[CHINA ALUMINUM NETWORK] The rare earth elements are located in the third sub-group of the periodic table, with a large atomic radius, a unique 4f electronic structure, large electron magnetic moments, and strong spin-orbit coupling. Its chemical properties are very lively. Its chemical activity is second only to alkali metals and alkaline earth metals in metal elements, and it can form chemically stable oxides and sulfides with almost all non-metal elements (oxygen, sulfur, halogen elements, etc.). , halides, etc.; in the formation of rare earth complexes, the coordination number varies from 3 to 12, and has a diverse crystal structure. The unique physical and chemical properties determine the various roles of rare earth elements in aluminum alloys.
3.1 Metamorphism
Deterioration treatment refers to the process of changing the microstructure and properties of an alloy by changing the crystallization conditions of the alloy by adding a small amount or a trace amount of a modifier to the metal and the alloy. Studies have shown that rare earth has a good metamorphism in the alloy, mainly in the refinement of grains and dendrites. The atomic radius of rare earth elements is greater than the atomic radius of aluminum, and the properties are relatively lively. It melts in the aluminum liquid and can easily fill the surface defects of the alloy phase. This reduces the interfacial tension between the old and new phases, and increases the growth rate of the crystal nuclei. A surface active film is formed between the alloy and the alloy liquid to prevent the grown grains from growing and refine the alloy structure. Addition of rare earth can also reduce the spacing between columnar and secondary dendritic arms, improve grain morphology, and to a certain extent control the grain size of the material. Experiments have shown that rare earth metamorphism has a certain incubation period, and rare earths can play a large role in metamorphism only if they are kept at high temperatures for a certain period of time.
3.2 Purification and purification
Rare earth has a good purification effect on the aluminum alloy melt. First, rare earth elements such as RE2O3, RE2S3, RES, RES2, RE3S4, REH2, REH3, and REX3 (X is a halogen element) and other chemically stable compounds are easily formed with O, S, halogen elements, and the like, and are used at 250 to 300C. The N action produces a refractory REN. At high temperatures, rare earths react with C, Si, and B to produce REC2, RE2C3, REC, RE2C, RE3C, RE4C, RESi2, REB4, REB6, and the like. At the same time, rare earths have a particularly high hydrogen adsorption capacity and can adsorb and dissolve hydrogen in large amounts, and can remove hydrogen from aluminum alloys. The compounds of rare earths and hydrogen have higher melting points and are dispersed in aluminum liquids. These compounds are formed as compounds. The hydrogen does not form bubbles and greatly reduces the hydrogen content and pinhole rate of aluminum. Secondly, rare earths can be combined with Sn, Bi, Pb, and Zn, which are low melting point elements in aluminum alloys, to form binary or multicomponent compounds with high melting point and light density. When the metal smelting temperature is lower than their melting point, these compounds float on slags. In order to purify the aluminum liquid, their tiny particles become heterogeneous crystal nuclei in the aluminum crystallization process, thereby refining the crystal grains. Later, the addition of rare earth can improve the physical and chemical properties such as the surface tension, fluidity, and viscosity of the aluminum alloy melt and slag, which is beneficial to the spheroidization of non-metallic inclusions and promote their floating, so that non-metallic inclusions can be effectively removed.
3.3 Alloying
There are three main forms of rare earths in aluminum alloys: solid solution in the matrix α (Al); segregation in the phase boundary, grain boundary and dendritic boundary; solid solution in the compound or exist as a compound. Its existence form has a great relationship with the amount of addition. When the rare earth content is low (less than 0.1%), the rare earth is mainly distributed in the former two forms, and the reinforcement is promoted by the limited solid solution and increased deformation resistance, which promotes the proliferation of dislocations; When the content of rare earth is more than 0.3%, it exists mainly in the third existing form. The rare earth and other elements in the alloy form many new phases containing rare earth, distributed in the grain or grain boundary, and make the shape and size of the second phase. Changes have occurred (most of the rare earth-containing second phases have been characterized by particle formation, spheroidization, and refinement), and a large number of dislocations have appeared, reinforcing the aluminum alloy to some extent.
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