Liquid phase separation and rapid dendritic growth of highly undercooled ternary Fe_62.5Cu_27.5Sn₁₀ alloy

The phase separation and dendritic growth characteristics of undercooled liquid Fe_62.5Cu_27.5Sn₁₀ alloy have been investigated by glass fluxing and drop tube techniques. Three critical bulk undercoolings of microstructure evolution are experimentally determined as 7, 65, and 142K. Equilibrium peritectic solidification proceeds in the small undercooling regime below 7K. Metastable liquid phase separation takes place if bulk undercooling increases above 65K. Remarkable macroscopic phase separation is induced providing that bulk undercooling overtakes the third threshold of 142K. With the continuous increase of bulk undercooling, the solidified microstructure initially appears as well-branched dendrites, then displays microscale segregation morphology, and finally evolves into macrosegregation patterns. If alloy undercooling is smaller than 142K, the dendritic growth velocity of γFe phase varies with undercooling according to a power function relationship. Once bulk undercooling exceeds 142K, its dendritic growth velocity increases exponentially with undercooling, which reaches 30.4m/s at the maximum undercooling of 360K (0.21T_L). As a comparative study, the liquid phase separation of Fe_62.5Cu_27.5Sn₁₀ alloy droplets is also explored under the free fall condition. Theoretical calculations reveal that the thermal and solutal Marangoni migrations are the dynamic mechanisms responsible for the development of core-shell structure.