| As the lightest engineering metal material, magnesium alloys have been named the "green engineering materials in the 21 st century",which possess high specific strength, high specific rigidity, good machinability, excellent damping capacity, and easy to recovery.Owing to these excellent properties, magnesium alloys have beenwidely used in aerospace,automotive industry and 3C digital products. However, magnesium alloys have poor plastic deformation ability, low strength, easy oxidation and their absolute high temperature creep resistance is poor, restricting their further application in industry. Mg-Zn-Y icosahedral quasicrystal phase(I-phase) has high hardness, high elastic modulus and good wettability with magnesium matrix, so it can be used as the strengthening phase of the magnesium alloys, which provide a new way for the research of strengthening magnesium alloys.In this work, Mg-Zn-Y quasi-crystalline master alloys were prepared by conventional cast technology, Mg-Zn-Y quasicrystal reinforced AZ91 D magnesium matrix composites were prepared by means of squeeze casting, the effects of different applied pressures and pouring temperature on mechanical properties and microstructures of the composites were investigated; Moreover, the in situ quasicrystal reinforced Mg93Zn6Y1 magnesium matrix composites were prepared by squeeze casting, the influence of different applied pressures on the microstructures and tensile properties of composite materials were studied.The experimental results show that stable icosahedral quasicrystal is obtained in Mg-47Zn-x Y Mg-45Zn-5.5Y by using conventional casting. With the increase of Y content in the alloy, the amount and size of the special morphology petals-like I-phase increases,break away and then develop single petal; With Zn reduction(Zn=45wt%), the morphology of I-phase changed from petal shaped to irregular polygonal shape, the size black of ?-Mg crystal increased. With the slow cooling rate, petals-like I-phase fully growth, break away,and then develop new morphology single icosahedrons.Under gravity casting(0 MPa) process, I-phase can distribute evenly in ?-Mg matrix grain boundaries or intracrystal. The mechanical properties of AZ91 D magnesium alloy improved significantly with the addition of I-phase. The tensile strength and elongation of quasicrystal reinforced AZ91 D magnesium matrix composites are 146.3MPa and 4.08%,respectively, which are 44.68% and 41.63% higher than AZ91 D magnesium alloy. The fracture mechanism of the composites is mainly composed of the cleavage plane and tear ridges, presenting typical brittle fracture features. Compared with gravity casting, squeeze casting process is more helpful to improve the mechanical properties of the composites.When the pouring temperature is 700 ?, the applied pressure is 100 MPa, the tensile strength and elongation of the composites reached the maximum value, which are 201.5MPa and 8.20%, respectively, and 37.7% and 100.9% higher than the corresponding samples prepared by gravity casting. After solid solution and aging heat treatment, the tensile strength and elongation of the composites increased to 233.5MPa and 9.81%.The microstructures of Mg93Zn6Y1 magnesium alloy prepared by squeeze casting are composed of ?-Mg phase and I-phase. I-phase exists in(I-phase+?-Mg)lamellar eutectic structure, and distributes in ?-Mg dendrite. With the applied pressures increasing, lamellar eutectic structure becomes smaller, and changed from continuous network fracture network to fracture networks, distributing more uniform. When the applied pressure increased from0 MPa to 150 MPa, the tensile strength of Mg93Zn6Y1 alloy increased from 148.3 MPa to193.5 MPa, and the elongation increased from 2.41% to 4.23%, an increase of 30.48% and75.52% respectively. Mg–Zn–Y alloy embody the characteristic of quasi-cleavage fracture. |
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