Formation Of Spherical Mg-based Quasicrystal And Its Strengthening Behavior On Magnesium Alloys

To improve the strength-toughness and heat-resistance, spherical quasicrystal contained Mg-based (SQCM) master alloy has been fabricated under conventional casting conditions by designing chemical compositions and optimizing processing parameters. Mg-based quasicrystal-particle-reinforced magnesium alloys were obtained by introducing SQCM master alloy into the magnesium alloy melt. The formation mechanism, microstructure evolution and heredity effects of Mg-based quasicrystal was investigated in this paper. Besides, its effects on magnesium alloys and the microscopic essence of strengthening behavior of Mg-based quasicrystal were also discussed.Based on the investigation of Mg-Zn-Y quasicrystal alloy, effects of alloying elements such as Ca and Mn on formation and macro-hardness of Mg-Zn-Y quasicrystal were investigated. Experiment results showed that the formation mechanism and macro-morphology of quasicrystal have changed with the addition of Ca or Mn elements. Spherical quasicrystal can be obtained by adding 0.3% Ca into Mg-Zn-Y quasicrystal alloy. The morphology of quasicrystal changes from petal-like to sphericity as the content of Mn increases, with its diameter decreasing and its quantity increasing. However, with the addition of Mn surpassing 2.0%, the quantity of spherical quasicrystal decreases and its morphology varies from sphericity to polyhedron or petal-like. Both the macro-hardness of these two kinds of quasicrystal master alloys gradually decreases with the increase of Mn or Ca addition. It was also found that the addition of Mn influences growing process of dendriteα-Mg phase. As the content of Mn increases, dendrite arm space ofα-Mg phase tends to be smaller and coarse primary dendrite disappears. Spherical quasicrystal contained Mg-based master alloy can be achieved under conventional casting condition with mould diameter less than 30mm.It was found that cracks exist in petal-like Mg-Zn-Y quasicrystal and spherical Mg-Zn-Y-Mn quasicrystal. Although adding these kinds of relatively low melting-point quasicrystal leads to obvious grain refinement and enhances mechanical properties both at room and elevated temperature, the improvement of elevated-temperature mechanical properties is limited. Since Nd can improve the elevated-temperature heat-resistance, a new spherical quasicrystal contained Mg-Zn-Nd master alloy was fabricated under conventional casting condition to improve the mechanical properties of magnesium alloys at elevated-temperature. Compared to spherical Mg-Zn-Y-Mn quasicrystal, no cracks in the spherical Mg-Zn-Nd quasicrystal and no eutectic phase surrounds the quasicrystal while divorced eutectic and individual lamellar eutectic phase appears, besides its melting point, spherical ratio and spherical rating become much higher. All these features indicate that superior heat resistance, better processing tolerance and lower production cost can be achieved than that of spherical Mg-Zn-Y-Mn quasicrystal when adding Mg-Zn-Nd quasicrystal into master alloy with the spherical Mg-Zn-Nd quasicrystal acting as reinforced phase of heat resistant Mg-based alloy.Because of its low surface energy and high thermal stability, a multi-phase Mg-based alloy with homogeneously distributed quasicrystal particles was obtained under conventional casting condition after adding SQCM master alloy into Mg-based alloy melt. The as-cast microstructure was obviously refined and room-temperature strength-toughness was dramatically improved due to the strengthening effect of quasicrystal particles. It was found that spherical Mg-Zn-Y-Mn quasicrystal exhibits superior strengthening effect than petal-like Mg-Zn-Y quasicrystal. After the addition of spherical quasicrystal containing Mg-Zn-Y-Mn master alloy into AZ91 alloy, room-temperature tensile strength, yield strength, percentage elongation, impact toughness was improved by 38%, 34%, 40% and 60%, respectively; heat resistance was also improved greatly. Moreover, excellent grain-refining and strengthening effects can be achieved after adding spherical quasicrystal containing Mg-Zn-Y-Mn master alloy into ZA85 alloy or high Zinc magnesium alloy. In this paper, both quasicrystal master alloy and quasicrystal reinforced magnesium alloys were fabricated, microstructure evolution rule of quasicrystal was discussed, and heredity effect and grain-refining effect of spherical quasicrystal during the solidification of magnesium alloy were investigated. The results showed that dispersive phase which distributes in the interior ofα-Mg matrix and in the vicinity ofβ-Mg₁₂Al₁₇ phase can strengthen the matrix and the grain boundaries, slow down the diffusion of elements inα-Mg matrix, impede the dislocation movement in the matrix, and prevent grain boundary sliding. Besides, morphology ofβ-Mg₁₂Al₁₇ phase changes from continuous nets to discontinuous nets or even sphere, and its aging precipitation changes too. Consequently, the strength and toughness at room temperature and the heat-resistance at elevated temperature can be improved efficiently.