The effect of ECAP die shape on nano-structure of materials E. Hosseini, M. Kazeminezhad * Department of Materials Science and Engineering, Sharif University of Technology, Azadi Avenue, Tehran, Iran article info Article history: Received 15 June 2008 Accepted 7 July 2008 Available online 15 August 2008 PACS: 81.05.Bx 81.40.Ef 81.07.Bc Keywords: ECAP Curved die Sharp die Model Nano-structure abstract A general flow line model is developed to investigate the deformation behavior of Cu and Al through Bc route of Equal Channel Angular Pressing process at curved and sharp dies. Considering the Taylor theory, the obtained strain and strain rate from the flow line model and also using a modified version of ETMB model, the evolutions of nano-structure in the processed Cu and Al are predicted. Comparison between the modeling results and experimental data is carried out and a reasonable agreement is achieved. The results show that the deformation in the sharp die occurs in a narrow zone with higher strain rate than that in the curved die. Also, the cell size of the processed Cu is smaller than that of the processed Al. Moreover, the cell size of the materials processed in the sharp die is finer than that in the curved die. It should be mentioned that in a specific range of strain the difference between the achieved cell sizes in the curved and sharp dies is small. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Nanocrystalline materials exhibit superior mechanical, physical and chemical properties in comparison with conventional coarse- grained polycrystalline materials, which have grain sizes in the range of 10–300 lm [1–3]. Nanocrystalline materials can be syn- thesized by considering either bottom-up or top-down approaches. In the bottom-up approach, the structure is arranged atom-by- atom and layer-by-layer. The bottom-up synthesis often resulted in porosity, which is known to be detrimental to the properties of nanocrystalline materials [4]. In the top-down approach the coarse microstructure changes to nano-structure. The top-down approach involves severe plastic deformation (SPD), which can be successfully used for the processing of bulk nanostructural materi- als [5,6]. The principle of SPD includes increasing dislocation den- sity by heavily deforming materials, formation of dense dislocation walls and transforming dislocation walls into high-angle grain boundaries [1]. Equal Channel Angular Pressing (ECAP) is a technique of bulk SPD [7–9]. In ECAP process, a simple shear deformation is imposed to the material when passed through two intersecting channels of equal cross-sectional area [10]. Since the size of workpiece remains unchanged after the pressing, this process can be executed indefi- nite number to achieve desired microstructure and strength [10– 12]. Three important physical parameters of this process are [13]: (a) ECAP rout, (b) intersecting channels angle of die, u and (c) outer curve angle of die, w. Generally, three routes in the ECAP pressing are usual: route A, without any rotation of the sample around its axis upon repeated deformation; route Bc, with sequential rotation of the sample by 90° around its axis; and route C, with the rotation of sample by 180° around its axis during repeated deformations [11]. Transmis- sion electron microscopy (TEM) examination and finite volume method (FVM) modeling showed that the ECAP process by route Bc with four passes and an angle of 90° between the intersecting channels is the most favorable arrangement of this process and caused to equiaxed nanocrystalline structure [13,14]. However, as shown in Fig. 1 the ECAP dies are usually built with two outer curve conditions, (a) w = 20  and (b) w =0  that impose the strain magnitude of about 1.06 and 1.15 in each pass, respectively [15]. The main interest of recent studies on ECAP is the understand- ing of the evolution of the microstructure and grain refinement process [16,17]. Indeed, grain refinement by SPD is mainly related to the evolution of subgrain (dislocation cell) boundaries [18,19]. Although there are some models on dislocation density evolution, microstructure and strength of materials through ECAP in sharp dies [19–22], there is not any modeling work on microstructure evolution during ECAP process with outer curve of die wall. In this study, to better understanding of the effect of ECAP die shapes on the microstructure evolution of materials, some modifications and extensions on the existent mechanical and dislocation density 0927-0256/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2008.07.002 * Corresponding author. Tel.: +98 21 66165227; fax: +98 21 66005717. E-mail address: mkazemi@sharif.edu (M. Kazeminezhad). Computational Materials Science 44 (2009) 962–967 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci