The Microstructure And Properties Of In-situ Quasicrystal Reinforced Mg-Zn-Y And Mg-Zn-Y-Al Alloys Fabricated By Common Solidification Technique

Icosahedral quasicrystal phases possess attractive mechanical and physical properties such as high strength, low friction coefficient, low surface energy and high thermal conductivity due to their peculiar lattice structure. Therefore, the quasicrystals will have potential application future in the development of new materials. Recently, great interest has been directed towards quasicrystals. As for Mg-Zn-Y alloys, the present researches are mainly concentrated on the phase equilibrium, the effect of alloying elements on microstructures, and the preparation of Mg-Zn-Y alloys with high strength. But the researches on the growth of icosahedral quasicrystal phases and the effect of the cooling rate on the microstructures are rare, on which the further researching is necessary. In addition, the friction and wear behavior of Mg-Zn-Y alloys has not been reported, and the quasicrystal reinforced Mg-Zn-Y-Al alloys have been researched rarely. Based on the above insufficiencies in the microstructure and properties researching of quasicrystal reinfored magnesium alloys, some valuable researches were done and some innovative results were obtained in this dissertation.Firstly, in-situ quasicrystal reinforced Mg-Zn-Y and Mg-Zn-Y-Al alloys were successfully prepared with common casting technique. The effects of quasicrystal phases on the microstructure of Mg-Zn-Y and Mg-Zn-Y-Al alloys were investigated systematacially. In addition, the growth form of icosahedral quasicrystal and the effects of the cooling rate and heat treatment on the microstructure were studied. The results are as follows:1. In Mg_60-3xZn_40+2xY_x alloys, I-phases are difficult to nucleate. I-phases can be formed in the alloy only when Y content is 1.5at.%. However, Mg7Zn3 phase (the 1/1 approximant phase of I-phase) are easy to be formed in other alloys.2. In MgZn₂₅Y_x alloys, I-phases are easy to nucleate. When the cooling rate is about 20℃/s, with increasing Y content, the content of I-phase increases gradually, the morphologies of I-phases vary from block to petal-like particles gradually, and the growth form transforms from facet to non-facet growth. When Y content is 1at.%-2.5at.%, the matrixes of the alloys are Mg₇Zn₃ phase, and (I-phase +α-Mg) eutectic phases exist in the microstructures. When Y content reaches 2.8at.%, Mg-25Zn-2.8Y alloy comprised I-phase,α-Mg and (I-phase +α-Mg) eutectic phases, and Mg₇Zn₃ phase disappears completely. With the further increase of Y content, the matrix of Mg-25Zn-3.5Y alloy is changed to MgZn phase, moreover, the quasicrystalline morphologies show more various characteristics. When the cooling rate is about 45℃/s and Y content is 1.5at.%-3.5at.%, all alloys comprise I-phase,α-Mg and (I-phase +α-Mg) eutectic phases, and MgZn and Mg₇Zn₃ phases disappear completely. When Y content is 1at.%, the morphologies of I-phase vary from petal-like particles to lamellar (I-phase +α-Mg) eutectic phases, and the size ofα-Mg increases gradually.3. All MgZn_6xY_x alloys comprise the dendrites ofα-Mg matrix separated by interdendritic I-phase and lamellar (I-phase +α-Mg) eutectic phases. With the increase of Y content, the content of I-phase increases; the content, size and lamellar space between I-phase andα-Mg in (I-phase +α-Mg) eutectic cell increase; but the grain size ofα-Mg matrix decreases.4. (I-phase +α-Mg) eutectic disappears in Mg-25Zn-2.5Y-2.5Al alloy by adding 2.5at.% Al to Mg-25Zn-2.5Y alloy. The morphologies of most I-phase are blocky particles, and the morphologies of little I-phase are petal-like particles. However, it has not the influence on the microstructure of Mg-9Zn-1.5Y-2.5Al alloy to add 2.5at.% Al to Mg-9Zn-1.5Y alloy. 5. In MgZn_6xY_xAl₆ alloys, the main phases of Mg-1.5Zn-0.25Y-6Al and Mg-3Zn-0.5Y-6Al alloys areα-Mg, I-phase and M₁₇Al₁₂. With the increase of Y content, the main phases of Mg-6Zn-1Y-6Al alloy areα-Mg, I-phase, M₁₇Al₁₂ and Al₂Y. With the further increase of Y content, the main phases of Mg-9Zn-1.5Y-6Al alloy areα-Mg, I-phase, M₁₇Al₁₂, Al2Y, and W-phase. With increasing Y content, the content of I-phase increases, and the grain size ofα-Mg matrix decreases.6. In MgZn₉Y_xAl₆ alloys, the main phases of Mg-9Zn-0.25Y-6Al and Mg-9Zn-0.5Y-6Al alloys areα-Mg, I-phase and M₁₇Al₁₂. With increasing Y content, Al2Y phase appear in all other alloys. Moreover, with the increase of Y content, the content of Al2Y phase increases firstly and then decreases.7. In Mg-Zn-Y-Al alloys, with increasing time and temperature of the heat treatment,α-Mg matrix is spheroidized gradually, but the second Al2Y particle sets back the process of spheroidization. Moreover, the effect of Al2Y grain size on the spheroidezation is obvious. With the increase of grain size, the spheroidization is more difficult gradually. With the same grain size of Al)2Y particles,α-Mg matrix is easier to be spheroidized, when the Al2Y particles contact the negative curvature ofα-Mg grain boundary compared with the positive curvature one. When the time of heat treatment is at 430℃, the (I-phase +α-Mg) eutectic is disappear completely in Mg-Zn-Y-Al alloys.Secondly, the effects of quasicrystal phases on the mechanical properties of Mg-Zn-Y and Mg-Zn-Y-Al alloys were investigated by measureing the tensile strengths, elongations and analyzing the dependence of the microstructures on the mechanical properties of Mg-Zn-Y and Mg-Zn-Y-Al alloys. The results are as follows:1. In MgZn_6xY_x alloys, with increasing Y content, the tensile strengths increase greatly when Y content is lower than 1.0at.%, and then increase slowly when Y content is more than 1.0at.%; the elongation rates increase firstly and then decrease. When Y content reaches 1.0at.%, the elongation rate achieves the highest value.2. In MgZn(6x)Y_xAl₆ alloys, the tensile strength and elongation rate increase firstly and then decrease with increasing Y content. When Y content reaches 1.0 at.%, the tensile strength and elongation rate achieve the highest value.3. The tensile strengths of MgZn_6xY_xAl₆ alloys are higher than those of MgZn_6xY_x alloys when Y content is 0.25at.% and 0.5at.%. Whereas, the tensile strengths of MgZn_6xY_xAl₆ alloys are lower than those of MgZn_6xY_x alloys when Y content is 1.0at.% and 1.5at.%. The failure modes of MgZn_6xY_x and MgZn_6xY_xAl₆ alloys are quasi-cleavage fracture.Thirdly, based on the changes of the microstructures of Mg-Zn-Y and Mg-Zn-Y-Al alloys, the effects of quasicrystal phases on the friction and wear behaviors of Mg-Zn-Y and Mg-Zn-Y-Al alloys are investigated by measuring the wear mass losses and friction coefficients and analyzing the worn surfaces and wear debris. The results are as follows:1. The friction coefficients decrease with increasing applied loads in all MgZn₂₅Y_x alloys. High applied load brings about high plastic deformation on the worn surface of the MgZn₂₅Y_x alloys. The friction coefficients and the wear mass losses of the MgZn₂₅Y_x alloys decrease firstly and then increases with increasing Y content. The Mg-25Zn-2Y alloy exhibits the lowest friction coefficient and the best wear resistance at all applied loads.2. The friction coefficients decrease with increasing applied loads in all MgZn_6xY_x alloys. High applied load brings about high plastic deformation on the worn surface of the MgZn_6xY_x alloys. The friction coefficients and the wear mass losses of the MgZn_6xY_x alloys decrease gradually with increasing Y content.3. The friction coefficients decrease with increasing applied loads in all MgZn_6xY_xAl₆ alloys. High applied load brings about high plastic deformation on the worn surfaces of the MgZn_6xY_xAl₆ alloys. The friction coefficients and the wear mass losses of the MgZn_6xY_xAl₆ alloys decrease firstly and then increases with increasing Y content. The Mg-3Zn-0.5Y-6Al alloy exhibits the lowest friction coefficient and the best wear resistance at all applied loads.