Microstructure and Mechanical Properties of As-cast Sn-Bi Eutectic Solder Alloys
The microstructure and mechanical properties of Sn-Bi eutectic alloys with four different Bi compositions(45％,50％,55％and 60％)were studied under free solidification.The crystal growth mechanism of Sn-Bi eutectic alloys and the effect of Bi content on the microstructure of the alloy were investigated.The results showed that the microstructure of as-cast Sn-Bi eutectic alloys consists of primary solid solution phase of Sn and clusters of Sn/Bi eutectic lamellae.With increasing Bi content,the quantity of primary Sn phase with dendritic growth morphology decreased,while its size increased.A large amount of rod-like or dot-like Bi phase precipitated from the solid solution phase of primary Sn,forming a rod-like eutectic structure.The as-cast microstructure of Sn-Bi hypereutectic alloys consists of primary Bi phase and clusters of Sn/Bi eutectic lamellae.The primary Bi phase does not dissolve Sn elements,and it grows in a small plate-like manner,surrounded by a layer of Sn halo phase.At room temperature,the hardness and strength of Sn-Bi eutectic alloys increase with increasing Bi content,while the elongation first increases and then decreases with increasing Bi content.
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