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Mg-RE-Zr alloys, such as ZM3, ZM4, and ZM6, are known for their enhanced properties due to the addition of rare earth elements. These elements help reduce magnesium's oxidation tendency during both liquid and solid states. In Mg-RE binary systems like Mg-Ce, Mg-Nd, and Mg-La, the phase diagrams in the magnesium-rich region show similar eutectic reactions, resulting in low-melting-point eutectic structures that typically form at grain boundaries. These eutectic networks are believed to play a role in suppressing micro-shrinkage, improving the material’s overall integrity.
These alloys exhibit excellent casting characteristics, with low porosity and reduced sensitivity to wall thickness. They can be repaired using nitrogen arc welding, showing good weldability and machining performance similar to ZM5. ZM6, for instance, is widely used in aerospace applications, including helicopter rear engine mounts, aircraft wing ribs, and hydraulic constant-speed device mounts. It is also employed in rotor lead platens for large turbine generators. ZM3 finds use in compressor casings and centrifuge cartridges, while ZM4 is commonly used in hydraulic constant-speed device housings. Due to its high damping capacity, ZM4 is suitable for instrument chassis and housing in radio engineering to minimize vibration effects.
In Mg-RE alloys, the addition of Zn often enhances strength, while Zr refines the grain structure, improves purification during smelting, and boosts corrosion resistance. For example, alloy EZ33 (with 3% RE, 2.5% Zn, and 0.6% Zr) offers high strength and creep resistance, suitable for temperatures up to 250°C. Manganese is sometimes added to further improve solid-solution strengthening, reduce atomic diffusion, and enhance heat and corrosion resistance.
Yttrium (Y) is another significant rare earth element in magnesium alloys, with a maximum solubility of 12.5% in Mg. Its solubility varies with temperature, indicating a strong age-hardening potential. Mg-Y-Nd-Zr alloys demonstrate superior room temperature strength and high-temperature creep resistance compared to other magnesium alloys, capable of operating up to 300°C. After heat treatment, their corrosion resistance surpasses all other magnesium alloys. However, pure Y is challenging to use due to its high cost and melting point, along with its strong affinity for oxygen.
Mg-Th alloys also offer exceptional creep resistance, with forgings and castings usable up to 350°C. Like rare earths, thorium improves casting and welding properties. The simplest Mg-Th-Zr ternary alloy, such as HK31 (3% Th, 0.7% Zr), has a microstructure similar to Mg-RE-Zr alloys. Proper heat treatment leads to continuous precipitation of Mg-Th compounds, enhancing mechanical properties at room temperature. Discontinuous Mg-Th dispersoids on grain boundaries inhibit grain boundary rotation at high temperatures, improving creep resistance. Adding Zn to Mg-Th-Zr further increases creep resistance through acicular phase formation on grain boundaries. Although Mg-Th alloys were once used in missiles and aircraft, they are now rarely used due to thorium’s radioactivity and associated health risks.