Characterization of Open Volume Regions in a Simulated Cu-Zr Metallic Glass
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BULK metallic glasses (BMGs) combine an impressive array of mechanical properties with unique forming capabilities at moderate temperatures, making them attractive for numerous applications.[1–4] Because of the lack of long-range atomic order and therefore crystalline defects, these alloys exhibit very different flow characteristics from their crystalline counterparts. Understanding the atomic structure of the glass is necessary for the development of a physically based flow model and optimization of these alloys as structural materials. One of the most studied models is based on the free volume concept, initially proposed for liquids by Cohen and Turnbull.[5] ‘‘Free volume’’ is defined as the atomic volume in excess of the ideal densely packed, but still disordered, structure.[6] Historically, it has been ASHWINI BHARATHULA and WEIQI LUO, Graduate Research Assistants, WOLFGANG WINDL, Associate Professor, and KATHARINE M. FLORES, Assistant Professor, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Contact e-mail: fl[email protected] This article is based on a presentation given in the symposium entitled ‘‘Bulk Metallic Glasses IV,’’ which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee. Article published online April 1, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
described as a macroscopic average based on a hardsphere model of glass structure. This average value is often used as a fitting parameter to describe the role structure plays in properties such as elastic constants,[7] electrical resistivity,[8] viscosity,[9] and the glass transition event.[10] While valuable as a fitting parameter, this average value provides little insight into the atomic structure or the local processes controlling these properties. Based on the early works of Spaepen[6,11] and Argon,[12] flow in metallic glasses is typically modeled as a diffusion-like process involving a stress-induced reshuffling of small groups of atoms. While related, Spaepen’s and Argon’s models contain subtle differences in how they describe the defect responsible for flow; Spaepen proposed that flow is associated with the jump of a single atom into a neighboring site with slightly smaller volume, resulting in the creation of free volume, followed by some subsequent structural relaxation.[6] Thus, a flow defect is associated with an atom-hole pair, and the probability of a hole of sufficient size existing is described by Cohen and Turnbull’s distribution and the average free volume. Experimental results confirm an increase in volume with deformation.[13–15] On the other hand, Argon’s model depicts the flow defects as small groups of atoms, referred to as ‘‘shear transformation zones’’ (STZs), in regions of low atomic density (locally VOLUME 39A, AUGUST 2008—1779
high free volume), that undergo spontaneous and cooperative shuffling or rearrangement under the combined influence of external stress and te
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