Effects of endogenous Al and Zn phases on mechanical properties of Sn58Bi eutectic alloy

Sn58-xBi-xAl-xZn were prepared by adding different addition (x = 0, 1.0, 2.0, and 3.0 wt%) of Al and Zn, and their response to melting point, microstructure, phase structure and mechanical properties were investigated. The presence of Sn-Bi-Zn eutectic significantly reduces the melting point of the alloy, but the heat fusion is increased with the increase of Zn and Al addition. The Al element is dispersed in the Sn–Bi eutectic matrix in the form of dendritic Al-rich phase, and the Zn element is dispersed in the form of needle-like Zn-rich phase. Besides, the addition of Al and Zn leads to the refinement of the eutectic structure and the reduction of the interlaminar spacing. The Al, Zn, and Bi can be dissolved in Sn to form β-Sn phase. The competition of Al, Zn, and Bi solubilizes in Sn results in the diffraction peak of the alloy to shift. The doping of Al and Zn in Sn causes the diffraction peak of the alloy to shift to the right, but when the proportion of Bi solubilizes in Sn increases, the diffraction peak of the alloy shifts to the left. The phase interface of Sn and Bi is the non-coherent interface. The mismatch degrees between (200) Sn and (102¯) Bi is 27.5%. The liquid metal induced embrittlement (LMIE) phenomenon can be observed between Al grain and Sn phase, which leads to brittle fracture of Al grains. With the increase of Al and Zn content, the mechanical properties are significantly improved. When the Al and Zn addition reaches 2 wt%, the ultimate tensile strength (UTS) of Sn56Bi2Al2Zn reaches 68.5 MPa, while the elongation reaches 49.04%, which is 22.76% and 46.34% higher than that of Sn58Bi. However, when the Al and Zn addition reaches 3 wt%, the Sn56Bi3Al3Zn possess the best UTS (70.5 MPa), but the plasticity is reduced to 26.80%. Moreover, the macroscopic hardness of Sn55Bi3Al3Zn is 25.3 HV, which is 22.2% higher than that of Sn58Bi (20.7 HV). Results show that the endogenous Al and Zn phases plays an important role in the mechanical properties of Sn58Bi.