Study On Microstructure And Properties Of Mg-Zn-Y With Quasicrystal And LPSO Phase

Magnesium alloys were the lightest structural metal materials in the engineering applications, with the preferable properties such as high specific strength, high specific stiffness, high damping coefficient, good mechanical processing performance and easy recycling, etc. However, its poor mechanical properties, poor corrosion resistance and so on, limit its broader applications. The cast magnesium alloys with rare earth elements exhibite excellent properties, which are becoming more and more attractive metal materials in the recent years. Adding Y to Mg-Zn alloy with heat treatment and new preparation technology can produce quasicrystalline phase(I-phase) and long period stacking ordered structure phase(LPSO phase) with good comprehensive performance.In this paper, the as-cast and rapidly solidified Mg-Zn-Y quasicrystal alloys were prepared through the conventional ingot casting and single-roller equipment, and the effect of cooling rates on solidification, microstructure and corrosion resistance of rapidly solidified quasicrystal alloys were investigated. The as-cast method was used to prepare Mg-Zn-Y alloy with long period stacking ordered structure phase, and heat treatment was carried out simultaneously. The alloy microstructures and solidification process were analyzed to research the phase types and components, phase transformation mechanism as well as properties transformation. The MgZn-Y icosahedral quasicrystal master alloy was added into the long period stacking ordered Mg-Zn-Y alloy in order to study the interrelation between I-phase and LPSO phase as well as properties transform.The Mg-Zn-Y icosahedral quasicrystals have been prepared successfully by conventional casting and rapidly solidified methods, and the quasicrystal alloys under different cooling rates have great different microstructures. The icosahedral quasicrystals can directly from liquid of Mg-Zn-Y alloys by conventional casting and rapidly solidified methods. The preferred growth directions of icosahedral quasicrystalline phase(I-phase) are along five-fold axes and the vertex of icosahedron is the best location for I-phase forming again. The cooling rates play a crucial role in both growth process and nucleation of I-phase. The icosahedral short-range order(ISRO) with five-fold symmetry was regarded as the nucleation particle of I-phase. A higher cooling rate leads to a larger degree of supercooling, and further results in the increased existence probability of ISRO in the melt. The corrosion resistance of Mg57Zn37Y6 alloys in the 3.5%(wt.%) Na Cl solution has a typical morphology of pitting, and the large volume fraction of quasicrystalline phases in rapidly solidified Mg57Zn37Y6 alloy play a dominant role in improving the corrosion resistance of alloy.The 18 R long-period stacking ordered structures are formed in as-cast Mg94Zn2Y4(at.%) alloy and its composition is Mg10Zn1Y1. The microstructural evolutions of as-cast Mg94Zn2Y4 alloys during solution treatment were systematically investigated. The results show that the 18 R and 14 H long-period stacking ordered structures can coexist in Mg94Zn2Y4 alloy after 788K-15 h solutioning treatment. The composition of 14H-LPSO and 18R-LPSO are Mg12Zn1Y1 and Mg10Zn1Y1, respectively, and the electron diffraction analysis show that the orientation relationship exists between 14H-LPSO and ?-Mg matrix, i.e.,(0001) 14 H //(0001) Mg and [0110] 14 H //[1120] Mg. The 14H-LPSO transformed from 18R-LPSO, and also formed directly through a precipitated behavior in ?-Mg matrix. TEM image and corresponding SAED patterns indicate the absence of stacking faults(SFs) in ?-Mg grains, which is the nucleation site of the 14 HLPSO. Then, the 14H-LPSO grows up with diffusion and migration of solute atoms. The tensile strength, yield strength and elongation of the Mg94Zn2Y4 alloy after 788K-15 h solution treatment were 245 MPa, 157 MPa and 13.8%, respectively, with an increase of 34.6%, 16.3% and 35.3% compared with as-cast Mg94Zn2Y4 alloy, respectively. The improvement of mechanical properties is due to the following aspects: firstly, a large number of fine needle like lamellar structure phases appear in the Mg94Zn2Y4 alloy after 788K-15 h solutioning treatment; secondly, the volume fraction of the second phase was significantly increased compared with as-cast Mg94Zn2Y4 alloy; thirdly, the grains were refined markedly. The block LPSO phases in the Mg94Zn2Y4 alloy after artificial aging following solid solution(T6) dissolved into ?-Mg or changed into smaller pieces, at the same time, the lamellar structure was more fine and the length and width of the fine needle phase were reduced significantly. The macro hardness of the Mg94Zn2Y4 alloy by solid solution at 788 K for 15 h and subsequent artificial aging at 473 K for 12 h reached a maximum.The different quality of Mg-Zn-Y icosahedral quasicrystal master alloy were added into the long period stacking ordered Mg-Zn-Y alloy cause significant changes in microstructure of Mg-Zn-Y alloy. The quasicrystalline phases existed in the(I-phase+?-Mg) eutectic structures which were the layer structure of independent existence or existed between in the thin plate like phases. The macro hardness of the Mg-Zn-Y alloy is closely related to the phase component of the alloy. The phase component in Mg-Zn-Y alloy is ?-Mg+Mg10Zn1Y1(0wt.%) ??-Mg+Mg10Zn1Y1+I-phase(4wt.%) ??-Mg+Mg10Zn1Y1+I-phase(8wt.%) ??-Mg+W-phase(12wt.%) and the macro hardness of the alloy is firstly increased and then reduced with increasing the mass fraction of Mg-Zn-Y icosahedral quasicrystal master alloy.