Met. Mater. Int., Vol. 19, No. 5 (2013), pp. 927~932 doi: 10.1007/s12540-013-5004-4 Grain Refinement and Tensile Strength of Carbon Nanotube-Reinforced Cu Matrix Nanocomposites Processed by High-Pressure Torsion Eun Yoo Yoon 1,2 , Dong Jun Lee 3 , Byungho Park 3 , M. R. Akbarpour 4 , M. Farvizi 4 , and Hyoung Seop Kim 1,3, * 1 Pohang University of Science and Technology (POSTECH), Center for Advanced Aerospace Materials, Pohang 790-784, Korea 2 Korea Institute of Materials Science (KIMS), Light Metal Division, Materials Deformation Department, Changwon 641-831, Korea 3 Pohang University of Science and Technology (POSTECH), Department of Materials Science and Engineering, Pohang 790-784, Korea 4 Materials and Energy Research Center (MERC), P.O. Box 14155-4777, Tehran, Iran (received date: 25 September 2012 / accepted date: 10 December 2012) In recent years, the processing of metallic materials via severe plastic deformation has been widely applied to manufacture bulk specimens of ultrafine grained/nanocrystalline structures. In this study, bulk nanocom- posites of carbon nanotube-reinforced Cu were manufactured by consolidation of mixtures of coarse grained Cu powders and CNTs of two volume fractions (5 vol% and 10 vol%) using high-pressure tor- sion, a typical SPD method. The effects of CNT reinforcements on the microstructural evolution of the Cu matrix were investigated using electron backscatter diffraction and scanning/transmission electron micros- copy; the results showed that the Cu matrix grain size was reduced to ~114 nm, and the CNTs were well dispersed in the matrix. Due to the effect of the UFG Cu and CNTs, the tensile strength (350 MPa) of the nanocomposite was higher than that (190 MPa) of Cu processed by the powder HPT process without CNTs. However, the Cu-CNT 10 vol% indicated a decreased tensile strength due to an increased interface area between the matrix and CNTs at high volume fractions of CNTs. Key words: composites, severe plastic deformation, grain refinement, electron back scattering diffraction (EBSD), scanning/transmission electron microscopy 1. INTRODUCTION Carbon nanotubes (CNTs) [1,2] have drawn great attention from the material science and engineering community dur- ing the last two decades due to their superior properties, such as mechanical, electric, optical, and electronic properties. The superior mechanical properties, particularly their extremely high elastic modulus, high tensile strength, and light weight make CNTs an ideal reinforcement material for light-weight and high-strength metal matrix composite (MMC) materials [3-7]. However, there are several difficulties in powder den- sification and in interface bonding between the metallic matrix and CNT particles for bulk manufacturing using con- ventional powder metallurgy methods, typically compaction and sintering or pressure assisted densification. In recent years, severe plastic deformation (SPD) methods have been developed as a new technique for manufacturing bulk metallic materials having ultrafine grained (UFG) and nanocrystalline structures [8-10] by imposing intense strains on the target materials. Among various SPD methods, high- pressure torsion (HPT) is considered the most effect process for imposing high strain and achieving grain refinements. In the HPT process, disk shaped samples are placed between two rigid anvils, which subject the sample to a high applied hydrostatic pressure and simultaneous torsional straining [11,12]. Processing by HPT has been applied mostly for the grain refinement of bulk solid materials; however it can also be used for consolidation of powders to fully dense materials [13,14]. Hence, it may be possible to synthesize fully dense materials without high-temperature sintering using the HPT process. Indeed, cold sintering under high hydrostatic pres- sure over several GPa showed many successful results [15]. Several papers have reported the HPT process for fabricat- ing metal-matrix CNT nanocomposite [16-19]. However, the effect of CNT volume fraction on grain refinement and mechanical properties in the HPT process has not been reported as far as the authors know. In this paper, we investi- *Corresponding author: hskim@postech.ac.kr ©KIM and Springer, Published 10 September 2013