Evolution of initial grain boundaries and shear bands in Cu bicrystals during one-pass equal-channel angular pressing W.Z. Han, H.J. Yang, X.H. An, R.Q. Yang, S.X. Li, S.D. Wu * , Z.F. Zhang * Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Received 25 September 2008; received in revised form 29 October 2008; accepted 1 November 2008 Available online 24 November 2008 Abstract The evolution of grain boundaries (GBs) and shear bands in Cu bicrystals during one-pass equal-channel angular pressing (ECAP) was systematically investigated by various techniques. Four Cu bicrystals were designed to make GBs with angles of 0°, 45°, 90° and 135° with respect to the intersection plane (IP) of the ECAP die. After ECAP, the shear bands and the GB orientations in the four bicrystals displayed distinct behaviors due to the difference in the initial GB directions and the special crystallographic orientation of the compo- nent grains. Based on the experimental results, it is suggested that the initial GBs have a remarkable influence on the shear deformation behaviors of the adjacent regions, and the deformation regions far from the GBs are mainly controlled by the crystallographic orienta- tions. The present investigations further demonstrate that shear deformation along the normal of the IP plays an important role in the deformation of Cu bicrystals. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Cu bicrystals; Equal-channel angular pressing (ECAP); Grain boundary (GB); Shear bands 1. Introduction For metals and alloys, the primary role of grain bound- aries (GBs) is thought to be that of obstacles to dislocation motion, and the widely referenced Hall–Petch relation [1,2] predicts that decreasing the average grain size, effectively increasing the fraction of GBs, results in increased strength [1–5]. It is well known that there are several severe plastic deformation (SPD) methods to refine the grain size or increase the volume fraction of GBs in metallic crystalline materials, e.g. equal-channel angular pressing (ECAP), high-pressure torsion (HTP) and surface mechanical attri- tion treatment (SMAT) [6–10]. ECAP, as one of the major SPD methods, has been widely used to fabricate ultrafine- grained materials for two decades [6,7]. The mechanism of grain refinement via ECAP has been extensively investi- gated in numerous materials, including both polycrystals and single crystals [11–23]. However, the investigations using polycrystals cannot clearly discern the individual influences of numerous GBs and the different orientations of the component grains on the grain refinement mecha- nism. Therefore, the use of various kinds of single crystals provides a unique opportunity to obtain a direct evaluation to reveal the effect of crystallographic orientation on grain refinement [11–15]. However, studies using single crystals also have some obvious limitations. For example, it is not clear what the roles of the initial high-angle GBs (HAGBs) are during the process of ECAP because there are no GBs in single crystals. In order to get a better understanding on the effect of both GB and crystallographic orientation on grain refine- ment, bicrystals with one flat HAGB plane are the ideal model materials. In the past decade, bicrystals have been widely employed as model materials to reveal the GB effects under monotonic and cyclic loading conditions. Winning et al. [24,25], Cahn et al. [26,27], Molodov et al. [28] and Zhang and Wang [29,30], have employed bicrystals to study the GB migration process and the GB fatigue 1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.11.001 * Corresponding authors. E-mail addresses: shdwu@imr.ac.cn (S.D. Wu), zhfzhang@imr.ac.cn (Z.F. Zhang). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 57 (2009) 1132–1146