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 epsilon(Cu3Sn) phase, an intermediate layer of eta(Cu6Sn5) phase and an outer layer of beta(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 epsilon(Cu3Sn) phase grows by non-faceted mode into dendrites as droplet diameter decreases. Especially, solidification path alters in the smallest droplet with 50 mu m diameter, in which eta(Cu6Sn5) and Sn3Sb2 phases form directly from the metastable liquid phase by suppressing the primary epsilon phase formation and the following peri-eutectic transformation.