Long-Term Microstructural Stability of Sn-40Bi and Sn-40Bi-10In Alloys

With the emergence of new applications for low-melting-point solder alloys, such as self-healing and semisolid additive manufacturing, understanding their long-term microstructural stability has become necessary. Sn-Bi-In alloys provide an attractive option for such applications, with a melting point below 150°C; however, little is known about their long-term stability and microstructural evolution. In this research, we conducted extended stability experiments for Sn-40Bi and Sn-40Bi-10In alloy compositions over three different heat-treatment periods of 12 h, 168 h, and 360 h. The microstructure of the two alloys prominently featured Sn-rich globules, which were observed to be finer in the Sn-40Bi-10In alloy than in the Sn-40Bi composition. The finer globules in Sn-40Bi-10In may be attributed to initial Sn-rich islands, which facilitated improved globule refinement during the semisolid stage. Furthermore, both the size and fraction of the globules were found to increase with increasing heat treatment time in Sn-Bi-In alloy; it was also observed that the Sn-40Bi-10In alloy globules exhibited a gradual rise in overall Bi content with time, whereas the interglobular Sn/Bi/BiIn eutectic regions showed a decrease in Bi content and an increase in Sn content.