Bauschinger Effect and Back Stress in Gradient Cu-Ge Alloy
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Bauschinger Effect and Back Stress in Gradient Cu-Ge Alloy XIANZHI HU, SHENBAO JIN, HAO ZHOU, ZHE YIN, JIAN YANG, YULAN GONG, YUNTIAN ZHU, GANG SHA, and XINKUN ZHU Using surface mechanical attrition treatment (SMAT), a gradient structure composed of two gradient structure (GS) layers and a coarse grain (CG) layer was generated from a Cu-5.7 wt pct Ge alloy, significantly improving the yield strength of the sample. Unloading–reloading tests showed an unusual Bauschinger effect in these GS samples. The back stresses caused by the accumulated geometrically necessary dislocations (GNDs) on the GS/CG border increased with increasing strain. As found by electron backscatter diffraction (EBSD), the GNDs are mainly distributed in the gradient structured layer, and the density of the GNDs increase with increasing SMAT time. The effect of the back stress increased with increasing SMAT processing time due to the increase in the strain gradient. The pronounced Bauschinger effect in a GS sample can improve the resistance to forward plastic flow and finally contributes to the high strength of GS samples. DOI: 10.1007/s11661-017-4176-9 The Minerals, Metals & Materials Society and ASM International 2017
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INTRODUCTION
HIGH strength and good ductility can be achieved simultaneously in metals with a gradient structure (GS).[1,2] Compared to bulk metals with coarse grains, gradient structured materials exhibit a substantially different deformation behavior and mechanical properties.[3–7] This property has drawn increasing attention in recent years. Various mechanisms, including mechanically driven grain growth,[5] strain gradient, multiaxial stress state and extra strain hardening,[8] and synergetic strengthening[9,10] have been proposed to elucidate the intrinsic effects associated with the strain and stress gradient. Wu[11] found an unusual Bauschinger effect (BE) in heterogeneous lamella (HL) structured Ti, which is responsible for its high strength and good ductility.
XIANZHI HU, ZHE YIN, JIAN YANG, YULAN GONG, and XINKUN ZHU are with the Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming Yunnan 650093 China. Contact e-mail: [email protected] SHENBAO JIN and GANG SHA are with the School of Materials Science and Engineering, Nanjing University of Science and Technology, RM307 Building 340, Nanjing, Jiangsu 210094 China. HAO ZHOU is with the The Institute of Microstructures and Properties of Advanced Materials in Beijing University of Technology, Beijing 100124 China. YUNTIAN ZHU is with the School of Materials Science and Engineering, Nanjing University of Science and Technology and also with the Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 276957919. Manuscript submitted January 1, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
A distinct Bauschinger effect that was considered to make a significant contribution to high strength has been found in freestanding Cu thin films,[12–15] conformally passivated copper nanopillars,[16] and hete
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