Numerical Study of the Effect of the Sample Aspect Ratio on the Ductility of Bulk Metallic Glasses (BMGs) Under Compress
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BULK metallic glasses (BMGs) have many unique properties, e.g., exceptionally high strength, large elastic limit, high hardness, good corrosion resistance, reduced sliding friction, etc., and are therefore regarded as potential candidates for engineering materials. However, their structural applications are severely stymied by their intrinsic limited plasticity that confined to narrow regions near dilute shear bands at room temperature. A multi-axial stress state can effectively induce the shear band over the BMG sample, and therefore, the plasticity is significantly enhanced. Furthermore, many researchers have pointed out that the sample aspect ratio greatly affects the shear band evolution and the plasticity of BMGs. Feng et al.[1] analyzed the effects of aspect ratios of Zr-based metallic glasses on the shear band arrangements through molecular dynamics (MD) simulations and experiments, and their results demonstrate that plasticity increases with the decrease in the aspect ratio. Wu et al.[2] decreased the sample dimension with an identical scale ratio and found that the compressive plastic strain of a Zr-based metallic glass increases from near zero to as high as 80 pct without failure. They proposed the critical shear offset to explain the strong size effect on the enhanced plasticity of metallic glass by taking the shear fracture energy density into account. The failure mode is very sensitive to the notch depth and notch sharpness. Zhao et al.[3] experimentally confirmed that the plasticity of Ti-based metallic glasses with different aspect ratios can be
YUNPENG JIANG, Professor, is with the College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P.R. China. Contact e-mail: [email protected]. Manuscript submitted November 5, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
improved by introducing two semicircular notches on the edges of the samples, owing to the interactions of shear bands (SBs) under conventional compression tests. More importantly, the idea of inducing an interaction of two major SBs caused by a proper stress gradient might result in a steady shear deformation instead of the fast fracture in conventional compression tests. Wu et al.[4] investigated the quasi-static compressive deformation behavior of a Vitreloy 1 BMG with an aspect ratio of 0.25. It is found that the friction and the confinement at the specimen–loading platen interface will cause the dramatic increase in the compressive strength. The formation of the strongly interacted, deflected, wavy, or branched shear bands can be attributed to the triaxial stress state in the glassy specimens with a very small aspect ratio. Kou et al.[5] investigated the compressive plasticity of Ti40Zr25Ni8Cu9Be18 BMGs with different specimen aspect ratios, and they also found that the specimen aspect ratio significantly affects the performance of the plasticity of a Ti-based BMG. The difference in the plasticity is a result of shear band behavior due to the geometrical constraints. Tao et al.[6] pointed that w
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