Mechanical behavior and microstructure of a thermally stable bulk nanostructured Al alloy
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I. INTRODUCTION
INTEREST in the field of nanostructured materials has grown tremendously in recent years. The potential for improved mechanical properties, by reducing the grain size to the nanostructured regime, of both metals and ceramics has been demonstrated by several researchers.[1–4] Evidence of the incredible amount of interest in nanostructured materials is found in an overview article by Rittner and Abraham.[5] Rittner and Abraham state that “More than 2775 nanomaterials-related research publications have appeared in print since 1991. . . . Since 1991, the U.S. government has spent more than $20 million funding work to develop and commercialize nanostructured materials through Small Business Innovation Research and Technology Transfer programs. . . . More than 300 U.S. patents have been awarded. . . .”[5] Most of this research has generated samples which are thin films or extremely small samples, using either hardness data or compression data to project mechanical behavior. The ductility data have been especially susceptible to question, due to the inherent differences between tension and compression tests. The surface condition has also been shown to play an important role in data collection for extremely small samples.[1] Very few data have been presented on tensile samples due to the difficulty in manufacturing a large quantity of material with a nanostructured (⬍100 nm) grain size. Significant data have been produced by a technique referred to as equal-channel angular extrusion (ECAE) or equal-channel angular pressing (ECAP). References 6 through 8 present a significant amount of research into the microstructure and mechanical behavior of ultrafine-grained (UFG) or submicrometer-grained (SMG) Al-Mg alloys produced by ECAE and ECAP. This technique, however, has V.L. TELLKAMP, Adjunct Assistant Professor, and E.J. LAVERNIA, Chair/Professor, are with the Chemical and Biochemical Engineering and Materials Science Department, University of California, Irvine, Irvine, CA 92697-2575. A. MELMED, Professor, is with the Materials Science and Engineering Department, Johns Hopkins University, Baltimore, MD 21218. Manuscript submitted September 6, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
produced materials with grain sizes in the range of 200 to 500 nm.[9] The microstructure of the UFG or SMG materials have been shown to be thermally unstable at relatively low temperatures.[6] In related studies, several researchers have examined the thermal stability of nanostructured powders generated by mechanical alloying or mechanical milling.[10,11] The materials that were found to have the highest thermal stability were either pure Al or alloys that contained significant quantities of Al. In the present study, a commercial aluminum alloy, 5083, was selected to promote thermal stability of the nanostructured grains while simultaneously providing a well-characterized material to compare mechanical behavior. The objective of this study was to accurately characterize the mechanical behavior of a nanostructured materia
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