Molecular Dynamics Based Study on Ductility Enhancement Effect of Nano-scale Void in Fine-grained Metallic Materials
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Molecular Dynamics Based Study on Ductility Enhancement Effect of Nano-scale Void in Fine-grained Metallic Materials Shin Taniguchi1, Toshihiro Kameda2, and Toshiyuki Fujita3 1 Department of Engineering Mechanics and Energy, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8573, Japan 2 Faculty of Engineering, Information and Systems, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8573, Japan 3 College of Engineering Systems, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 3058573, Japan ABSTRACT In fine-grained metallic materials, the dominant grain boundary (GB) process, such as dislocation emission, dislocation absorption, and dislocation pile-up, causes non-uniform deformation, which results in high yield stress and low ductility. When a nano-scale void is introduced, the dislocation activity enhancement around the void could inhibit GB fracture and enhance ductility. In this study, by considering nanocrystalline Cu models, the influence of an intragranular nano-scale void on the fracture process has been investigated through molecular dynamics simulation. The dependence of ductility enhancement on the grain size and void size has especially been discussed at low and room temperatures. Sufficient dislocation activity enhancement accompanied by optimal void growth causes a fracture mode transition from GB fracture to transgranular fracture. While the ductility enhancement strongly depends on the void size at low temperature, it depends on the grain size at room temperature. The strong dependence of ductility enhancement on the temperature is found in the case of relatively small grains. INTRODUCTION The dominant deformation mechanism in polycrystalline metallic materials varies with the grain size [1]. For example, a grain boundary (GB) process, such as dislocation emission, dislocation absorption, and dislocation pile-up, can be found in fine-grained metallic materials with a grain size less than 1 m. The GB process suppresses intragranular dislocation activity and causes the non-uniform deformation. Because of this mechanism, fine-grained metallic materials generally show high yield stress and low ductility [2]. The dominant GB process causes brittle fracture, which is accompanied by rapid crack growth [3]. The GB fracture is initially caused by void nucleation and void growth around the GB [4]. Thus, the suppression of intragranular dislocation activity is closely related to low ductility in fine-grained metallic materials. Nano-scale defect clusters generally cause the degradation of metallic materials, reducing the lifetime of the materials [5]. This is because the presence of dislocation activity around the clusters decreases the plastic deformation stability [6]. Thus, nano-scale defect clusters adversely affect the ductility. However, by considering the variation of the dominant deformation mechanism with the grain size, the unique influence of nano-scale defect clusters in fine-grained metallic materials can be determined. For example, a previous study has reported that fi
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