Microstructure and Deformability of Sn-Zn-Bi Alloys

With addition of Bi, wettability of Sn-Zn alloys can be obviously improved. As a low melting point alloy, it is expected that Sn-Zn-Bi alloy come to being one of commercial solders. But, there is an unfavorable effect on plasticity of Sn-Zn-Bi alloys by adding Bi, so Bi concentration is usually required below 5mass% to maintain the alloys enough elongation. This alloying method can not match with the demand for satisfactory wettability. For example, wettability of typical Sn-8Zn-3Bi alloy is not good enough for air condition. This solder usually needs nitrogen atmosphere help it wet on substrate. Embrittlement of Sn-Zn-Bi alloy must be solved before it can be deformed into solder wire. In this paper, effect of microstructure evolution on elongation of Sn-Zn-Bi alloys has been investigated. Adding Bi into Sn-9Zn, primary Zn phase dispersed in eutectic matrix. With addition of Bi, quantity and size of the coarse plate Zn phase in those alloys have increased, and then elongation of the alloys has decreased. Adding 4mass% Bi into Sn-9Zn, elongation of the alloy is below 8%. Fracture analysis shows that intergranular crack is liable to occur along with the interface between primary Zn and matrix. The weak cohesion between primary Zn and matrix cause decrease of elongation. To restrain primary Zn phase from forming, Zn concentration of Sn-Zn-Bi alloys has been deduce. Results display that Sn-Zn-Bi alloys without primary Zn phase obtain higher deformability. Elongation of Sn-6Zn-6Bi alloy is above 8%