Mechanism of liquid-liquid phase separation and crystal nucleation in Fe-Cu immiscible alloy doped with trace Zr

Immiscible alloys exhibit liquid-state miscibility gaps, where micro-alloying elements play a crucial role in controlling microstructure by influencing liquid-liquid phase separation (LLPS) and crystal nucleation. However, the micro-alloying mechanisms remain poorly understood. In this study, we combine experiments, theoretical analysis, and molecular dynamics simulations to reveal how Zr regulates LLPS and crystal nucleation from a microscopic perspective. Our results show that Zr exerts both suppressing and promoting effects on γ-Fe nucleation. At low Zr contents, the Zr-induced increase in viscosity dominates, leading to suppressed γ-Fe nucleation. Beyond a certain concentration, the effect of Zr on viscosity becomes limited, while its promoting influence on nucleation becomes more pronounced, resulting in enhanced γ-Fe nucleation. For LLPS, Zr enhances phase separation by stabilizing the Cu/Fe interface, thereby increasing the LLPS temperature, while its preferential interactions reduce Cu-Fe mutual solubility despite the accompanying rise in melt viscosity. These dual effects arise from Zr-modulated atomic mobility and bonding, ultimately affecting the alloy microstructure. This study contributes to a deeper understanding of the micro-alloying mechanism, thereby offering valuable guidance for the design of immiscible alloys.