Spark Plasma Sintering of Cryomilled Nanocrystalline Al Alloy - Part I: Microstructure Evolution
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INTRODUCTION
IN recent years, spark plasma sintering (SPS) technology has emerged as a viable approach to sinter composites, nanocrystalline materials, and amorphous alloys.[1–5] Much of this work has been motivated by the premise that the rapid heating, and hence reduced sintering times, that are possible with SPS should help retain the initial microstructure, and thereby avoid or minimize undesirable reactions, such as coarsening or crystallization. The technique is similar to traditional hot pressing; however, in this case, the sample is heated by a high-intensity, low-voltage pulsed DC electric current flowing directly through the sample (if electrically conductive) and a graphite die, and by simultaneously applying a uniaxial pressure during consolidation. The SPS process has the advantages of sintering samples at a high heating rate and with high pressure. Thus, samples
YUHONG XIONG, YING LI, and BAOLONG ZHENG, Postdoctoral Researchers, ENRIQUE J. LAVERNIA, Distinguished Professor, and JULIE M. SCHOENUNG, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Contact e-mail: [email protected] DONGMING LIU, Visiting Assistant Researcher, is with the Department of Chemical Engineering and Materials Science, University of California, and is also an Associate Professor, with Department of Materials Science and Engineering, Shandong University, Jinan, Shandong 250061, P.R. China. CHRIS HAINES, Senior Metallurgist, JOSEPH PARAS, Materials Engineer, DAROLD MARTIN, Senior Materials Engineer, and DEEPAK KAPOOR, Group Leader, are with the United States Army, RDECOM-ARDEC, Picatinny Arsenal, NJ 07806. Manuscript submitted February 24, 2011. Article published online October 29, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
can be sintered at comparatively lower temperature in a shorter time and can retain finer microstructure than is achievable with conventional methods. Aluminum alloys, such as Al 5083 (Al-4.5Mg0.57Mn-0.25Fe, wt pct), are widely used because they provide a combination of light weight and high strength. Nanostructured and ultrafine-grained (UFG) Al alloys, in which grain sizes are typically from less than 100 nm to 800 nm, have the potential to revolutionize traditional materials design via atomic-level structural control to tailor engineering properties. Consideration of recent advances and discoveries indicates that such materials provide an exciting new approach for the development of advanced systems suited to military applications. For instance, UFG Al 5083 consolidated using cryomilled nanostructured powder has demonstrated an ultimate tensile strength (UTS) greater than 700 MPa (over twice that of a conventional Al 5083), and preliminary ballistic data showed that the UFG Al 5083 has a V50 that is 33 pct higher than conventional Al 5083.[6] Improvements to material properties greatly depend on the corresponding microstructure of the materials. Relevant studies for bulk Al 5083 samples made by SPS and other more conventiona
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