Atomistic study of the mechanical properties of metallic-glass nanowires
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Atomistic study of the mechanical properties of metallic-glass nanowires K. Koshiyama and K. Shintani Department of Mechanical Engineering and Intelligent Systems, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan ABSTRACT Melt-growth simulations based on the molecular-dynamics method for both the Cu-Zr and Ni-Al crystalline nanowires of B2 structure are performed to produce metallic-glass nanowires of amorphous structure. Next, tensile deformations of these nanowires are simulated at various temperatures. For the sake of comparison, Cu-Zr and Ni-Al crystalline nanowires of B2 structure are also elongated. It is revealed that the tensile strength of the metallic-glass nanowires is third or fourth of the tensile strength of the crystalline nanowires. Increasing tensile strain, the Cu-Zr crystalline nanowires of B2 structure change their structure twice, whereas the metallic-glass nanowires only decrease their thicknesses locally, and necking takes place. INTRODUCTION Metallic glass is an amorphous metal which shows clear glass transition. Compared with other crystalline metals, metallic glass has superior mechanical properties such as high strength, high elasticity, high hardness, etc. Recent researches have revealed that metallic-glass nanowires are created during the breaking of bulk metallic glass by viscous flow. Metallic-glass nanowires can be bent elastically, and they exhibit an extremely high strength. Furthermore, metallic glass of various alloy compositions can be processed to become nanowires. Such nanowires will be applied to highly sensitive sensors for hydrogen and magnetism [1]. However, their deformation mechanism has not been clarified yet because plasticity theory for crystals cannot be applied to amorphous structure. In this paper, formation of metallic-glass nanowires through heating-cooling process and their tensile deformation are investigated using molecular-dynamics (MD) simulation code LAMMPS [2]. In many cases, metallic glasses are alloys. Thus, Cu-Zr and Ni-Al metallic-glass nanowires are the objects of our simulation. METHOD OF SIMULATION The initial setup for a simulation model is shown in figure 1. The Cu-Zr of B2 structure consists of Cu atoms at eight apexes and a Zr atom at a center in a unit lattice. The Ni-Al B2 structure consists of Ni and Al atoms at the positions of Cu and Zr atoms, respectively. The initial lattice constants are 3.24ύ and 2.8712ύ for the Cu-Zr and Ni-Al structures, respectively. The parameters of the nanowire models employed in our simulations are shown in table I. The two kinds of nanowires with large and small sizes are adopted for each structure. Their tensile simulations are performed at the five temperatures shown in table I.
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Figure 1. Nanowire model of B2 structure. Figure 2. Obtained bulk amorphous structure. The magnified window shows a unit lattice. The embedded atom method (EAM) potential is adopted to calculated atomic interactions. Its parameters for Cu, Zr, and Cu-Zr and for Ni, Al, and Ni-Al are referre
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