Surface tension of aluminumrich Al-Cu liquid alloys
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TRUE STRAIN Fig. 5--Biaxial flow stress of aluminum alloy 5182-O after sudden changes in strain rate during controlled strain rate testing. Base true strain rate ( I x ) is 2.5 x 10-4 s-L Jumps axe between 10x and 100x of base strain rate. z~
plastic flow takes place throughout a volume 20 mm or greater in length. This has been confirmed experimentally by observation of moving Liiders bands within this volume until the onset of necking. The zone of plastically deforming material decreases during local necking to a length about equal to the current sheet thickness. It further decreases when flow becomes localized within a sample-scale shear band. Metallography shows that flow within sample-scale shear bands extends along the direction of sheet extension for a distance of about 20/xm. Thus, the average strain rate for the volume of plastically deforming material increases by 103 during the development of necking and sample-scale shear bands. On the basis of bulge test results, one might expect a greater than 20 MPa decrease in flow stress due to changing strain rate. The conservative estimate shown in Table I shows that flow stress softening is far greater due to strain rate change as compared to temperature change. It is known that many factors can potentially decrease flow stress during plastic flow. 16 Thermal softening is always a negative contributor to flow stress, but the magnitude of softening depends upon material factors, rate of heat extraction, and deformation rate. This study has shown that the temperature rise during past experiments by the authors contributes less than 1 pct of the softening that occurs prior to formation of sample-scale shear bands. This demonstrates that sample-scale shear bands develop readily under conditions where thermal softening makes a negligible contribution to their formation. While thermal softening can be a significant factor promoting shear band formation in some circumstances, it is not the only softening mechanism influencing shear band development. I I56--VOLUME 18A, JUNE 1987
1. S.L. Semiatin, R. A. Ayres, and J. J. Jonas: Metall. Trans. A, 1985, vol. 16A, pp. 2299-2308. 2. S.L. Semiatin, N. Frey, N. D. Walker, and J. J. Jonas: Acta Metall., 1986, vol. 34, pp. 167-76. 3. E. Rauch, G. R. Canova, J. J. Jonas, and S. L. Semiatin: Acta Metall., 1985, vol. 33, pp. 465-76. 4. S.L. Semiatin and J. J. Jonas: Formability and Workability of Metals: Plastic" Instability and Flow Localization, ASM International, Metals Park, OH, 1984. 5. S.L. Semiatin and G. D. Lahoti: Metall. Trans. A, 1982, vol. 13A, pp. 275-88. 6. S.L. Semiatin and G. D. Lahoti: Metall. Trans. A, 1983, vol. 14A, pp. 105-15. 7. H.C. Rogers: Ann. Rev. Mater. Sci., 1979, vol. 9, pp. 283-311. 8. A.L. Wingrove: Metall. Trans., 1973, vol. 4, pp. 1829-33. 9. P.W. Leech: Metall. Trans. A, 1985, vol. 16A, pp. 1900-03. 10. U. Gerlach: MetaU. Trans. A, 1986, vol. 17A, pp. 435-42. 1I. S. Kuriyama and M
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