Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea Short communication An insight into the strain rate dependence of tensile ductility of an ultrafine grained Cu matrix nanocomposite Dengshan Zhou a,b,c, ⁎ , Hongwei Geng a,b , Deliang Zhang a,b,c , Wei Zeng c , Charlie Kong d , Paul Munroe e a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China b Institute of Ceramics and Powder Metallurgy, School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China c State Key Laboratory for Metal Matrix Materials, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China d Electron Microscope Unit, The University of New South Wales, Sydney 2052, Australia e School of Materials Science and Engineering, The University of New South Wales, Sydney 2052, Australia ARTICLE INFO Keywords: Ultrafine grained copper matrix nanocomposites Tensile ductility Strain rate sensitivity Deformation mechanisms ABSTRACT In this study, we observed a strong strain rate dependence of tensile ductility for an ultrafine grained Cu-5vol% Al 2 O 3 nanocomposite prepared by powder compact extrusion. This dependency of tensile ductility of the material to strain rate, combined with detailed materials characterization, suggests that the strain at which nearly ideal plastic deformation occurs is associated with the magnitude of the flow stress. The measured apparent activation volume and observed alignment of Cu grains suggest that plastic deformation during this nearly perfect deformation stage is dominated by the co-operative grain boundary sliding. 1. Introduction The lack of sufficient tensile ductility has been recognized as a major barrier to the application of high strength nanocrystalline (NC) (grain sizes≤100 nm) and ultrafine grained (UFG) (100 nm < grain sizes≤1 μm) metallic materials [1–4]. It appears that most high strength NC and UFG alloys exhibit an elongation to fracture of less than 10% at room temperature. This is much lower than that of their coarse grained (CG) counterparts [5,6]. It has been accepted that for processing artifact-free NC and UFG alloys, their low tensile ductility mainly results from the lack of strain hardening capability. Such limited strain hardening ability is associated with both insufficient dislocation accumulation inside nanograins/ultrafine grains and high dynamic recovery rate at grain boundaries [7]. In addition, the instability of crack nucleation and/or propagation also causes NC and UFG metallic materials to fail at a very small plastic strain in tension [2–4]. However, for strain rate sensitive UFG metals, although the strain hardening effect decays rapidly with plastic strain and soon becomes trivial, the strain rate hardening effect is still capable of delaying the onset of severe localization of plastic deformation and, hence, could render materials with enhanced tensile ductility [8,9]. It has been shown that the strain rate sensitivity of the flow stress of face centered cubic (FCC) structured metals, such as Cu, can be elevated by approximately an order of magnitude through reducing grain sizes from the micron level to the nanometer/sub-micron level [10–13]. Such drastically increased strain rate sensitivity allows NC/ UFG Cu to achieve good tensile ductility at a relatively low strain rate of 1×10 -5 s -1 or lower [9,10,14–17]. For instance, a NC Cu sample, with an average grain size of 54 nm, exhibited a tensile strain rate sensitivity of 0.0272 in the strain rate range of 1×10 -2 to 1×10 -4 s -1 , and its elongation to fracture increased from 6% to 12% by decreasing the strain rate from 1×10 -2 to 1×10 -4 s -1 [15]. Similarly, the strain rate sensitivity of an UFG Cu, with an average grain size of around 300 nm produced by equal channel angular pressing (EACP), was measured to be 0.025 by strain rate change tensile test (tensile jump test) at 6×10 -7 s -1 [9,18]. The sample showed a perfectly flat true stress-strain curve over the first 12% of strain without exhibiting necking at a low strain rate of 1×10 -6 s -1 . In our investigation of the strain rate dependence of the tensile ductility of an UFG Cu-5vol%Al 2 O 3 nanocomposite, which was pre- pared by powder compact extrusion and had grain sizes in the range of ~70 to ~800 nm and Al 2 O 3 nanoparticle sizes in the range of ~5 to ~350 nm, we observed an interesting phenomenon. The elongation to fracture of the sample increased drastically from 8.1% to 15%, i.e. by a factor of about two, as the strain rate decreased from 1×10 -4 to 5×10 -5 s -1 , while it only increased slightly from 6.5% to 8.1% with http://dx.doi.org/10.1016/j.msea.2017.02.006 Received 20 November 2016; Received in revised form 1 February 2017; Accepted 2 February 2017 ⁎ Corresponding author at: Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China. E-mail addresses: zhoudengshan@mail.neu.edu.cn (D. Zhou), genghongwei@stumail.neu.edu.cn (H. Geng), zhangdeliang@mail.neu.edu.cn (D. Zhang), zengwei@sjtu.edu.cn (W. Zeng), c.kong@unsw.edu.cn (C. Kong), p.munroe@unsw.edu.cn (P. Munroe). Materials Science & Engineering A 688 (2017) 164–168 Available online 03 February 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved. MARK