2018 Volume 59 Issue 2 Pages 303-310
The s-orbital energy level (Mk) of alloying elements in a Bi cluster was used to determine the composition for alloys of this system for use as Pb-free high-temperature solders. Binary Bi-Cu and Bi-Ag alloys with ⊿Mk of 0.013–0.343 and ternary Bi-2.0Ag-0.5Cu and Bi-5.0Ag-0.5Cu alloys with ⊿Mk of 0.180 and 0.379, respectively, were fabricated and tensile tested at 423 K; here, ⊿Mk is the compositional average of Mk. The flow stress and fracture strain at 423 K increased after the alloying elements were added to the alloys. The relationships between the 0.2% proof stress, ultimate tensile strength or fracture strain, and ⊿Mk were similar to those determined previously through tests performed at 293 K. Thus, these relationships could be useful for predicting the stress and fractures strain levels based on ⊿Mk, regardless of the temperature and alloy composition. Moreover, a transition from ductility to brittleness was observed at 348–373 K for both ternary alloys. In addition, the melting points of the ternary alloys lay between 536 and 538 K, indicating that the alloys would be suitable as high-temperature solders. The contact angles of molten droplets of 10 of the experimental binary and ternary alloys on a Cu plate as determined at 973 K were 24–30°. This confirmed that the alloys exhibited good wettability with respect to Cu. Finally, the ternary Bi-2.0Ag-0.5Cu and Bi-5.0Ag-0.5Cu alloys showed thermal conductivities of 12.1 and 15.9 W/m/K, respectively, at 373 K; these were lower than that (30.4 W/m/K) of Pb-5Sn.