Highly variable chemical short-range order in a high-entropy metallic glass

High-entropy metallic glasses (HE-MGs) recently joined the high-entropy materials and metallic glasses families as a novel class of alloys. The high-entropy effect that is believed to be crucial to HE-MGs, however, has not been well explored yet in amorphous forms. In this work, we chose a Zr20Nb20Cu20Ni20Ti20 quinary HE-MG as a model system and studied its structural evolution as a function of temperature by in situ synchrotron high-energy x-ray diffraction (XRD) and extended x-ray absorption fine structure spectroscopy (EXAFS) techniques. The HE-MG exhibits irreversible structural crossover upon heating, specifically, from a relatively disordered high-energy glass state to a more ordered low-energy glass state in the supercooled liquid region but below its crystallization temperature. The pair distribution function (PDF) derived from XRD and EXAFS data suggests that the highly variable chemical short-range order (CSRO) is the underlying mechanism of the structural ordering crossover in the HE-MG. The initial ribbon sample obtained by melt-quenching has a relatively high chemical disorder with nearly random nearest atomic neighbors due to the high-entropy effect. However, the high chemical disorder is metastable upon post-fabrication heating at medium temperatures, which gradually degrades driven by enthalpy with the random neighbors replaced by more energy-favored ones. The chemical complexity of the HE-MG prevents further development of the ordering into typical crystallizations. In contrast, a conventional quinary MG with a close composition but lower entropy (Vit106, Zr57Nb5Cu15·4Ni12·6Al10) does not show similar variable CSRO during heating. These findings demonstrate that the high-entropy effect does play an essential and unique role in HE-MGs. Competitions between entropy, enthalpy, and atomic-level stress could result in high variability in CSRO and properties, which may provide us another unexplored dimension for effectively tuning structures and tailoring properties for various applications.