Macrosegregation pattern and microstructure feature of ternary Fe–Sn–Si immiscible alloy solidified under free fall condition

Ternary Fe37Sn32Si31 immiscible alloy was rapidly solidified in the form of droplets with 97–978 μm diameter inside a drop tube. In addition to the two major phases, FeSi compound and (Sn) solid solution, nonequilibrium solidification yields three minor phases: Fe5Si3, α1Fe and Fe3Sn2. The two- or three-layer core–shell structure represents the typical macrosegregation pattern under reduced gravity condition. If the droplet diameter is sufficiently small, the evolution of macrosegregation is suppressed so as to form a homogeneously dispersed microstructure. The dendritic growth of primary FeSi compound dominates the droplet solidification process, which is characterized by a remarkable grain refinement effect with the decrease in droplet diameter. Moreover, FeSi compound is found to entrap as much as 1.14 at.% Sn owing to the combined influences of rapid cooling and substantial undercooling. Temperature field analyses indicate that there exists a great temperature gradient difference from droplet center to its surface, which initiates phase separation preferentially at droplet surface and stimulates thermal Marangoni convection. The concentration gradient induced by liquid phase separation brings about even stronger solutal Marangoni migration for Sn-rich globules. Phase field simulation reveals that surface segregation and Marangoni migration are the dynamic mechanisms responsible for the development of the core–shell structure.