Abstract
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[1]) which, in a collective motion manner, reduce dramatically the atomic mobility.[2] 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.[2, 3] 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.[4-6] 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.