Microstructure and Mechanical Properties of Ultrafine-Grained Copper Produced Using Intermittent Ultrasonic-Assisted Equ
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INTRODUCTION
EQUAL-CHANNEL angular pressing (ECAP) is a severe plastic deformation technique used to refine grain size of a material without changing its shape. The strength of a material improves as grain size decreases in accordance with the Hall–Petch effect.[1,2] In conventional polycrystalline materials, grain boundaries can effectively block dislocation motion, necessitating a stronger external stress to drive the slip deformation. Many researchers have made further contributions to the development of the technique since Segal[3] invented ECAP in 1977. For example, Valiev et al.[4] and Langdon[5] investigated the principles of grain refinement. Cetlin et al.[6] discussed how to avoid cracks and inhomogeneities in billets processed by ECAP. Some methods were proposed to assist the ECAP technique to improve the properties of materials. It was reported that JIANXUN LU and ZHAOZHI WU, Ph.D. Students, XIAOQIANG CHEN, Undergraduate Student, BIN XU, Assistant Professor, XIAOYU WU, Professor, and ZHIYUAN LIU, Associate Professor, are with the Guangdong Provincial Key Laboratory of Micro/Nano Optomechatronics Engineering, Shenzhen University, Nanhai Ave 3688, Shenzhen 518060, Guangdong, P.R. China and also with the Shenzhen Key Laboratory of Advanced Manufacturing Technology for Mold & Die, Shenzhen University, Nanhai Ave 3688, Shenzhen 518060, Guangdong, P.R. China. Contact e-mail: [email protected] SHUANGCHEN RUAN, Professor, is with the Guangdong Provincial Key Laboratory of Micro/Nano Optomechatronics Engineering, Shenzhen University. Manuscript submitted January 20, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
back-pressure could prevent the fracture of low ductile materials during ECAP.[7] In addition, a beveled-edge punch was introduced to improve the homogeneity of ultrafine-grained materials.[8] Furthermore, ECAP was used to combine with other processes. Zhilyaev et al.[9] reported the evolution of microstructure and microtexture in ultrafine-grained materials by ECAP combined with HPT (high-pressure torsion). Recently, ECAP was used to improve the performance of various metallic materials, including aluminum, copper, titanium, low carbon steel, nickel, and many alloys.[10–15] However, there are still many flaws during ECAP, such as large extrusion force, heterogeneous grain sizes, poor thermal stability, which needs to be further ameliorated. Ultrasonic-assisted plastic processing has been applied to metal forming processes since the 1960s. Comprehensive studies of this method have been performed. It was reported that Huang et al.[16] found that ultrasonic vibration decreases the forming load and improves the friction condition with increases in temperature. Liu et al.[17] also revealed that ultrasonic vibration can heat a specimen to about 507 K (234 C). Furthermore, Pal et al.[18] and Zhang et al.[19] illustrated that ultrasonic vibration can increase dislocation density. Ahmadi et al.[20] and Djavanroodi et al.[21] experimented with combining ECAP and ultrasonic vibration for use in pure Al. Ahmadi et al.[2
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