Kinetic–thermodynamic coupling in secondary liquid–liquid phase separation of Cu–Fe alloys

Cu–Fe alloys with metastable immiscibility were examined under conventional solidification at different diameters. Both primary and secondary liquid–liquid phase separations were observed during the solidification. The size of Fe-rich spherulites and the internal morphology of Cu-rich minority phases were strongly affected by cooling rate, reflecting coupled thermodynamic and kinetic effects. Distinct Cu-rich morphologies within Fe-rich spherulites revealed the dynamic evolution of self-driven secondary phase separation. Migration, coalescence, coagulation, and Ostwald ripening were identified as the main mechanisms shaping these morphologies. Direct experimental evidence confirmed asynchronous crystallization of Fe-rich spherulites. Phase-field simulations further clarified the evolution of Cu-rich domains during the separation. The alloys also displayed soft ferromagnetic behaviour containing low coercivity and high saturation magnetisation. These findings demonstrate a pathway for tailoring microstructures in immiscible Cu–Fe alloys to achieve controlled properties and functional performance.