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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 (RE) and zirconium (Zr). Rare earth elements play a crucial role in reducing magnesium's oxidation tendency during both liquid and solid states. In Mg-RE systems like Mg-Ce, Mg-Nd, and Mg-La, the binary phase diagrams exhibit similar characteristics in magnesium-rich regions, typically featuring simple eutectic reactions. These eutectic structures often reside at grain boundaries, forming low-melting-point networks.
These eutectic phases, arranged in a network-like structure along grain boundaries, are believed to help suppress micro-shrinkage during solidification. The casting properties of Mg-RE-Zr alloys are generally excellent, with low porosity and minimal sensitivity to wall thickness. ZM3, ZM4, and ZM6 can all be repaired using nitrogen arc welding, demonstrating good weldability and machining performance comparable to ZM5. ZM6, in particular, is widely used in aerospace applications, including helicopter rear engine mounts, aircraft wing ribs, hydraulic constant-speed device mounts, and rotor lead platens for large turbine generators. It is commonly employed in critical structural components that require high strength and durability.
ZM3 has found use in engine compressor casings and centrifuge cartridges, while ZM4 is often applied in the housing of hydraulic constant-speed devices. Due to its high damping capacity, ZM4 is also utilized in radio engineering for instrument chassis and housings, helping to reduce harmful vibrations. In many Mg-RE alloys, zinc (Zn) is added to enhance strength, while zirconium (Zr) is used to refine grain structure, improve purification during smelting, and boost corrosion resistance. For instance, the alloy EZ33 (with 3% RE, 2.5% Zn, and 0.6% Zr) exhibits both high strength and creep resistance, suitable for use up to 250°C.
Manganese (Mn) is sometimes incorporated into Mg-RE alloys to provide solid-solution strengthening, reduce atomic diffusion, and improve heat and corrosion resistance. Another key rare earth element is yttrium (Y), which has a maximum solubility of 12.5% in magnesium. Its solubility varies with temperature, indicating a strong tendency for age hardening. Mg-Y-Nd-Zr alloys show significantly higher room temperature strength and high-temperature creep resistance compared to other magnesium alloys, making them suitable for temperatures up to 300°C. Additionally, these alloys demonstrate superior corrosion resistance after heat treatment.
Pure yttrium, however, presents challenges in practical application due to its high cost and high melting point (around 1500°C), as well as its strong affinity for oxygen. Magnesium-thorium (Mg-Th) alloys also offer excellent creep resistance, with forging and casting applications capable of withstanding temperatures up to 350°C. Like rare earths, thorium improves casting and welding properties. Simple Mg-Th-Zr ternary alloys, such as HK31 (3% Th, 0.7% Zr), have microstructures similar to Mg-RE-Zr alloys. Proper heat treatment allows for continuous precipitation of Mg-Th compounds within grains, enhancing mechanical properties at room temperature, while discontinuous dispersoids on grain boundaries help inhibit grain boundary rotation at high temperatures, improving creep resistance.
The addition of zinc to Mg-Th-Zr further increases creep resistance through the formation of acicular phases on grain boundaries. Despite these advantages, Mg-Th alloys are rarely used today due to the radioactive nature of thorium, which poses health risks to humans.