the specimen shoulder. The engineerVOLUME 28A, AUGUST 1997—1711
Fig. 5—SEM micrograph showing a typical area of the surface of a superplastically deformed specimen. The surface was mechanically polished before tensile test, and scratches were scribed on it with 0.25mm diamond paste.
Fig. 3—The effect of initial strain rate on the stress-strain characteristics of Zn-0.3 wt pct Al alloy at room temperature.
Fig. 4—A logarithmic plot of flow stress estimated at an engineering strain of 10 pct vs initial strain rate for Zn-0.3 wt pct Al alloy at room temperature. 1712—VOLUME 28A, AUGUST 1997
ing flow stress-engineering strain curves are summarized in Figure 3 for each test. All curves exhibit no noticeable strain hardening effect, and the peak value of flow stress, like total elongation, is also very sensitive to the initial strain rate. To obtain the value of the strain-rate-sensitivity parameter, flow stresses estimated at an engineering strain of 10 pct are plotted in Figure 4 for respective initial strain rates. As easily noted in Figure 4, the stress vs strain rate curve for Zn-0.3 wt pct Al alloy at room temperature can be divided into two regimes: region II and region III. In region II, the strain-rate-sensitivity parameter, the slope of the stress vs strain rate curve, is about 0.4 and very close to the critical value for superplasticity of 0.5, at which superplasticity by GBS can be expected. The value of the strain-rate-sensitivity parameter obtained in this study is somewhat smaller than those of typical superplastic materials in spite of strikingly large tensile elongation of about 1400 pct. It is, therefore, easily noted that the strain-ratesensitivity parameter is not a sufficient condition for su
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