| Magnesium alloys have wide application prospect to be utilized as the lightest metalstructural materials in aerospace, electronic communication and automotive fields, etc.However, the application of most magnesium alloys has been greatly restricted due to theirpoor strength, oxidation and creep resistance. Icosahedral quasicrystalline phase (I-phase) hasattractive mechanical properties attributed to its unique atomic structure, such as highthermodynamic stability, high hardness and high strength, etc. Recently, it has been reportedthat magnesium alloys containing I-phase as a secondary solidification phase exhibit goodmechanical properties, which provide a new approach for magnesium alloy strengthening.Firstly, Mg-Zn-RE (Y, Nd) quasicrystal master alloys were prepared by means ofconventional casting. The effects of alloy elements and cooling rate on microstructures andmechanical properties of Mg-Zn-RE alloys were investigated. In addition, evolution rule ofmicrostructure and thermal stability of I-phase during heat treatment were analyzed. Then, theformation mechanism and growth pattern of I-phase were discussed. Secondly, Mg-Zn-Ndalloys containing I-phase were fabricated by using squeeze casting method and the influencesof applied pressures on microstructures and hardness of alloys were studied. Finally,Mg-Zn-Y quasicrystal-reinforced AZ91D magnesium matrix composites were prepared bysqueeze casting method. The effects of applied pressures, pouring temperatures andquasicrystal master alloy contents on microstructures and mechanical properties ofcomposites were investigated. Moreover, the strengthening mechanism was discussed.The results showed that, microstructures of conventional casting Mg-Zn-Y alloys were affected by the contents of Zn and Y elements, the influence of Y was greater than that of Zn.Mg-45Zn-xY alloys mainly consisted of α-Mg, I-phase and Mg7Zn3phase. Petal-likequasicrystal phase can be obtained with the increase of Y content in Mg-Zn-Y alloys. Themorphology of quasicrystal converted into polygon-like structure when the content of Y was10.0wt%and the hardness reached the maximum value of89.0HRB. Mg-xZn-2.5Y alloysconsisted of α-Mg matrix, MgZn dendrite and lamellar (α-Mg+Mg7Zn3) eutectic structure asthe contents of Y element were30.0wt%and40.0wt%, while the microstructures werecomposed of Mg7Zn3matrix, dendrite α-Mg and (α-Mg+I-phase) eutectic structure when Ycontents were45.0wt%and50.0wt%. Besides, petal-like quasicrystals existed inMg-50Zn-2.5Y alloy, which made hardness reach the maximum value of84.5HRB. MgZn6xYxalloys consisted of α-Mg matrix and (α-Mg+I-phase) eutectic structure which distributedaround the grain boundaries. With the increase of Y addition, the matrix grain size decreased,the content and the size of eutectic structure increased, the morphology of which alsotransformed from discontinuous distribution to continuous distribution. When the content of Yelement was2.0at%, MgZn6xYxalloy had the maximum hardness value of54.0HRB. A largenumber of dispersive and high temperature-stable quasicrystals precipitated in Mg95.1Zn4.2Y0.7alloy through heat treatment at400℃for12hours.The results also showed that, microstructure of Mg-45Zn-10.0Y alloy was significantlyinfluenced by cooling rate. The sizes of quasicrystal particles were30μm,15μm,12.5μm and9μm by adopting number1,2,3and4mould respectively, which indicated that the size ofquasicrystal decreased with cooling rate increasing. It was also found that the surfacemicrostructure of Mg-45Zn-10.0Y alloy contained more petal-like quasicrystals with size ofabout35μm and the bottom of which appeared as smooth plane. However, the coremicrostructure contained more polygon-like and less petal-like quasicrystals, the sizes ofwhich were20μm and50μm respectively.Mg-45Zn-xNd and Mg-50Zn-xNd alloys consisted of Mg7Zn3matrix, black α-Mgdendrite, lamellar (α-Mg+MgZn) structure, spherical Mg40Zn55Nd5I-phase and rod-like phaseunder conventional casting condition. The morphology of ternary phase was determined by Nd and Zn contents. Heat treatment experiment results of Mg-45Zn-1.5Nd alloy indicated that,α-Mg dendrite dissolved into matrix, lamellar structure disappeared completely and thedistribution of microstructure became more homogeneous with the increase of holding time.Meanwhile, the content of elevated temperature-stable spherical I-phase increased and thesize of which reduced. The mechanical properties were improved by increasing heat treatmenttime, the hardness and tensile strength were85.5HRB and168MPa respectively in case ofMg-45Zn-1.5Nd alloy was heat-treated at330℃for6hours, which were6.2%and48.7%higher than those of as-cast alloy. Microstructure of squeeze casting Mg-45Zn-2.5Nd alloywas composed of Mg7Zn3matrix, α-Mg dendrite, lamellar (α-Mg+MgZn) structure, sphericalMg40Zn55Nd5quasicrystal and polygon-like Mg36Zn60Nd4phase. Large quantity of sphericalI-phase formed in alloy and the hardness reached the maximum of85.2HRB when the appliedpressure was150MPa.It was showed that icosahedral quasicrystal phase precipitated directly from liquid ofMg-Zn-RE alloys. The morphologies of I-phase in Mg-Zn-Y alloys mainly appeared as fivepetals-like, six petals-like and polygon-like shapes, while in Mg-Zn-Nd alloy it existed asspherical shape.Microstructures of squeeze casting Mg-Zn-Y quasicrystal-reinforced magnesium matrixcomposites comprised α-Mg matrix, β-Mg17Al12phase that dispersed on grain boundaries andMg3Zn6Y quasicrystal particles. It was showed that squeeze casting process was an effectivemethod to refine grain. With the increase of applied pressure, the contents of β-Mg17Al12phase and Mg3Zn6Y quasicrystal phase increased and the morphology of α-Mg changed fromdendrite structure to equiaxed grain structure. β-Mg17Al12phase on boundaries refined andα-Mg matrix tended to be coarse with the increase of pouring temperature. Microstructures ofmagnesium matrix composites were also influenced by the content of quasicrystal masteralloy. Fine and uniform microstructure can be obtained and β-Mg17Al12phase was brokenwhen the content of Mg-Zn-Y quasicrystal master alloy was5wt%. However, microstructuresbecame unhomogeneous and exhibited distinct regions of fine and coarse structure (Bi-modalstructure) when addition contents were3wt%,7wt%and10wt%. The results showed that the optimum content of quasicrystal master alloy was5wt%, the best applied pressure andpouring temperature were100MPa and700℃respectively. Under such conditions, the tensilestrength and elongation of composite reached the maximum values, which were194.3MPaand9.2%respectively. The reinforcement mechanism was analyzed on the base of experimentresults, it was concluded as fine-grain strengthening and quasicrystal particle strengthening.The study of this thesis provides a new kind of quasicrystal strengthening phase andsqueeze casting technology for preparing magnesium matrix composites. High propertycomposites can be obtained by optimizing the content of Mg-Zn-Y quasicrystal master alloyand squeeze casting processes, which has great theoretical and practical significance topromote rapid development and wide application for high-strength and heat-resistantmagnesium matrix composites. |
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