Fracture of a Commercial Zr 41.2 Ti 13.8 Cu 12.5 Ni 10.0 Be 22.5 Bulk-Metallic Glass
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INTRODUCTION
BULK-METALLIC glasses (BMGs) usually exhibit extraordinarily high yield strength, low ductility, and very low strain rate sensitivity.[1–4] The macroscopic plastic flow of the BMG alloys at room temperature is microscopically associated with the initiation and propagation of the shear bands. Although the macroplastic strain to failure of monolithic BMG alloys is very low (typically less than 0.2 pct) during plastic deformation, the local plastic strain within the shear bands can be sufficiently high to give rise to some microstructural changes (i.e., relaxation, crystallization, nanosized voids, etc.) within the shear bands, as previously reported.[5–11] The localized plastic deformation within the shear bands causes a sudden temperature rise in the BMG alloys,[9,12–19] which results in a dramatic softening of the material. Such softening of the materials can lead to a catastrophic propagation of a single dominant shear band and the eventual fracture of the BMG alloys. Reports of the presence of a liquidlike pattern and droplets on the fractured surface[14,15,20–26] provide support to the premise of a temperature rise with the accompanying viscous flow during the fracture of the BMG alloys. However, the precise magnitude of the temperature increase, DTFracture, during the fracture of the BMG alloys remains a controversial issue. For example, Liu et al.[14] argued that the large elastic energy stored in the specimen could be converted into the heat during the shear-band propagation and the fracture of Zr-based BMGs. They reported a temperature rise of DTFracture = 900 K. On the other hand, by measuring G.J. FAN, Researcher, H.H. LIAO, Researcher, H. CHOO, Professor, and P.K. LIAW, Professor, are with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA. Contact e-mail: [email protected] N. MARA, Researcher, A.V. SERGUEEVA, Researcher, A.K. MUKHERJEE, Professor, and E.J. LAVERNIA, Professor, are with the Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA. Manuscript submitted September 10, 2006. Article published online August 1, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
the light emission during the fracture of a Zr41.2Ti13.8 Cu12.5Ni10.0Be22.5 BMG alloy, Gilbert et al.[21] reported a temperature rise of DTFracture = 3175 K for the specimen fractured in the air and DTFracture = 1400 K for the specimen fractured in the nitrogen atmosphere. Such an environmental effect on the temperature rise and the fracture of the BMG alloys has been noticed in other reports.[14,24] In contrast to the aforementioned reports, the temperature rise during the fracture of BMG alloys was estimated to be only several tens of Kelvin by Flores and Dauskardt,[22,23] which alone cannot explain the liquidlike patterns frequently observed on the fractured surface. They argued that the plastic deformation creates the free volumes and, therefore, decreases the viscosity of the BMG alloys. They attributed the liquidlike pattern of the fractured surf
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