Chemical Short-Range-Ordering Effect on Structural Relaxation in Metallic Glasses
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INTRODUCTION
PLASTIC stability, or ductility, of metallic glasses depends sensitively on the alloy chemistry (e.g., Reference[1]) and microscopically involves evolution of flow defects, or disordered regions, that are considered to carry any shear deformation (e.g., Reference[2]). A good first step to understand how alloying chemistry affects the plasticity is to characterize chemical effects on the formation and destruction of the flow defects with an absence of an external stress, that is, the structural relaxation. Atomic structures of glass materials are difficult to observe directly. For glasses with metallic atomic bonding, the hard-sphere dense-random-packing (DRP) structures are considered as the ideal amorphous state with possibly the lowest entropy and the slowest atomic transport in the frozen liquid structures. The deviation of glasses and undercooled liquids from the ideal DRP state can be depicted as defects associated with structural and chemical disordering. The defects are different from those in their corresponding crystalline counterparts and are considered to be diffusive, short lived, and spatially delocalized around the glass transition temperature, Tg.[3] Two major classical models have been developed to phenomenologically explain why structural relaxation slows near the kinetic glass transition: the free-volume (FV) theory developed by Cohen and Turnbull[4,5] and extended by Cohen and Grest[6] and by Spaepen,[7] and the configurational entropy (or cooperatively rearrangement region, CRR) theory suggested by Adams and Gibbs.[8] In the FV theory, which accounts for density fluctuations, the disordering is indicated by the free volume, which is the difference between the glass or AIWU ZHU, Senior Scientist, and G.J. SHIFLET, Professor, Department of Materials Science and Engineering, and S.J. POON, Professor, Department of Physics, are with the University of Virginia, Charlottesville, VA 22932. Contact e-mail: [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 February 16, 2008 1958—VOLUME 39A, AUGUST 2008
liquid volume and the DRP, with a size distribution among atoms (or their Voronoi polyhedra). For metallic glasses, two issues remain for application of the FV model in the structural relaxation analysis: (1) the activation volume and energy in the elemental process of atomic transport are too large for the single atom jumping mechanism; and (2) it is difficult to logically account for the bimolecular reaction mechanism for the annihilation of the FVs as generally observed, even for metallic glasses. The collective processes of atomic or molecular rearrangement may account for the large activation energy and volume as possibly indicated in the CRR theory where local configurational entropy fluctuations among molecular clusters are considered to c
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