Effects of liquid separation on the microstructure formation and hardness behavior of undercooled Cu-Co alloy

Applying a melt-fluxing method, a substantial degree of undercooling up to 220 K has been obtained for Cu50Co50 immiscible alloy and the non-equilibrium solidification microstructure is investigated considering the successive processes of undercooling, liquid separation, non-equilibrium and near-equilibrium solidification. For samples undercooled by 62, 79 and 105 K, grain refinement occurs and becomes more serious without the occurrence of liquid separation and peritectic transformation. In contrast, metastable liquid demixing occurs upon the undercooling reaching 125 K, and it proceeds completely with the further increase of undercooling to 188 and 220 K, which can be ascribed to the sufficient time through the immiscible region. After liquid separation, versatile solidification microstructure for Co-rich and Cu-rich regions can be observed due to the different non-equilibrium effect. Moreover, second liquid separation becomes more serious for higher undercooling because of the enhanced immiscible effect. Finally, the solute content of primary Co-rich phase was analyzed with the consideration of solute trapping and liquid immiscible effects, which gives an excellent explanation for the variation of hardness behavior.