Pseudobinary eutectics in Cu–Ag–Ge alloy droplets under containerless condition

Abstract

Pseudobinary eutectic generated by pseudobinary eutectic transition or peri-eutectic transition is a crucial structure in ternary alloy systems. Its formation mechanism strongly influences mechanical properties of these metallic materials. However, it was customarily neglected. In this paper, two pseudobinary eutectics, i.e. (Ag+ζ) and (Ag+ε2), were investigated during the rapid solidification of Cu–Ag–Ge ternary alloy in a 3m-drop tube. The sharp temperature variations and dramatic kinetic activities of the falling alloy droplets before solidification cause special microstructural characteristics. (Ag) dendrite is the heterogeneous nucleus for anomalous (Ag+ζ) pseudobinary eutectic in large droplets. Lamellar (Ag+ζ) pseudobinary eutectic grain forms independently on condition that primary (Ag) dendrite cannot form and its eutectic morphology becomes anomalous with the decrease of droplet size. Nanoscaled (Ag+ε2) pseudobinary eutectic generating at the last stage of solidification is the product of both peri-eutectic and pseudobinary eutectic transitions. It distributes in the gaps of (Ag+ζ) pseudobinary eutectic grains and its morphology remains lamellar regardless of droplet size.