Rapid solidification mechanism of highly undercooled ternary Cu₄₀Sn₄₅Sb₁₅ alloy

The rapid solidification of ternary Cu₄₀Sn₄₅Sb₁₅ peri-eutectic type alloy was realized by glass fluxing and drop tube methods, and the corresponding maximum undercoolings are 185 K (0.22T_L) and 321 K (0.39T_L), respectively. The phase constitution of Cu₄₀Sn₄₅Sb₁₅ alloy in these two rapid solidification experiments deviates from the two equilibrium phases (Sn + Cu₆Sn₅). In glass fluxing method, the structural morphology of Cu₄₀Sn₄₅Sb₁₅ alloy is mainly characterized by a three-layer lamellar structure, which is comprised by an inner layer of long strips of primary ε(Cu₃Sn) phase, an intermediate layer of η(Cu₆Sn₅) phase and an outer layer of β(SnSb) phase. As undercooling rises, this lamellar structure is remarkably refined. When small alloy droplets are containerlessly solidified during free fall in drop tube, the primary ε(Cu₃Sn) phase grows by non-faceted mode into dendrites as droplet diameter decreases. Especially, solidification path alters in the smallest droplet with 50 μm diameter, in which η(Cu₆Sn₅) and Sn₃Sb₂ phases form directly from the metastable liquid phase by suppressing the primary ε phase formation and the following peri-eutectic transformation.