Effects of Mg on Microstructure and Solidification of a Hypereutectic Zn–8 wt.%Al Alloy

Hypereutectic ZnAl alloys are used as coatings and self‐lubricating components because of their wear and corrosion resistance. Although ZnAl and ZnAlCu alloys have been extensively studied, the influence of Mg additions on macrosegregation and grain structure evolution during transient directional solidification remains poorly understood. This study investigates the effects of Mg on the thermal parameters, macrostructure, microstructure, and macrosegregation of directionally solidified hypereutectic Zn–8 wt.%Al alloys. Optical microscopy, SEM/EDS, X‐ray fluorescence, X‐ray diffraction, and thermodynamic calculations using the CALPHAD method were employed. The additions of Mg reduced the cooling rate (ṪE) and growth velocity (VE) from 60.0 °C/s and 2.5 mm/s (Zn–8 wt.%Al) to 15.5 °C/s and 1.1 mm/s (Zn–8 wt.%Al–0.5 wt.%Mg) and to 12.7 °C/s and 1.0 mm/s (Zn–8 wt.%Al–2 wt.%Mg). Furthermore, Mg promoted fully equiaxed grain structures throughout the castings, demonstrating its influence on grain nucleation under transient solidification conditions. The microstructure consisted of a Zn‐rich phase, lamellar Al + Zn dendrites, and the ternary eutectic constituent (Al + Zn + MgZn ₂ /Mg ₂ Zn ₁₁ ). Finally, Mg altered the macrosegregation behavior from an inverse profile (Zn–8 wt.%Al–0.5 wt.%Mg) to a normal profile (Zn–8 wt.%Al–2 wt.%Mg), providing new insights into the role of Mg in controlling solidification microstructures and solute redistribution in hypereutectic ZnAl alloys.