Efficient local atomic packing in metallic glasses and its correlation with glass-forming ability

A liquid freezes into a glass when bypassing crystallization upon cooling. The slower an atom moves around in the liquid, the more likely the atom escapes the capture of crystallization. In glass-forming liquids such as polymer and silica liquids, rather than individual random walk, atoms are encaged in molecular units (such as the SiO4 tetrahedra in liquid silica) which, in a collective motion manner, reduce dramatically the atomic mobility. These local structural units thus account for the excellent glass-forming ability of the liquids, and remain as the building blocks of polymer and silica glasses. Analogously, metallic glasses also exhibit signs of local structure units such as short range orders (SROs) (or solute-centered atomic clusters), as proposed in recent works of structural modeling and computer simulation. However, there is a lack of key experimental determination of local atomic structure in metallic glasses, and the correlation between local atomic packing with glass-forming ability has yet to be demonstrated experimentally over the years. In this work, we have probed local atomic structure of metallic glasses using time-of-flight neutron and synchrotron X-ray diffraction techniques with high resolution. Our results provide evidence for a new scheme of efficient local atomic packing where atomic clusters encompass multiple types of atoms in the first coordination shell. We also demonstrate the first experimental evidence of a clear correlation between the number of unlike atom bonds and the glass-forming ability. Our findings have important implications for understanding atomic structure of metallic glasses, liquids and other disordered materials, and may provide insights into a broad range of scientific problems where efficient space filling by packing spheres is essential.