Evolution of hierarchical microstructures in Cu–Fe immiscible alloy driven by liquid-state mixing

Copper–iron (Cu–Fe) immiscible alloys are known for their potential to form hierarchical microstructures with superior mechanical properties under rapid solidification conditions. However, the formation of these microstructures during Cu/Fe melting and mixing—typically occurring in processes such as arc- and laser-induced melting‒remains poorly understood, despite its relevance to the integration of structural materials across various industries. This study showed that hierarchical and homogeneous microstructures in Cu–Fe alloys can be tailored in situ with two distinct regimes governed by the degree of Fe dilution through non-equilibrium solidification. In the high-Fe content sample, phase separation during the liquid state, followed by Marangoni-driven motion, led to the formation of a hierarchical structure comprising DO3-ordered Fe-rich particles with embedded Cu-rich grains, along with uniformly distributed L12 nanoparticles. In contrast, the low-Fe sample exhibited more uniformly dispersed, smaller DO3-ordered Fe-rich particles with a lower number density, along with dispersed L12 nanoprecipitates. The formation of such microstructures, including Cu/DO3 Fe-rich particles and L12 nanoprecipitates, was primarily governed by surface energy–driven mechanisms and solute trapping under rapid cooling. These microstructures enhanced the local hardness and elastic modulus, primarily due to the increased number density of Fe-rich particles, highlighting their dominant role over size or morphology in strengthening Cu–Fe alloys. This study provides new insights into the microstructural evolution of immiscible alloy systems. The findings offer a foundation for microstructural tailoring to enhance mechanical performance and expand the potential applications of Cu–Fe alloy systems in advanced engineering technologies.