Coupling of structural preordering and density fluctuation: A unified picture of nucleation in metallic glasses

Crystallization is ubiquitous in nature and technology, occurring in pharmaceuticals, biomacromolecules, polymers, metallurgy and glass formation. Yet how crystallization proceeds, particularly at its earliest stage, remains elusive. Studies on glass-forming liquids increasingly suggest that crystallization initiates in structurally preordered regions, with density fluctuations playing a secondary role, a view widely generalized from hard-sphere colloids. Here we show, across disordered metallic systems from pure elements and intermetallic compounds to glass-forming alloys, that density fluctuations and structural preordering emerge simultaneously during nucleation. This coupling reflects local atomic rearrangements in which free-volume reduction, denser packing, and bond-orientational order develop together. Our findings provide a physical basis for a unified nucleation framework for disordered materials.