Adding trace amounts of active elements to high-temperature alloys, such as rare earth elements, Ti, Zr, Hf, etc., can significantly reduce the oxidation rate and increase the adhesion between the oxide film and the matrix, thus significantly improving the oxidation resistance of high-temperature alloys. This effect is called the Reactive Element Effect (REE). Active elements generally refer to those elements whose oxides are more stable than the matrix oxides.
Mechanism of active element effect
The mechanisms by which oxygen-active elements improve the oxidation resistance of high-temperature alloys include the following:
pinning effect. The oxides of active elements penetrate along the grain boundaries or directly into the alloy matrix, “nailing” the continuous outer oxide film and the alloy matrix together to increase the adhesion of the oxide film.
Improve the plasticity of the oxide film. Rare earth elements and their oxides can refine the grains of the oxide film and the matrix grains, increase the plasticity of both, especially the oxide, so that the stress in the oxide film can be eliminated through plastic deformation, thereby improving the strength of the oxide film. Adhesion.
Promote the rapid and large formation of Cr2O3. Since rare earth elements can reduce the diffusion activation energy of Cr3+ and increase the diffusion coefficient, it is conducive to the rapid formation of a complete Cr2O3 oxide film.
Active elements reduce the critical concentration for selective oxidation of Cr or Al to form a single oxide film, which is very beneficial to the self-repairability of the oxide film.
Active elements can inhibit harmful impurities, segregate at grain boundaries, improve the bonding strength of the oxide film interface, etc.
However, for a specific high-temperature alloy, there can only be one or several mechanisms that work together to improve the oxidation resistance of the alloy. Many grades of superalloys actually produced contain one or two active elements. The recently developed four hot-corrosion resistant casting superalloys K444, K446, K435 and K452 even contain Hf, Zr, La, Y, Ce, etc. an active element. Therefore, the Cr2O3 oxide film of these alloys has excellent adhesion and good hot corrosion resistance.
Applications of active element effects
The application of active element effects is not limited to high-temperature alloy materials, but is also widely used in other fields. For example, the effect of mixed rare earth elements (mainly containing La and Ce) on the oxidation resistance of Incoloy801 alloy was studied. The chemical composition of Incoloy 801 alloy is C, 0.05; Si, 0.80; Mn, 0.94; Cr, 22.33; Ni, 32.10; Ti, 0.75; Fe balance. The results show that adding 0.2% mixed rare earth elements significantly reduces the oxidation weight gain, oxidation rate and oxidation activation energy QP2 of 801 alloy. The rare earth-containing 801 alloy oxide film is relatively thin. At the interface between the oxide film and the matrix, there are gray strip-shaped rare earth oxides containing 47.6% lanthanum, which extend into the matrix and tightly combine the oxide and the matrix, thereby improving the oxidation rate. membrane adhesion. At the same time, because the atomic radius of rare earth elements such as lanthanum and cerium is larger than that of elements such as nickel and chromium, the g matrix causes lattice distortion, reduces the diffusion activation energy of Cr3+, increases the diffusion coefficient of Cr3+, and promotes the rapid and large amount of Cr2O3 formed, playing a good protective role. In addition to solid solution in the alloy matrix, rare earth elements are also dissolved in trace amounts in the oxide film, increasing the binding energy of Cr3+ in the oxide film, thereby improving the thermal stability of the oxide film.
in conclusion:
Although the mechanism by which active elements improve antioxidant properties is not yet clear, it is certain that improving the adhesion ability of the oxide film is its main role. Whether it is a Cr2O3 oxide film or an Al2O3 oxide film, active elements change the diffusion process through the segregation of grain boundaries in the oxide film, that is, promoting the inward diffusion of oxygen along the grain boundaries, while delaying the outward diffusion of chromium or aluminum, making Cr2O3 Or Al2O3 grows at the interface between the alloy matrix and the oxide film, reducing the formation of holes at this interface and reducing the peeling of the oxide film.
The research and application of active element effects are of great significance for improving the oxidation resistance and heat resistance of materials. In future research, the mechanism and application of active element effects need to be further explored to meet the needs for material performance in different fields.
Tianjin Anton Metal Manufacture Co., Ltd. is a company specializing in the production of various nickel-based alloys, Hastelloy alloys and high-temperature alloy materials. The company was established in 1989 with a registered capital of 10.0 million, specializing in the production and sales of alloy materials. Anton Metal’s products are widely used in aerospace, chemical industry, electric power, automobile, nuclear energy and other fields, and can also provide customized alloy material solutions according to customer needs. If you need to know the price consultation of alloy materials or provide customized alloy material solutions, please feel free to contact the sales staff.
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Post time: Nov-18-2023
