Effect of composition and cooling rate on microstructure and tensile properties of Sn-Zn-Bi alloys

The effects of the addition of Bi to Sn-Zn near eutectic alloy and cooling rate on the thermal and mechanical properties were investigated. With the addition of up to 8 mass% Bi to Sn-8 mass% Zn, the melting temperature decreases from 198.4 to 186.1 degreesC, but the pasty range increases from 2.36 to 6.72 degreesC. Sn-9 mass% Zn and Sn-8 mass% Zn-(1,2,3,6,8) mass% Bi alloys have far lower undercooling than the other Sn-based lead-free alloys, and the temperature decreases with Bi content. It was also confirmed that the difference of cooling rates significantly affects the microstructure and mechanical properties. In the case of fast cooling, alloys have fine Sn, Zn, and Bi phases with smooth surfaces. But in the case of slow cooling, Sn-Zn alloys exhibit coarse dendrite structure and large needle or rod-like Zn-rich precipitates with rough surface and voids, and extensive segregation of massive Bi toward a surface. (C) 2002 Elsevier Science B.V. All rights reserved.