Tensile Failure Modes in Nanograined Metals with Nanotwinned Regions
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STRENGTH and ductility of metals are two mutually exclusive properties.[1] The traditional strengthening approaches based on grain refinement compromise ductility inevitably. Specifically, bulk ultrafine-grained or nanostructured metals fabricated by severe plastic deformation usually have high strength but relatively low ductility.[2,3] Recently, as a novel approach for synthesizing bulk nanostructured metals, dynamic plastic deformation (DPD) has been used to generate nanotwins achieving high strength while maintaining
X. GUO is with the School of Mechanical Engineering, Tianjin University, Tianjin, China, with the Tianjin Key Laboratory of Nonlinear Dynamics and Control, Tianjin 300072, China, and also with the State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an 710049, China. Contact e-mail: [email protected] Y. LIU is with the School of Mechanical Engineering, Tianjin University. G.J. WENG is with the Department of Mechanical and Aerospace Engineering, Rutgers University, New Brunswick, NJ 08903. L.L. ZHU is with the Department of Engineering Mechanics, School of Aeronautics and Astronautics, Zhejiang University, Hangzhou 310027, Zhejiang, China. Manuscript submitted February 7, 2018. Article published online July 2, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A
substantial ductility.[4] This is attributed to the fact that twin boundaries (TBs) not only serve as strong obstacles to dislocation motion, but provide ample room for dislocation slip and storage to accommodate plastic strains.[5–9] Therefore, nanotwinned (NT) metals with large density coherent TBs have drawn much attention.[10–14] In fact, benefited from the effective synthesizing approaches of nanotwin for metals with low stacking fault energy, such as DPD[4,15,16] and electrodeposition,[17–22] numerous studies have revealed essential roles of the dislocation–TB interactions in enhancing the combination of strength and ductility, including experimental investigations,[5,21–25] theoretical analysis,[26,27] and numerical simulations.[28–30] In general, it has been uncovered that mechanical properties of NT metals can be improved by further increase of coherent TBs density or decrease of the average twin spacing. For instance, Lu et al.[31,32] revealed that decreasing twin spacing enhances work-hardening ability, leading to enhanced strength, ductility, and fracture toughness of NT Cu. Furthermore, Qin et al.[33] found that both tensile strength and fracture toughness of the DPDed Cu are enhanced with an increasing volume fraction of nanotwin bundles. On the other hand, via the TB affected zone, Jerusalem
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et al.[27] and Dao et al.[34] proposed a crystal plasticity model of the ultrafine-grained Cu with nanotwin and the model can capture the increase in strength, rate sensitivity, and ductility with the decrease in twin spacing. However, with the twin spacing decreasing to a few nanometers, NT metal shows softening phenomenon. For instance, Lu et al.[19] found
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