Study of the influence of copper and magnesium additions on the microstructure formation of Zn–Al hypoeutectic alloys

In this paper the influence of copper and magnesium content in the microstructural evolution during the solidification of Zn–4 wt.% Al hypoeutectic alloys was investigated using the CA-CCA method (Computer-Aided Cooling Curve Analysis) and SEM (Scanning Electron Microscopy). The identification of chemical composition of the phases and microconstituents was done by SEM using the EDS (Energy Dispersive X-Ray Spectroscopy) operation mode. For that purpose, ternary and quaternary alloys were prepared with different amounts of copper and magnesium. The influence of both copper and magnesium amounts on the transformation temperatures of the Zn–Al based hypoeutectic alloys was evident in the distinct microstructures formed during solidification as well as in the cooling curves obtained by thermal analyses, promoting modifications in solid–liquid temperature range, in the kinetic and also in the chemical compositions of the phase transformation. The addition of extra copper promoted the formation of significant quantities of the copper-rich phase (CuZn4 precipitate) in the interdendritic region, while the addition of extra magnesium promoted the formation of the magnesium-rich phase and changed not only the morphology of the primary dendrites but also its relative content. Besides, an increase in the relative primary eutectic structure and a decrease in the quantity of the lamellae eutectoid structure were observed. Additionally, the secondary lamellar eutectic became more refined in the presence of higher magnesium content. All the cooling curves are in agreement with the observed microstructure. Both elements, copper and magnesium, also promoted an increase in the hardness of the Zn–4Al hypoeutectic alloys due to the formation of CuZn4 phase and to the secondary lamellae eutectic refinement, respectively.