Strength and strain hardening of aluminum matrix composites with randomly dispersed nanometer-length fragmented carbon nanotubes S.J. Yoo, a S.H. Han b and W.J. Kim a,⇑ a Department of Materials Science and Engineering, Hongik University, 72-1 Mapo-gu, Sangsu-dong, Seoul 121-791, Republic of Korea b Korea Institute of Materials Science, 797 Changwondaero, Seongsan-gu, Changwon, Gyeongnam 642-831, Republic of Korea Received 9 December 2012; revised 6 January 2013; accepted 7 January 2013 Carbon nanotube (CNT)-reinforced aluminum composites were fabricated through ball milling combined with rolling. The composites exhibit high strength and high strain-hardening ability, the combination of which has been rarely reported. During the ball-milling process, CNTs were broken and became low aspect ratio tubes. The processed composites had the CNTs (a few dozen nanometers in size) randomly and uniformly dispersed in their grain interiors. This type of CNT distribution contributed to work hardening and strengthening by the Orowan mechanism. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ball mill; Rolling; Metal matrix composites; Carbon nanotubes; Work hardening Carbon nanotube (CNT)-reinforced composites have great potential for use in load-bearing applications and electronic packaging because CNTs exhibit high specific strength, high thermal conductivity and a low coefficient of thermal expansion [1,2]. CNT-reinforced metal matrix composites (MMCs) have attracted much attention over the past decade. Among CNT–MMCs, aluminum matrix composites have been the most exten- sively studied type with the goal of developing light structural materials with excellent mechanical properties [3–10]. In synthesizing CNT–Al composites, ball-milling methods have been widely used to produce powders in which CNTs are well dispersed. The ball-milled powders are then consolidated by hot pressing [3], spark plasma sintering [4] or plasma spraying [5]. Rolling [6] and extrusion [7–10] have often been used as post-sintering processing steps to enhance the density of the bulk com- posites as well as the CNT dispersion in their matrices. In interpreting the strengthening effect of CNTs, the shear lag models for short-fiber composites, in which applied stress is transferred to the fibers through an interfacial shear stress, have been widely adopted. Srin- ivasa and Agarwal [11] showed that the Kelly and Tyson model [12] and the modified Halpin–Tsai model [13] pre- dict the strength of CNT–Al composites up to 2 vol.% CNT, while the generalized shear lag model [14], which takes into account the distribution of fiber orientation, gives a better prediction at higher CNT contents. The Orowan strengthening mechanism has also been pro- posed as a potential mechanism of strengthening in CNT–Al composites [9] because CNTs are very strong and have nanosized diameters, but the operation of such a mechanism has not yet been reported. In this paper, we report the fabrication of CNT–Al composites with CNTs randomly and uniformly dis- persed in their grain interiors. Through analysis of the relation between microstructures and mechanical prop- erties of the 1 and 3 vol.% CNT–Al composites with this type of CNT distribution, the strengthening mechanism of the composites was proposed. The strength and strain-hardening behavior of the processed composites were also compared with those of the CNT–Al compos- ites studied by other investigators. CNT–Al composite powders were fabricated using high-energy ball milling. Mixtures of pure aluminum powder (99.5% purity, 100–150 lm in diameter) and 1 or 3 vol.% multi-walled carbon nanotubes (MWCNTs, 20 nm in diameter and 10–15 lm in length) were ball milled in an attrition mill at 400 rpm for 6 h under an ar- gon atmosphere. Hardened stainless steel balls were used, 1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2013.01.013 ⇑ Corresponding author. Tel.: +82 2 320 1468; fax: +82 2 325 6116; e-mail: kimwj@wow.hongik.ac.kr Available online at www.sciencedirect.com Scripta Materialia xxx (2013) xxx–xxx www.elsevier.com/locate/scriptamat Please cite this article in press as: S.J. Yoo et al., Scripta Mater. (2013), http://dx.doi.org/10.1016/j.scriptamat.2013.01.013