The Evolution of Strain Gradient and Anisotropy in Gradient-Structured Metal
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The Evolution of Strain Gradient and Anisotropy in Gradient-Structured Metal XIANGDE BIAN, FUPING YUAN, XIAOLEI WU, and YUNTIAN ZHU Gradient-structured metals have been reported to possess superior mechanical properties, which were attributed to their mechanical heterogeneity. Here we report in-situ observation of the evolution of strain gradient and anisotropy during tensile testing of a gradient-structured metal. Strain gradients and anisotropy in the lateral directions were observed to increase with increasing applied tensile strain. In addition, the equivalent Poisson’s ratio showed gradient, which evolved with applied strain. The gradient structure produced higher deformation anisotropy than coarse-grained homogeneous structure, and the anisotropy increased with increasing tensile strain. The strain gradient and anisotropy resulted in strong back-stress hardening, large strain gradients, and a high density of geometrically necessary dislocations, which helped with increasing the ductility. DOI: 10.1007/s11661-017-4199-2 Ó The Minerals, Metals & Materials Society and ASM International 2017
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INTRODUCTION
IT has been a challenge to produce both high strength and high ductility in metals and alloys.[1–5] The strength can be easily enhanced by well-known strategies such as grain refinement, work hardening, solution strengthening, second-phase particle strengthening, and deformation twins.[6–11] However, high strength is usually accompanied with the sacrifice of ductility in homogeneous metals. For example, ultrafine-grained (UFG) or nanostructured (NS) metals obtained by severe plastic deformation (SPD) can have strengths an order of magnitude higher than those of their coarse-grained (CG) counterparts. However, they usually show limited strain hardening and very low (near zero) uniform elongation.[6,7,12,13] Recently, several promising strategies for achieving simultaneous high strength and high ductility have been proposed by tailoring microstructures through heterogeneous and/or hierarchical structures.[2–5,14–29] Among them, the gradient structure, where the grain size or
XIANGDE BIAN, FUPING YUAN, and XIAOLEI WU are with the State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, P.R. China, and also with the School of Engineering Science, University of Chinese Academy of Sciences, Beijing 100190, P.R. China. Contact e-mail: [email protected] YUNTIAN ZHU is with the Nano Structural Materials Center, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, P.R. China, and also with the Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695. Manuscript submitted March 19, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the substructure size changes gradually along the depth,[5,20–26] has great potential in engineering applications due to their superior combinations of strength and ductility. The tensile properties and underlying deformation m
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