| Quasicrystal has become the promising materials in the materials science research field and attracted broad attentions in the world, since quasicrystal was found in the Al-Mn system for the first time in the 1980 s. After decades of unremitting efforts, many other quasicrystal systems were found successively. Magnesium quasicrystal materials become one of the research hotspot due to its unique structure type in recent years. Because of high hardness, low surface energy and well corrosion resistance, the magnesium quasicrystal is always utilized as reinforcement phase. As a new magnesium-based quasicrystal, Mg-Zn-Nd spherical icosahedral quasicrystal breaks a new path in the development and research of high performance magnesium alloys. The Mg-Zn-Nd icosahedron spherical quasicrystal belongs to the stable three dimensional quasicrystal of thermodynamics, and its study is still in infancy especially in the effect of different freezeing rate and heat treatment temperature. Therefore, through the preparation of Mg-Zn-Nd quasicrystal alloy, the growth mechanism of spherical icosahedral quasicrystal phase and impact factors were studied, and it has agreat importance for the theory and application of the icosahedron spherical quasicrystal.In this paper, Mg-Zn-Nd quasicrystal alloy was prepared and heat treatment through the normal casting solidification and rapid solidification. To investigate the microstructure of the different composition of the alloy, the Mg-Zn-Nd spherical quasicrystal alloy phase formation mechanism, to discuss the microstructure of different solidification conditions and solidification rate of the Mg-Zn-Nd quasicrystal alloy solidification path, solidification microstructure and properties of spherical quasicrystal phase, the number and composition impact, and the impact of heat treatment phase transformation of the Mg-Zn-Nd quasicrystal alloy microstructure, as well as the impact on the spherical morphology quasicrystalline phase components, to further study the thermodynamics Mg-Zn-Nd spherical quasicrystals stability, X-ray diffraction analysis(XRD), scanning electron microscopy(SEM), differential thermal analysis(DSC), transmission electron microscopy(TEM) and other testing methods were used. The results showed as follows:The Mg-Zn-Nd quasicrystal can be obtained by ordinary casting solidification and rapid solidification methods. Different solidification rate, quasicrystalline phase can be precipitated directly from the undercooled melt. By ordinary preparation of alloy casting solidification, the contents of element Nd and atomic ratio of Mg/Zn has a great influence on the alloy microstructure. When the alloy atomic ratio of Mg and Zn are maintained between 2.5 to 2.6 and the atomic ratio of Nd element is 1.2%, the alloy microstructures obtained the most parts of spherical quasicrystal phase and uniformly distributed in the matrix phase Mg7Zn3, the alloy contained no other phases. The spherical icosahedral quasicrystal phases also follow the rules of nucleation and growth, and the adsorption of Nd element in formation process, the type of growth interface and interfacial stability and other factors together lead to the final morphology.By ordinary casting method solidified casting molds in different Mg-Zn-Nd quasicrystal alloy, its microstructures, properties and microstructures of tissue coagulation in I-phase quasicrystal phases are different. In this experiment, different alloy dies possess different casting solidification speed, copper mold has the highest alloy solidification speed leads to the largest number of spherical quasicrystal, smallest grain size and highest roundness. As the decreasing of solidification rate, the number of spherical quasicrystalline alloy phase is reduced, increasing grain size and the morphology began transforming. Mg-Zn-Nd quasicrystalline alloy ribbon prepared by rapid solidification methods. With the increasing rotating speed and solidification rate, the spherical quasicrystalline phase turns to be more, and smaller grain size. At this point, the solidification rate increases will inhibit nucleation spherical quasicrystalline phase is not conducive spherical quasicrystalline phase of growing up. Therefore, the nucleation spherical icosahedral quasicrystals have one of the best critical solidification rate, lower than the optimum solidification speed, with increasing solidification rate, and promote the nucleation quasicrystalline phase, higher than the optimum coagulation When the speed, with the solidification rate increase, inhibit nucleation quasicrystalsThe Mg-Zn-Nd quasicrystalline alloy at 300℃ for 50 h after heat treatment, spherical I-phase keeps stable with no changed, and the phase of cast alloy matrix Mg7Zn3 decomposed into MgZn eutectic phase and α-Mg phase organization, of which there are a small number of MgZn2 phase. The ball I-phase at 300℃ had good thermodynamic stability, well decomposition and no melting. The Mg-Zn-Nd quasicrystalline alloy at 350℃ for 30 h after heat treatment, alloy structure changed significantly, and spherical I-phase melting phenomenon occurs, some grains form incomplete, and the others have melted away, at 350℃ for 30 h heat eutectic alloy after the other phase is MgZn phase and black α-Mg phase as well as the presence of a small amount of MgZn2 phase. These facts show that the spherical I-phaseis thermodynamically unstable at 350℃ and can`t be stable existence. |
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