Microstructure and properties of a bismuth-indium-tin eutectic alloy

Abstract

A ternary eutectic alloy with a composition of 57.2%Bi, 24.8%In and 18%Sn was continuously cast into wire of 2 mm diameter with casting speeds of 14 and 79 mm min−1 using the Ohno continuous casting process. The microstructures and mechanical properties of the wires were compared with those of statically cast specimens. Extensive segregation of massive bismuth crystals, bismuth complex structures, and tin rich dendrites was found in specimens which were statically cast. The bismuth complex-regular structures, which are a ternary eutectic constituent, existed along the boundaries of the BiIn dendrite cells forming a double binary eutectic. In the continuously cast wires, primary tin dendrites coupled with a fine bismuth phase were uniformly distributed within the Bi-In alloy matrix. With this novel, net-shape, casting process, the formation of massive bismuth crystals, bismuth complex-regular structures and BiIn eutectic dendrite cells was prevented, resulting in a more uniform microstructure which was in contrast to the heavily segregated structures of the statically cast specimens. These differences in structure significantly affected the mechanical properties. The continuously cast wires exhibited considerable ductility in contrast with the statically cast specimens which had lower toughness and exhibited cleavage fracture with little or no elongation at higher strain rates.