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Trace amounts of La were introduced into Al-20Mg2Si composites to adjust their microstructure and corrosion properties. The modification mechanism of trace amounts of La and the corrosion behavior of composites were systematically studied by thermal analysis, microstructure observation and electrochemical testing. The study shows that trace La refines the size of the primary Mg2Si phase by forming a tiny LaAlSi phase, thereby providing an effective heterogeneous nucleus for the primary Mg2Si phase during the solidification process. The excess La atom can be selectively adsorbed on the {100} surface of the primary Mg2Si phase, which inhibits the preferential growth of the Mg2Si phase. The enhanced corrosion resistance of La microalloyed Al-20Mg2Si composites can be attributed to the formation of a stronger protective surface oxide film and a finer primary Mg2Si phase, which effectively hinders the penetration of corrosive media and attenuates the local corrosion tendency of Al-Mg2Si composites. This study provides new enlightenment for the preparation of highly corrosion-resistant aluminum alloys and their composites.
With the global environmental pollution and energy crisis, the demand for high-performance lightweight materials, especially aluminum alloys and their composite materials, has increased significantly. When the hard Mg2Si reinforcement is embedded in the soft Al matrix, the Al-Mg2Si alloy has high specific strength, good pouring ability, excellent wear resistance and heat resistance. Al-Mg2Si alloy with composite structure is considered a promising candidate material in the aerospace and automotive sectors. However, the primary Mg2Si phase in supereutectic Al-Mg2Si composites usually exhibits a rough dendrite shape, which seriously affects the properties of the materials and restricts their practical application. Therefore, controlling the size and morphology of the primary Mg2Si phase is the key to improving its performance.
In recent years, the research and application of high performance of metal materials through the introduction of rare earth elements have received great attention, but most of them focus on the microstructure and mechanical properties of composite materials. In practical engineering applications, Al-Mg2Si composites are often exposed to extremely corrosive environments, and their corrosion resistance is also crucial for prolonging their service life. At present, the mechanism of the effect of trace amount of La on the microstructure and corrosion resistance of Al-Mg2Si composites needs to be further studied.
Based on this, the team of researcher Fan Shuqian from the Chongqing Institute of Green and Intelligent Technology of the Chinese Academy of Sciences and the team of Wu Jing from the School of Bioengineering of Chongqing University studied the microstructure evolution and corrosion behavior of the La-microalloy Al-20Mg2Si composites. The modification mechanism caused by the addition of trace amount of La was discussed by combining differential scanning calorimetry analysis and microstructure observation. The corrosion behavior of Al-20Mg2Si composites under different amounts of La was revealed by impregnation, electrochemical and X-ray photoelectron spectroscopy analysis. The findings were published in the journal "Journal of Materials Science & Technology" titled "Microstructural evolution and corrosion behavior of Al-20Mg2Si composites with trace La addition".
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The results show that the addition of trace amounts of La changes the primary phase of Mg2Si from coarse dendrites to truncated octahedron. When the amount of La was 0.10%, the average size of the Mg2Si primary phase was refined from 77.6 μm to 26.7 μm, and the corrosion current density decreased from 5.96 μA·cm−2 to 3.03 μA·cm−2.
Trace amounts of La refine and modify the primary Mg2Si phase by promoting the heterogeneous nucleation of the primary Mg2Si phase and inhibiting the growth of the primary Mg2Si phase. The existing LaAlSi clusters provide efficient heterogeneous nuclei for the primary Mg2Si phase. The selective adsorption of La atoms on the surface of the {100} inhibited the preferential growth of the primary Mg2Si phase.
The addition of trace amount of La makes the oxide film on the protective surface of Al-Mg2Si composites stronger, and the primary Mg2Si phase becomes thinner, which can effectively hinder the penetration of corrosive medium and weaken its local corrosion tendency, thereby significantly enhancing its corrosion resistance.
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