Yield points and strain aging in hexagonal-based particulate composites
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RELATIVELY little has been reported concerning yield points and strain-aging effects with the hexagonal close-packed metals cadmium, magnesium, and zinc. Yield points have been observed with magnesium, l- 3 cadmium, 4 and zinc 5 single crystals containing interstitial or substitutional solutes. Yield points have also been reported for polycrystalline magnesium alloys 6-8 at low homologous temperatures (< 0.6T„2 ), where the phenomenon is thought to be due to the Cottrell locking mechanism. 7 The strain-aging yield points reported here for cadmium-, magnesium-, and zinc-based particulate composites appear above 0.5 to 0.6Tm , and persist at temperatures up to the melting temperature. This note presents the general features of these yield phenomena which apparently have never before been presented in the literature. The strain-aging yield points to be described are associated with the particles present in these particulate composites, and not with any solute atom/dislocation interaction. In an earlier note s it was shown that such particulate composites exhibit an inverse strain rate effect and thermal hardening, 10 effects frequently related to yield-point phenomena. MATERIALS AND EXPERIMENTAL PROCEDURE
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Strain-aging yield-point behavior was investigated for three particulate composites: Cd -30 vol pet B," Mg-25 vol pet B, 12 and Zn -15 vol pet Al,O 3 13 Similar yield points were observed with the two other composites studied, Cd -30 vol pet W, 11 and Zn -28 vol pet W.'4 The composites were prepared by a powder metallurgy process involving a series of warm rollings and extrusions. This process results in a uniform dis.
I. C. HUSEBY is Member of Technical Staff, General Electric Corporate Research and Development Center, Schenectady, NY. S. E. HSU is Section Leader in Materials Science, Chung Shan Institute of Science and Technology, Lung-Tan, Taiwan. T. R. McNELLEY is Assistant Professor, Department of Mechanical Engineering, University of Wyoming, Laramie, WY 82070. G. R. EDWARDS is Assistant Professor, Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93940. D. FRANCOIS is Professor, Department of Mechanical Engineering, Technical University of Compiegne, 60206 Compeigne, France. J. C. SHYNE and O. D. SHERBY are Professors, Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. Manuscript submitted February 4, 1975. METALLURGICAL TRANSACTIONS A
tribution of second phase particles and a fine grain 11-16 The boron particles varied in size ( 1 to 3 µm). size from submicron to 40 microns, while the Al,O, particles were 0.3 micron in size. Three control extrusions, which did not contain dispersed particles other than a small amount of oxide initially present on the powder particles, were prepared by the same process using only cadmium or magnesium or zinc powder. No yield-point phenomena or strain-aging effects were observed with these control extrusions. Cylindrical samples, 7.6 mm in length and 5.1 mm in diam, were machined from the extrusions. Const
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