Atomic packing and short-to-medium range order in metallic glasses.

摘要

Since its discovery in the late 1950's, metallic glass has grown from a singular observation to an extensive class of alloys with broad scientific and commercial importance. Tremendous effort has been devoted to research in this field. Unlike the well-defined long-range order in crystalline materials, the details of atomic arrangement in metallic glasses still remain mysterious. Over the years, many structural models have been proposed and it is believed that short-range order (SRO) is ubiquitous and medium-range order (MRO) is likely in metallic glasses. But none of the models provide a complete structural description of SRO, and information about MRO is even less tangible. In order to elucidate the atomic-level structure of metallic glasses in a comprehensive manner, we have used a combination of experimental and computational techniques to construct three-dimensional atomic arrangements in several model systems which involve different chemical bonding conditions and atomic size ratios. These model systems include an immiscible system with a positive heat of mixing (Ag-Ni), a metal-metalloid system with a negative heat of mixing (Ni-P), and a more generic model system of binary Lennard-Jones particles (A-B). Through a geometrical analysis on the atomic-level configurations, we demonstrate the prevailing existence of chemical and topological SRO among nearest neighbors, and icosahedral-like dense packing of solute-centered quasi-equivalent clusters as the basic medium-range packing scheme. As such, the intra-cluster SRO has been resolved, based on which a specific inter-cluster MRO is identified. The findings in this work have implications for understanding the nature, forming ability, and properties of metallic glasses.