Undercooling‐governed glass‐forming behavior in TiCu‐based bulk metallic glass and composite: evolution in local chemical heterogeneity and liquid phase stability

Abstract The influence of undercooling (Δ T ) on the glass formation of TiCu‐based glass‐forming alloys was systematically investigated by modulating the Cu mold temperature upon arc‐suction casting. Δ T of the undercooled liquids changed from 545 to 321 K depending on the mold temperature, which induced a significant change in the microstructure of alloys from a bulk metallic glass (BMG) composite containing crystals to a monolithic BMG. The unexpected enhancement in the glass‐forming ability of the TiCu‐based BMGs was mainly related to the evolution of the local chemical heterogeneity (LCH) of the glassy phases. The LCH in BMGs became more pronounced with increasing Δ T , and nuclei formed in Cu‐rich regions. On the other hand, the LCH in the glassy phases gradually weakened under small Δ T conditions. Accordingly, the melt solidified into a monolithic BMG without nuclei in both Ti‐rich and Cu‐rich regions under small Δ T conditions. Moreover, a change in the phase stability of undercooled liquids could be expected with increasing Δ T by considering a non‐classical model for the nucleation of glass‐forming systems. The evolution of the liquid phase stability by modulating Δ T significantly influenced the crystallization behavior of the undercooled liquid. Understanding the microstructural evolution in the TiCu‐based BMGs with changes in Δ T will assist in comprehending nanoscale chemical heterogeneities and the stability of the undercooled liquid, which could contribute to the optimization of the glass‐forming behavior of BMGs with liquid phase separation and to the control of crystalline phase in BMG composites.