Primary dendrite growth of Co₃Sn₂ intermetallic compound in rapidly solidified ternary Co₃₅Cu₃₅Sn₃₀ alloy

The rapid solidification of ternary Co₃₅Cu₃₅Sn₃₀ alloy droplets with 20–785 µm diameter was realized by drop tube technique, aiming to investigate the dendrite growth mechanism of primary Co₃Sn₂ intermetallic compound at high cooling rate. As alloy droplet diameter decreases, the surface segregation occurs prior to the solidification process with an increased probability, and the segregation shell becomes more apparent. The dendrite growth of primary Co₃Sn₂ compound dominates the droplet solidification process, which evolves from equiaxed dendrite to spherical grain and is accompanied by remarkable grain refinement with the decrease of droplet diameter. Meanwhile, a peculiar zone near the droplet surface in which the primary Co₃Sn₂ compound grows directionally is observed. This directional structure becomes more evident as droplet diameter decreases, which is attributed to the temperature gradient along the radial direction inside droplet. In each alloy droplet, owing to the increased cooling rate from the center to the surface, the grain size decreases accordingly.