On the universal model for medium-range order in amorphous metal structures

A critical assessment is provided of the proposition that a universal model exists for the medium-range structure of metallic glasses. Previous studies are reconsidered from a topological perspective. While support for universal medium-range order has been based on chemically distinct systems, it is shown that these alloys are topologically very similar regarding the relative size of solute and solvent atoms. The short-range structural order in metal–metalloid glasses – capped trigonal prisms – is shown to be unlikely in metallic glasses where the solute radius is greater than about 80% of the solvent. Other possible short-range ordered configurations, including icosahedra, are possible based on geometric arguments. While a universal model for medium-range order may exist in spite of different short-range ordering, it is nevertheless likely that alloys with significantly different atom radius ratios will possess distinct medium range ordering. Additional studies in topologically distinct systems are required to unambiguously assess the universality of medium-range order in metallic glasses.