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Magnesium-zinc (Mg-Zn) binary alloys have seen limited practical application due to their coarse microstructure and susceptibility to microporosity. However, these alloys offer a significant advantage: they can be strengthened through age hardening, making them promising for high-performance applications. To further develop Mg-Zn alloys, researchers are exploring the addition of third elements that can refine grain structure and reduce the tendency for microporosity.
One such element is zirconium (Zr), which has proven effective in refining grains in as-cast Mg-Zn alloys. It is widely used in industrial Mg-Zn alloys to improve their mechanical properties. These alloys typically undergo direct aging or solution treatment followed by aging, resulting in high yield strength and good load-bearing capacity.
Compared to Mg-Al-Zn alloys, Mg-Al-Zr alloys exhibit higher yield strength, better density, and a smaller wall thickness effect. Common alloys in this series include ZM1, ZM2, and ZM7. While ZM1 and ZM2 show good fluidity, they tend to form more porosity. The casting properties of ZM2 have been significantly improved with the addition of rare earth elements, particularly when zinc content is low and rare earth content is high. ZM1 alloys are commonly used in aircraft wheel castings and other simple-shaped structural components.
The effectiveness of grain refinement in Mg-Zn alloys is closely tied to the amount of dissolved alloying elements. This directly impacts the mechanical performance of the material. Therefore, precise smelting techniques and temperature control are crucial. ZM2 alloys are used in critical parts like front bearing housings and covers of worm gear jet engines, as well as aircraft motor and hydraulic pump casings.
The composition of ZM1 alloys plays a key role in determining their mechanical properties. Alloys with lower zinc and higher rare earth content offer better casting and welding properties but lower tensile strength. Conversely, higher zinc and lower rare earth content result in better tensile properties, though with some trade-offs in casting behavior.
Adding copper (Cu) to Mg-Zn alloys enhances toughness and age-hardening potential. The sand-cast alloy ZC63 (w(Zn)=6%, w(Cu)=3%, w(Al)=0.5%) is a representative example, showing tensile strength of 240 MPa, yield strength of 145 MPa, and elongation of 5% after aging—outperforming AZ91. Cu increases the eutectic temperature, allowing higher solution treatment temperatures and enhancing subsequent aging effects. It also modifies the eutectic structure, changing the morphology of the Schröter phase from irregular blocks to plate-like structures. However, Cu reduces corrosion resistance.
Adding rare earth elements to Mg-Zn alloys improves heat resistance, enabling operation at up to 150°C. These alloys are used in helicopter gearbox cases. Increasing Zn content leads to the formation of Mg-Zn-RE phases at grain boundaries, increasing brittleness and causing local melting during solution treatment. Some studies suggest these phases can be dissolved using nitrogen after prolonged heating, and they are successfully applied in thin-walled magnesium parts like ZE63.
Silver (Ag) has been found to enhance the age-hardening effect in Mg-RE alloys, leading to the development of QE22, QE21, and EQ21. These alloys show excellent high-temperature tensile and creep properties, especially between room temperature and 473K. Silver is one of the most effective elements for improving high-temperature performance in magnesium alloys, though thorium (Th)-based alloys face limitations due to radioactivity.
The addition of elements like selenium (Se) and yttrium (Y) has led to important advancements in magnesium alloys. Drits developed the WE-type heat-resistant, high-strength magnesium alloys, where Y can be added via Y-containing mixed rare earths. With 75% Y and the rest heavy rare earths, these alloys exhibit excellent mechanical properties and are widely used in racing and aerospace gearbox housings.