Microstructural, mechanical and thermodynamic study of the as-cast Zn–Al–Sr alloys at high Sr content

Abstract

A series of four Zn–Al–Sr casting alloys was performed in order to detail their microstructural evolution, thermodynamic transitions, and mechanical properties. The chemical composition of the studied alloys were Zn–1Al-0.05Sr, Zn-2.5Al-0.1Sr, Zn-4.5Al-0.5Sr, and Zn–10Al-1.5Sr (wt%). The microstructural evolution analysis through X-ray diffraction and scanning electron microscopy shows that ternary alloys are mainly constituted by three phases, the η-Zn, α-Al, and SrZn13. Also, it was observed that both particle size and distribution of the SrZn13 phase increase by the addition of Sr. Thermodynamic calculations reveal that this intermetallic phase could find the required conditions to precipitate and grow during the casting process of the alloys, which would explain the large particle size. By differential scanning calorimetry and thermodynamic calculations, we demonstrate that Sr slightly shifts the thermodynamic equilibrium of phase transitions in the Zn–Al system. This result is mainly attributed to the depletion of zinc atoms into the melt promoted by the precipitation of the SrZn13 phase. Additionally, the Sr improves the yield strength, ultimate tensile strength, and hardness of the Zn–Al alloys, but decreases their elongation. Nevertheless, the ductility can be enhanced by increasing the Al content, which counters the brittleness provided by the SrZn13 phase. Finally, the ternary Zn–Al–Sr alloy with 10 wt% Al and 1.5 wt% Sr exhibits better overall performance including yield strength and ultimate tensile strength, revealing the optimization of the microstructural properties.