| Magnesium alloys with series of excellent performance are the lightest structural materials in engineering applications and have wide application prospects in the automotive, aerospace,3C electronic products. However, there exists a huge gap between the practical application and their potential due to the poor corrosion and ignition-proof resistance. Therefore, it is significant to expand their industrial applications by improving and enhancing corrosion and ignition-proof resistance. In this paper, the secondary phase characteristics in Mg-xAl-xCa-0.6Mn alloys can be controlled by semi-continuous casting, alloying and thermal deformation. The microstructure, corrosion and ignition-proof resistance will be systematically studied by the methods of metallography, scanning electron microscopy, energy dispersion spectrum, X-ray diffraction analysis and electrochemical analysis. On the basic of the investigations mentioned above, the influence of the combined addition of Ca, Al and hot extrusion process on microstructures, corrosion and ignition-proof resistance have been clarified. Moreover, the mechanism of high temperature oxidation behavior and ignition-proof property of Ca-containing magnesium alloy has also been explored. The main results are as follows:(1) The microstructure features of semi-continuous casting Mg-xAl-xCa-0.6Mn alloys and influence of Ca, Al elements and hot extrusion on microstructures and corrosion resistance are studied. The results show that semi-continuous casting Mg-xAl-xCa-0.6Mn alloys consist of coarse ?-Mg dendrites, fine lamellar a-Mg/Mg2Ca and small coarse strip a-Mg/(Mg, Al)2Ca eutectic phases. With increase of Ca and Al elements, the grains can be obviously refined and the a-Mg matrix can be effectively segmented due to the increase of eutectic phases. After hot extrusion, coarse dendrites in semi-continuous casting microstructures are transformed into small equiaxed grains; reticular eutectic phases are broken into micron or nanoscale particles like bands along extrusion direction under three-dimensional pressure. With increasing extrusion ratio, the grains can be refined further and the reticular eutectic phases can be broken into smaller particles. Moreover, the distance between secondary phase bands becomes smaller due to the increase of extrusion ratio.(2) The influence of Ca, Al elements content and hot extrusion process on microstructures, corrosion resistance is studied. The intrinsic link between microstructures and corrosion resistance is analyzed by corrosion morphology, corrosion product characteristic. Moreover, the mechanism of Mg-xAl-xCa-0.6Mn alloys in different states has been explored. The results show that corrosion behavior of semi-continuous casting Mg-xAl-xCa-0.6Mn alloys is mainly intergranular corrosion. With the increase of Ca, Al elements content, corrosion potential increases, corrosion current density and weight loss corrosion rate decrease, corrosion resistance of semi-continuous casting alloys improves. After hot extrusion, corrosion resistance of as-extruded alloys initially increases and then decreases with Ca, Al elements increasing. Improved corrosion resistance of matrix and grain boundaries caused by grain refinement, corrosion barrier of bands with secondary phases, relatively stable and dense corrosion product film are the reasons of high corrosion resistance of as-extruded alloys. With extrusion ratio increasing, smaller size and more secondary phase particles increase galvanic corrosion, more scattered secondary phase bands weaken corrosion barrier, so corrosion resistance decreases.(3) The influence of Ca element addition on ignition temperature and high-temperature oxidation of Mg-xAl-xCa-0.6Mn alloy is studied. Mechanism of high-temperature oxidation and ignition-proof is analyzed by oxidation film morphology, phase, element distribution and other aspects. Simple models of high-temperature oxidation are also stetted up according to thermodynamic calculations. The results show that Ca element can effectively improve melt surface oxide film quality and ignition temperature of Mg-Al-Ca-Mn alloys. With the increase of Ca element content, ignition temperature increases and ignition temperature of Mg-5.5Al-5.5Ca-0.6Mn alloys is up to 1071?. High-temperature oxide film of Mg-Al-Ca-Mn alloys is a three-layer composite structure consisting of CaO, MgO-CaO composite film and Mg2Ca secondary phase particles. High temperature oxidation of Mg-Al-Ca-Mn alloy can be divided into three processes:Ca and O2 preferentially form loose CaO film, under that gathering micron or nanoscale particles Mg2Ca; after Ca activity reduces, Mg and O2 form MgO, and then combines with CaO to form CaO·MgO composite protective film; With the temperature increases, increment of Mg vapor pressure leads to partial oxidation film burning, and further extends to the whole sample. |
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