Abstract
The atomic structure of metallic glasses has been a long-standing scientific mystery. Unlike crystalline metals, where long-range ordering is established by periodic stacking of fundamental building blocks known as unit cells, a metallic glass has no long-range translational order, although some degrees of short- and medium-range order do exist.1,2,3 Previous studies1,2,3,4 have identified solute-centered clusters, characterized by short-range order (SRO) in favor of unlike bonds, as the fundamental building blocks of metallic glasses. However, how these building blocks are connected or packed to form the medium range order (MRO) remains an open question.1,2,3 Here, based on neutron and x-ray diffraction experiments, we propose a new packing scheme - the self-similar packing of atomic clusters. We show that MRO has the characteristics of a fractal network with a dimension of 2.38, and is described by a power-law correlation function over the medium-range length scale. Our finding provides a new prospective of order in disordered materials and has broad implications for understanding the structure-property relationship in metallic glasses, particularly those involving change in length scales due to phase transformation and mechanical deformation.