The density and packing fraction of binary metallic glasses

The present work develops a physical model of metallic glass structure that gives a reasonable estimate of density. The efficient cluster-packing model is used as a starting point, and is refined by a high-fidelity estimate of the size of structure-forming clusters and cluster–cluster separations. These are predicted as continuous functions of composition and relative atom radii. Predicted densities are all are within ±10% of measured densities for 200 binary metallic glasses, representing a precision in cluster–cluster separations of ±3%. New structural insights from this work include the importance of acknowledging the unique cluster topologies to estimate cluster–cluster separations; an improved ability to estimate the higher packing efficiency of unlike atoms in the first coordination shell of atomic clusters; and an improved estimate of metalloid–metalloid separations. The unusual, bilinear influence of composition on density in Fe–B glasses is explained by considering the sizes of β and γ sites in different metallic glass structures. Global atom packing fractions derived from measured densities range from about 0.62 to 0.76, and the most stable binary glasses all have packing fractions in excess of 0.70, supporting the idea that atom packing efficiency influences glass stability.