Effects Of Al₄C₃ And Ce, Ca, Sr On Microstructure And Performance Of AZ91D Magneisum Alloy

Magnesium alloys are widely used in automotive, electric and aeronauticalindustry due to their low density, good electromagnetic shielding characteristics, highdamping characteristics, excellent machinability and so on. However, the applicationof magnesium alloys is far less than other metal materials because of its inherentproperties, such as lower strength, poor corrosion resistance and high-temperatureperformance and so on. Therefore, it is important to explore active approach ofimproving the mechanical properties and formability through grain refinement, thus topush the application of magnesium alloys and develop its performance advantages. Inthis thesis, the separate addition of Al₄C₃ and the combined additions of Al₄C₃ andCe,Ca,Sr were added to AZ91D alloy, which is the most widely used magnesiumalloys to research their effect on microstructures, mechanical properties and corrosionbehavior.The effect of the addition of Al₄C₃ and alloying elements Ce, Ca, Sr on AZ91Dmicrostructure was investigated by optical microscope, scanning electric microscope,energy spectrum analysis and X-ray diffraction. The results indicate that the additionof Al₄C₃ and alloying elements Ce, Ca, Sr caused a large decrease of grain size. Andformed a new phase Al₄Ce when the combined addition of Al₄C₃ and Ce. The smallestgrain size of AZ91D alloy was obtained in the case of separate addition of1.2wt%Al₄C₃. Meanwhile, when the content of Al₄C₃ is 0.6wt%, the content of Ce,Ca, Sr was 0.2wt%,0.2wt%,0.1wt%, respectively, the refine effect of AZ91D alloywas best. In which the addition of Al₄C₃ caused to transform the eutecticmicrostructure morphology of AZ91D alloy from the fully-divorced eutectic intohoneycombα+βpartially-divorced eutectic. In contrast, the further addition of Ce,Ca and Sr inhibited this trend. Theβ-Mg₁₇Al₁₂ phases dissolved intoα-Mg matrixby Solution treatment, and then after aging treatment theβ-Mg₁₇Al₁₂ phases wereprecipitated fromα-Mg matrix. Fine-grained microstructure of AZ91D alloy can beobtained on the condition of lower undercooling after the addition of Al₄C₃ and Ce, Ca,Sr through DTA analysis.Tensile and hardness tests show that the addition of Al₄C₃ and alloying elementsCe, Ca, Sr can significantly increased the hardness and mechanical properties ofAZ91D alloy. In the case of T6 condition, the tensile strength have been enhancedremarkably while elongation have been improved with low values in the case of the addition of Al₄C₃, Ce, Ca and St. the fracture mechanism of AZ91D alloy have beenchanged from cleavage fracture to quasi-cleavage fracture with the addition of Al₄C₃.The strengthening and toughing mechanism of alloy perhaps included fine-grainstrengthening, solution strengthening and second-phase precipitation strengthening, inwhich fine-grain strengthening plays a leading role.The effect of Al₄C₃ and Ce, Ca, Sr on corrosion perfomance and corrosionmechanism in 3.5%NaCl solution were studied by polarization curve measurementand the weight-loss test. The experiment found that Al₄C₃ and Ce were benefit toimprove the corrosion resistance of AZ91D alloy while the additions of Ca and Srdeteriorated its corrosion resistance. The research of corrosion mechanism discoveredthat the refinement of microstructure weakens the cathodic effect ofβphase anddecreases the microgalvanic, which results in the improvement of corrosion resistanceof alloys. The formation of corrosion product film and purity effect which result in thecorrosion resistance further improved with the addition of Ce. And the standardelectrode potential of Ca and Sr are lower than that of Mg, the corrosion potentialdecrease after addition of Ca and St, which make the corrosion resistance becomerelatively poor.