| Structural refinement during solidification of AZ91 alloy in Mg-Al-Zn system has been studied with a focus on the effects of solute elements, such as Sr, Ca, Ce, Nd, in situ TiC particulate and electromagnetic stirring on microstructures and mechanical properties. The relations of chemical composition, microstructures and properties are discussed to improve its mechanical properties and corrosion resistance.The effects of Sr on microstructures and properties of AZ91 alloy are investigated. The results show that Sr addition leads to the α grain refinement, the increase in density, that is, decrease of microporosity. In addition, Sr addition results in improving the ambient strength properties and little influencing elongation for as-cast and T6 condition AZ91 alloy. The role of Sr in grain refinement of AZ91 alloy is also discussed.The microstructures and mechanical properties of AZ91 alloy added minor Ca are investigated. The results indicate that the α grain is fined, the room temperature strength properties are improved, but elongation becomes poor due to Ca addition. The mechanism of grain refinement of Ca on AZ91 alloy is attributed to Ca at the solid/liquid interface front during solidification strongly restricts the α grain growth.The influences of Ce and Nd on the as-cast microstructures and mechanical properties of AZ91 alloy are systematically studied. The results reveal that microstructures are markedly fined by Ce and Nd addition, Ce and Nd are combined with Al and respectively form Al3Ce needles and Al3Nd particles near the grain boundaries. Meanwhile, the morphology of β -Mg17Al12 phase are changed from continuous network to discontinuous one. Ce and Nd addition significantly improve ambient strength properties of as cast and T6 condition AZ91 alloy, and slightly increase its elongation. The mechanism of grain refinement of Ce and Nd on AZ91 alloy is mainly attributed to the increasing of constitutional undercooling in a diffusion layer ahead of the advancing solid/liquid during solidification, which results in the increase of the number of nucleation because nucleants in the melt are more likely to survive and be activated in the constitutionally undercooled zone.The mechanism of grain refinement and technique of Ce addition combined Ca or Sr are systematically investigated. Water-quenching method and metallographic observation are used to study the mechanism of grain refinement.A master alloy of Al-TiC is successfully prepared by cast contact reaction based on SHS principle. Thermodynamic calculation of chemical reaction is made to study process parameters of reactive synthesis.The effects of the level of Al-TiC on microstructures and mechanical properties of AZ91 alloy are systematically investigated. The results show that in stiu TiC particulate can fine the α grain, which results in improving strength properties. In addition, the role of TiC particulate in grain refinement of alloy is discussed.The effects of electromagnetic stirring on microstructures and mechanical properties of AZ91 and AZ91+0.8%Ce alloy are studied. The results show that electromagnetic stirring with appropriate input voltage has an important role in the refinement and spheroidization of the α phase, and reduces the amount of β -Mg17Al12 and the segregation tendency of Zn element in the β-Mg17Al12 phase. The mechanism of structural refinement of electromagnetic stirring and Ce element on alloy is presented.The effects of corrosion resistance of Ce Nd and Sr on AZ91 alloy immersed in NaCl solution are investigated using static weight loss test and polarization curve measurement. The results show that Ce and Nd can significantly reduce corrosion rate of AZ91 alloy, but Sr is disadvantageous to corrosion resistance. The relation between the microstructures and corrosion resistance is discussed.
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