Role of Zn in the Microstructure, Segregation, and Cytotoxicity of Sn-0.2 Ni Solders

Sn-Ni system alloys are promising alternatives to replace Sn-Pb alloys as they exhibit high corrosion resistance and good weldability. However, Sn-Ni alloys still have low mechanical strength and low reliability. Using the strategy of the addition of alloying elements can be a way to improve the properties of Sn-Ni alloys. Zinc (Zn) plays an essential role in the lead-free solder alloys sector by mitigating the growth of intermetallic compounds in soldered joints, refining the microstructure, enhancing the mechanical strength, and ultimately reducing the overall cost. This study aims to explore the impacts of Zn additions (0.2 and 0.5 wt %) on thermal parameters (growth rate-V and cooling rate-Ṫ), macrostructure, microstructure, phase transformation, macrosegregation, and cytotoxicity. All of these factors will be examined in directionally solidified Sn-0.2 wt % Ni alloys under transient heat flow conditions on a copper sheet. The samples underwent characterization using optical microscopy, scanning electron microscopy, X-ray fluorescence, and X-ray diffraction. Property diagrams and isopleths were generated by using the CALPHAD method. Cytotoxicity analysis involved assessing cell viability after 15 and 30 days of incubation for the alloys, followed by exposure of the extracts for 24 and 48 h. The Zn additions caused a significant increase in the melting temperature in the Sn-Ni-Zn alloys. Fully columnar macrostructures were observed for the Sn-Ni-Zn alloys. The as-cast microstructures of Sn-Ni-Zn alloys were completely dendritic, with an Sn-rich matrix (Sn-β) surrounded by a Ni₃Sn₄ + NiSn + Sn-β phase eutectic mixture. Zn additions did not change the dendritic arrangement of the Sn-Ni-Zn alloys when compared to the Sn-0.2 wt % Ni. Furthermore, increasing the Zn content did not affect the microstructural scale in the ternary Sn-Ni-Zn alloys. The toxicity of the examined alloys is not significantly influenced by the microstructural length scale. On the other hand, factors such as incubation time and chemical composition may have an impact on the cytotoxicity. Overall, the presence of Zn in the Sn-Ni-Zn alloys enhanced the cell viability.