Magnesium and its alloys are highly reactive during the smelting process, making them prone to gas absorption and oxidation inclusions. These issues can significantly reduce the quality of the alloy. Additionally, their limited corrosion resistance and high-temperature creep performance further restrict their application in various industries. To address these challenges, rare earth elements have been introduced into magnesium alloys due to their unique electron configuration and chemical properties.
Rare earth elements exhibit a stronger affinity for hydrogen and oxygen compared to magnesium. Their oxides also have a higher density than magnesium oxide, making them effective in improving the properties of magnesium alloys. By adding appropriate amounts of rare earth elements during smelting, it is possible to remove hydrogen, reduce oxide inclusions, prevent combustion, refine the microstructure, and enhance both high-temperature performance and corrosion resistance.
This paper explores the mechanisms and applications of rare earth elements in magnesium alloys, focusing on their roles in purifying the melt, acting as flame retardants, refining microstructures, enhancing high-temperature performance, and improving corrosion resistance.
**1. Purification of Magnesium Alloy Melt by Rare Earth Elements**
**1.1 Hydrogen Removal**
Magnesium is chemically active and reacts with water vapor during smelting, leading to hydrogen evolution. Hydrogen has high solubility in molten magnesium, which can result in porosity, shrinkage, and other defects in castings. Rare earth elements can effectively remove hydrogen by forming high-melting-point hydrides and oxides that float to the surface as slag. The reaction between rare earth elements and hydrogen is thermodynamically favorable, with negative Gibbs free energy values indicating strong reactivity. This makes rare earth elements an effective means of reducing hydrogen content in magnesium alloys.
**1.2 Removal of Oxide Inclusions**
Magnesium readily reacts with oxygen to form MgO, resulting in oxide inclusions that degrade the mechanical and corrosion properties of the alloy. Rare earth elements, having a greater affinity for oxygen, can react with these inclusions to form stable rare earth oxides, thereby removing them from the melt. Studies have shown that adding rare earth elements can significantly reduce the number and size of inclusions, improving the overall quality of the alloy.
In addition to hydrogen and oxygen, rare earth elements can interact with sulfur, nitrogen, and halogens to remove non-metallic impurities. At high temperatures, they also react with carbon, silicon, and boron to form carbides, silicides, and borides. These interactions improve the physical and chemical properties of the melt and slag, promoting better inclusion removal and enhancing the overall performance of the alloy.
**2. Flame Retardant Effect of Rare Earth Elements**
During the melting and casting of magnesium alloys, the formation of MgO leads to oxidation and potential combustion. Rare earth elements, when added to the alloy, can increase the ignition temperature by forming a dense protective oxide layer composed of MgO, Al₂O₃, and RE₂O₃. This composite film prevents further oxidation and reduces the risk of combustion. Experimental results show that adding rare earth elements can significantly raise the light-off temperature of magnesium alloys, improving their thermal stability and safety.
**3. Effect of Rare Earth on Microstructure Refinement**
The addition of rare earth elements can refine the as-cast microstructure of magnesium alloys by forming needle-like or strip-like intermetallic compounds with aluminum. These compounds tend to accumulate at grain boundaries, hindering grain growth and resulting in finer grains. This refinement enhances the mechanical properties of the alloy, including strength and ductility. Moreover, rare earth elements can influence the solid solution and aging behavior of the alloy, affecting the formation of precipitates and delaying age hardening.
**4. Impact on Mechanical Properties**
Rare earth elements contribute to the improvement of mechanical properties through grain refinement, solid solution strengthening, and grain boundary reinforcement. Fine-grained structures increase deformation resistance, while stable rare earth compounds enhance high-temperature strength and creep resistance. These effects make rare earth-modified magnesium alloys suitable for high-temperature applications.
**5. Enhancement of Corrosion Resistance**
The addition of rare earth elements can alter the structure of the corrosion layer on magnesium alloys, forming a more protective oxide film. This film is less reactive to corrosive media such as NaCl solutions, reducing the rate of corrosion. Rare earth elements also modify the electrochemical behavior of the alloy, decreasing the corrosion current and increasing polarization resistance. These effects collectively improve the corrosion resistance of magnesium alloys, making them more durable in harsh environments.
**Conclusion**
China possesses abundant resources of both magnesium and rare earth elements, providing a unique advantage in developing advanced magnesium alloys. By leveraging the beneficial properties of rare earth elements, it is possible to further enhance the performance of magnesium alloys, particularly in terms of high-temperature resistance and strength. This will enable broader applications in industries such as automotive, aerospace, and electronics, meeting the growing demand for high-quality, high-performance materials.
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**Label:** Rare Earth Magnesium Alloy
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