Ductility Enhancement in Sn-40Bi-X Alloys by Minor Additions of Alloying Elements

To improve the low ductility and high strain-rate sensitivity in Sn-Bi based solder alloys, the influences of the minor additions of alloying elements (Ag, Mn, In) were investigated. The strain-stress curves of various Sn-40Bi(-X) alloys, including a pre-suggested Sn-40Bi-0.1Cu composition were measured using a tensile testing machine. As a result, the elongation and ultimate tensile strength (UTS) values were compared. The small addition (0.5 wt.%) of Ag significantly enhanced the ductility and high strain-rate sensitivity of the alloys at strain rates of 10⁻4 to 10⁻2 s⁻1 mainly due to the increase and refinement of eutectic lamellar structures. The microstructure change increased the area of grain boundaries, thus ameliorating the grain boundary sliding mode. It was also found that Mn is an effective element in enhancing the ductility, especially at the strain rates of 10⁻3 to 10⁻2 s⁻1. The enhancement is likely attributed to the fine and homogeneous microstructure in the alloys containing Mn.