Consolidation of Carbon Nanotube Reinforced Aluminum Matrix Composites by High-Pressure Torsion
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INTRODUCTION
CARBON nanotubes (CNTs) have unique tubular structures with a few nanometer diameter and large aspect ratio that possess high elastic modulus (>1 TPa) and high tensile strength (>30 GPa).[1] These exceptional mechanical properties combined with a low density make them an ideal reinforcement in Al matrix composites (AMCs) for the automotive and aerospace industries where weight reduction for decreasing fuel consumption is a priority. However, CNTs have a strong tendency to agglomerate due to their high aspect ratio and their Van der Waals bondings.[2] This agglomeration is detrimental to the mechanical properties of AMCs and is a major challenge that inhibits the development of these composites. Many methods have been used in an effort to reduce the agglomeration of CNTs: coating CNTs on the Al powder surface,[3] in situ synthesis of CNTs in Al powders,[4] molecular level mixing of the CNT/Cu composite with Al powders,[5] and mechanical milling (MM) of Al/CNT powder mixtures.[6] MM, a solid state high-energy ball milling process that introduces a reinforcement phase into the matrix by repeated cycles of welding-fracture-welding of the particles,[7] has been used successfully to uniformly disperse a variety of reinforcements within an Al matrix.[8–10] Furthermore, the mechanical properties of a MM processed Al/CNT composite would be further improved by the Al grain HAMED ASGHARZADEH, Assistant Professor, is with the Department of Materials Engineering, Faculty of Mechanical Engineering, University of Tabriz, 51666-16471 Tabriz, Iran. Contact e-mail: [email protected] SOO-HYUN JOO, Ph.D. Candidate, and HYOUNG SEOP KIM, Professor, are with the Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea. Contact e-mail: [email protected] Manuscript submitted December 20, 2013. Article published online May 23, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
refinement to the nanoscale due to intense straining and by the incorporation and dispersion of the oxide layer initially present on the surface of Al powders.[2] MM processed Al/CNT powders have been consolidated by conventional methods, e.g., hot pressing,[11,12] hot extrusion,[13,14] and spark plasma sintering.[15,16] Since in these techniques densification occurs by heating at relatively high temperatures, an interfacial reaction between Al and CNTs may result in the formation of brittle Al4C3 phase.[17] Grain growth of nanocrystalline Al may occur as well. Both have a detrimental effect on the mechanical properties of Al/CNT composites. Severe plastic deformation (SPD), a top-down approach that applies intense strains to workpiece materials, can be used for production of bulk fully-dense ultrafine-grained materials. Recently, SPD processes like equal channel angular pressing (ECAP),[18,19] accumulative roll bonding (ARB),[20,21] friction stir processing (FSP),[22,23] and highpressure torsion (HPT)[24,25] have been successfully applied for uniform dispersion of CNTs in Al mat
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