Efeito de microadições de Zn na microestrutura, citotoxicidade e propriedades mecânicas de ligas eutéticas Sn-Cu e Sn-Ni

Abstract

The development of Pb-free solder alloys is an urgent demand for the electronics industry in order to comply with applicable laws and environmental guidelines and provide products with optimized mechanical and physical properties. Among the alternative lead-free solders, can be highlighted alloys of the Sn-Cu and Sn-Ni systems, with properties superior to the alloys of the Sn-Pb system, such as mechanical and corrosion resistances, in addition to the low cost. However, such alloys still exhibit relatively high melting point and insufficient oxidation resistance. One way to optimize its properties and microstructure is through the addition of alloying elements, such as zinc (Zn). Zn, which is also a low-cost element, and is used in Pb-free solder alloys to minimize intermetallic compounds growth in soldered joints, to refine the microstructure and to increase mechanical strength. Therefore, this study aims to understand the effects of Zn additions (0.2 and 0.5 wt.%) on thermal parameters (growth rate-V and cooling rate-Ṫ), microstructure, phases transformation, macrosegregation, mechanical properties (ultimate tensile strength-σu and yield tensile strength-δ) and cytotoxicity in the eutectic Sn-0.7wt.%Cu and Sn0.2wt.%Ni alloys directionally solidified under transient conditions against electrolytic copper substrate. Zn additions caused small alterations in the phase transformation temperatures of the Sn-Cu-Zn alloys, however significant for Sn-Ni-Zn alloys. Macrostructures with columnar-equiaxial transition (CET) and fully columnar were observed for the Sn-Cu-Zn and Sn-Ni-Zn alloys, respectively. The microstructures of the Sn-Cu-Zn alloys are predominantly dendritic with a tin-rich matrix (β-Sn phase) surrounded by a eutectic mixture composed of the β-Sn+Cu6Sn5+CuZn phases. In the final positions of the Sn-Cu-Zn castings, eutectic cells of the low growth rate have been observed. For the Sn-Ni-Zn alloys, the microstructure is completely dendritic, composed of a tin-rich matrix (β-Sn phase) surrounded by a eutectic mixture Ni3Sn4+ NiSn + β-Sn, in addition to a probable formation of the intermetallic (Cu,Ni)6Sn5 due to the dissolution of the Cu substrate. Zn additions refined the dendritic arrangement of the Sn-Cu-Zn alloys when compared to the Sn-0.7wt.%Cu alloy. However, increasing the Zn content did not affect the microstructural scale. On the other hand, both Zn additions did not change the scale of the dendritic arrangement, compared to the Sn0.2wt.%Ni alloy. Zn did not influence the values of σu and δ in the Sn-Cu-Zn alloys, with columnar growth. With the reduction of primary dendritic spacing (ʎ1), σu increased for both Sn-Cu-Zn and Sn-Ni-Zn systems, however, the behavior of δ was opposite. The Sn-0.2wt.%Ni-0.5wt.%Zn alloy exhibited higher σu values, which are associated with the solid solution hardening mechanism. A ductile fracture mode has been observed for all alloys examined. Cytotoxicity analyzes showed that the microstructural scale does not influence the toxicity of the examined alloys, but factors such as incubation time and chemical composition do. In general, Zn improved the cell viability of the eutectic Sn-Cu and Sn-Ni alloys, but still with moderate cytotoxic levels.