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THE number and type of atoms that surround a given central atom is the simplest description of atomic structure in metallic glasses. This first-neighbor constitution dictates the number and type of atomic bonds in the structure and so exerts a dominant influence on stability. The first-neighbor constitution also sets the local atomic packing efficiency,[1,2] which controls many physical and mechanical properties, including the mechanisms of deformation. The first-neighbor constitution is given by partial coordination numbers Zij, which are the number of j atoms in the first shell of a reference i atom. In binary alloys consisting of solute (a) and solvent atoms (X), there are four separate partial coordination numbers: Zaa, ZaX, ZXa, and ZXX. Only three of these are independent, because ZaX and ZXa are related fa ZaX ¼ fX ZXa

½1

where fi is the atomic fraction of atom i. Although recent structural descriptions focus on solute-centered clusters, a complete structural description requires that equal merit be given to both solute-centered and solvent-centered clusters. DANIEL B. MIRACLE, Senior Scientist, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, OH 45433. Contact e-mail: Daniel.miracle@ wpafb.af.mil KEVIN LAWS, Senior Research Fellow, is with the ARC Centre of Excellence for Design in Light Metals, School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia. OLEG N. SENKOV, Senior Researcher, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, and is also with UES, Inc., Dayton, OH 45432. GARTH B. WILKS, Scientist, is with the UTC, Inc., Wright-Patterson AFB, OH 45433. Manuscript submitted April 28, 2011. Article published online December 7, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A

Partial coordination numbers can be measured using diffraction techniques (see for example, Reference 3). Three independent measurements are needed to separate the overlapping contributions to the total scattering function from the three independent atom pairs in binary alloys, and six independent experiments are needed to resolve Zij values in ternary alloys. An integrated approach combining experimental data, reverse Monte Carlo calculations, and atomic simulation to deconvolute overlapping peaks has been developed recently to give Zij values for ternary metallic glasses.[4] The Zij values for more complex metallic glasses have been estimated[5] using structural concepts from the efficient cluster packing (ECP) model[6,7] to rationalize the insignificance of some weighting factors. As a result of the difficulties associated with the determination of Zij, relatively little data are available for binary glasses and almost no data are published for partial coordination numbers in higher order glasses. Measured partial coordination numbers are compiled in a recent assessment of binary metallic glasses.[8] These data are plotted in Figure 1, along with additional data not included in tha