Liquid-liquid phase transition and chemical phase separation in Cu-Zr-Al-Y bulk glass-forming supercooled liquid

Anomalous exothermic peaks before crystallization in metallic supercooled liquids are commonly attributed to chemical phase separation (CPS) or structural changes due to polyamorphic liquid-liquid phase transition (LLPT). In this work, we present experimental evidence of the simultaneous occurrence of LLPT and CPS during the anomalous exothermic reaction in a supercooled Cu-Zr-Al-Y liquid. The CPS results from the positive enthalpy of mixing between Y and Zr, while LLPT, characterized by medium-range structural ordering, originates from the two-phase field (fragile and strong phases) centered around Cu₅₀Zr₅₀ in the Cu-Zr system. These two aspects interact with each other and are considered to be a key factor in stabilizing the supercooled liquid against crystallization. To describe the structural and chemical evolutions in the supercooled liquid, we propose schematic Gibbs free energy surfaces for the (Cu, Al)-(Zr, Al)-Y pseudo-ternary system that incorporate both the two-phase field of LLPT and the local maximum of the CPS in the Y direction. The results of this work not only enrich our understanding of anomalous thermophysical signals in the supercooled liquid but also offer insights into the development of bulk metallic glasses with superior glass-forming ability and thermal stability.