Vacuum melting and equilibrium microstructure of Mg-La alloy with tantalum foil coating

Magnesium alloy is one of the most exploitive and promising light metal materials in metal materials, and the research of adding rare earth elements to magnesium alloy has attracted increasing attention in recent years. La element is one of the representative rare earth metals of lanthanide series. The study of Mg-La alloys is of great significance for understanding the effect of rare earth elements on magnesium alloys. However, Mg has an electron arrangement order of 1S²2 S²2P⁶3S². It is easy to lose the two outermost electrons. Mg is a particularly active metal, which is easy to be oxidized. The α value of the oxidation of Mg (the ratio of the metal oxide to the metal volume consumed to form the oxide) is 0.78, so the oxide film of Mg is not dense and easy to be further oxidized. Mg alloy with La as alloying element has good effect of solution strengthening and aging hardening at high temperature and room temperature, and has higher high temperature strength and excellent creep property. The high temperature mechanical properties and corrosion resistance of magnesium alloy can be improved by adding La element. The element La is also highly chemically active, tarnishing and blackening in the air in a very short time. The metal Mg and metal La are easy to oxidize in the air, and tend to react with many crucible made of refractory materials during smelting, which makes the melting of Mg-La alloy more difficult than the general magnesium alloy. In this study, tantalum foil cladding method was used to prepare Mg-La alloy under vacuum, in order to avoid the oxidation of Mg and La in the melting process, and to avoid the reaction between Mg and quartz tube. In addition, tantalum foil will not pollute the alloy, which can be confirmed by the subsequent radiographic analysis results.

The phase relation under equilibrium condition is very valuable for the design of alloy composition and its treatment. In this experiment, rare earth magnesium alloy with La as the added element was prepared to study the equilibrium phase of Mg-La alloy at 550℃.

Conclusion :(1) The method of tantalum foil cladding melting under vacuum can not only avoid the oxidation of alloy components, but also ensure that the alloy is not contaminated by refractory materials.

(2) The equilibrium microstructure of the Mg-La alloy prepared at 550 ℃ consists of two phases, one of which is the intermetallic compound Mg₁₇La₂ with more volume fraction, and the other is the intermetallic compound Mg₁₇La₂ formed by the cell crystal reaction between the liquid phase L and Mg₁₇La₂.