Size Effect on Flow Behavior of a Zr 55 Al 10 Ni 5 Cu 30 Bulk Metallic Glass in Supercooled Liquid State
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BULK metallic glass (BMG) has become an attractive candidate for structural application due to its high specific strength, excellent wear resistance, corrosion resistance, and other potential properties, since the formation method of large size BMG in centimeter or millimeter was invented in the early 1990s.[1–3] Therefore, the flow behavior of BMG has been paid much attention mainly concerning mechanism and optimal processing parameter in the last decade.[4–7] Temperature, strain rate, and specimen size were pointed out as the origin of variety of flow behavior.[8–10] Several researchers found that specimen size plays an important role in the plasticity of BMG systems at room temperature, meanwhile only a few reports focusing on the size effect on flow behavior in supercooled liquid state can be retrieved from the literature.[11] Unlike the size effect at room temperature that flow stress increases with the increasing of specimen size in millimeter scale and with the decreasing of specimen size in micro scale,[12–17] the flow stress in supercooled liquid state increases with specimen size decreasing in millimeter scale. Therefore, mechanisms presented on the grounds of experiment results at room temperatures, such as free volume theory, resilient property theory of test system, and flaw sensitivity theory, cannot be quoted directly to interpret the size-dependent phenomenon in supercooled liquid state. Indeed, the proposed room temperature mechanisms were based on the initial structure of the specimen at room temperature or on the modeling of the specimen as ideally rigid. Such considerations are not X.Y. WANG, Professor, L. DENG, Post-Doctoral, N. TANG, Graduate Student, and J.S. JIN, Lecturer, are with the State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science & Technology, Wuhan 430074, P.R. China. Contact e-mail: [email protected] Manuscript submitted July 22, 2013. Article published online April 12, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
relevant in the supercooled liquid state. On the other hand, utilizing the high viscosity and low strain rate sensitivity of supercooled liquid state, BMG can be fabricated into parts with complex three-dimension structures and has already been widely used in experimental processing, including stamping, extrusion, nano-indentation, and microforming with dimensions ranging from sub-millimeter to centimeter.[18–21] So, it is important to take specimen size into account while conducting plastic forming in supercooled liquid state. In the present study, uniaxial compression tests with different size specimens were conducted in supercooled liquid state to investigate the effect of specimen size on flow behavior and try to provide an insight into its mechanism. Based on the experimental results, processing maps were constructed to distinguish the workability of BMG with different sizes and determine the optimal processing condition during thermoplastic forming. II.
EXPERIMENTAL PROCEDURE
A. Material Preparation The Zr55Al10Ni5Cu30
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