Formation of four-membered rings in metallic glasses with minor additions

The structural nature of amorphous materials remains a long-standing challenge in condensed matter physics. The emergence of order within disorder has driven significant breakthroughs, including the discovery of quasicrystals and structural motifs in amorphous systems. Among these motifs, ring structures, which exhibit rotational symmetry without translational symmetry, play a crucial role in enabling fractal packing and long-range disorder in network glasses, such as oxide, chalcogenide, and halide systems. However, their universality in simple atomic systems, like metallic glasses, remains unsolved. Here, a four-membered Frank-Kasper (FK) cluster ring structure, denoted as 4M-Ring, has been proposed to be a consistent structural model for a metastable amorphous phase of Cu-Zr-Al-Y bulk metallic glasses. This medium-range structural motif, about 15 Å, is geometrically stable and energetically favorable. The addition of yttrium (Y) as a minor alloying element facilitates the formation of the 4M-Ring by occupying its central position and linking four edge-sharing FK polyhedra. Furthermore, evidence from electron microscopy and small-angle neutron scattering suggests that 4M-Rings organize into a nanoscale network with diffuse interfaces, reducing interfacial energy and enhancing exceptional thermal stability. Our findings suggest the possibility of the presence of novel Frank-Kasper medium-range order in metallic glasses with minor additions and reveal new structural principles that govern amorphous materials.