In situ scattering study of multiscale structural evolution during liquid–liquid phase transition in Mg-based metallic glasses

The glass-forming ability of Mg–Cu–Gd alloys could be significantly promoted with the addition of Ag. A calorimetric anomaly could be observed in the supercooled liquid region of the Mg–Cu–Ag–Gd metallic glass, indicating the occurrence of a liquid-state phase transition driven by entropy. However, the underlying mechanism of the polyamorphous phase transition remains unsettled. In the paper, in situ scattering techniques were employed to reveal multiscale structure evidence in a Mg65Cu15Ag10Gd10 metallic glass with an anomalous exothermic peak upon heating. Resistivity measurements indicate a reentrant behavior for the Mg–Cu–Ag–Gd metallic glass in the anomalous exothermic peak temperature region during heating. In situ synchrotron diffraction results revealed that the local atomic structure tends to be ordered and loosely packed first, followed by reentering into the initial state upon heating. Moreover, time-resolved small-angle synchrotron X-ray scattering (SAXS) results show an increase in nanoscale heterogeneity first followed by a reentrant supercooled liquid behavior. A core–shell structure model has been used to fit the SAXS profiles when polyamorphous phase transition occurs. In contrast, there is no structure anomaly for the reference Mg–Cu–Gd alloy system. The detailed multiscale structural evidence suggests the occurrence of a liquid–liquid phase transition followed by a reentrant behavior in the Mg–Cu–Ag–Gd metallic glass. Our results deepen the understanding of the structural origin of the glass-forming ability and shed light on the possibility of tuning the physical and mechanical properties by heat-treatment in the supercooled liquid region of Mg-based metallic glasses.