Aluminum-Affecting Solidification Behavior, Microstructure, and Tensile Properties in Tin–Zinc Eutectic Alloy

The Sn-9 wt %Zn eutectic alloy is a promising material as a soldering alloy. It has a eutectic temperature (198 °C) close to the Sn-Pb eutectic alloy, low toxicity, low cost, and excellent mechanical properties. However, the Sn-Zn alloys exhibit disadvantages such as low corrosion resistance and low wettability, which limits their practical use in soldering operations. One of the improvement alternatives is modification with aluminum (Al). This element promotes changes in the microstructure, mechanical properties, and corrosion resistance. Therefore, the present work aims to study the effect of adding Al (0.5% and 2.0% in wt %) on the solidification thermal parameters (cooling rate-Ṫ_L and growth rate-V_L), microstructure, macrosegregation, tensile properties, and fracture modes of directionally solidified (DS) Sn-9 wt % Zn alloys. The samples have been characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray fluorescence (XRF), X-ray diffraction (XRD), and tensile tests. Al additions to the Sn-9 wt % Zn alloy increase the <i>liquidus</i> (T_L) and reduce the eutectic (T_E) temperatures, thus increasing the solidification interval. Eutectic cell growth has been observed for the Sn-Zn-Al alloys, with cells composed of β-Sn, α-Zn and α-Al phases. Only the Sn-9 wt % Zn-2 wt % Al alloy promotes a change in the microstructural scale, significantly refining the eutectic cellular arrangement. The morphologies of the α-Zn phase particles forming the eutectic constituent are shown to be dependent on the Al content as well as on the cooling rate. The Portevin-Le Chatelier (PLC) effect and the twinning deformation mechanism have been observed in the stress-strain curve features in Sn-Zn-Al alloys, being impacted by the Al content. The Al additions promote a reduction in both the ultimate tensile strength (σ_u) and the elongation-to-fracture (δ). In comparison to the Sn-9 wt % Zn binary alloy, Al has not changed the fracture mode, remaining ductile, in the Sn-Zn-Al alloys as well. The major contribution lies in the wide variety of samples solidified at significantly different cooling rates, along with the two additions of Al for mapping, which corroborates industrial scales of soldering conditions.