Research Of Microstructures,Mechanical Properties And Applications Of Deformed Mg-4Zn-xY Alloys

In order to improve the mechanical properties of wrought magnesium alloys, the effect of ratio of Zn/Y, hot-extrusion, rolling and heat treatment on microstructure and mechanical properties of Mg-Zn-Y alloys was studied in this thesis. As the research objects, Mg-4Zn-xY (x=1,2,3) alloys were fabricated using permanent mold casting method. The ingots (130mm in diameter) were hot extruded into bars (20mm in diameter) under the temperature of 450℃. Microstructures and compositions were observed and analyzed by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), and energy dispersive spectrum (EDS); hardness and tensile properties were tested by Brinell hardness machine and electron tensile machine. Then the research object was focused on Mg-4Zn-1Y alloy. The extruded bars and sheet-metals were ageing treated and rolled respectively. The physical and chemical properties of MgZn2 and Mg17Al12 were calculated by First Principle Calculation. The variation of microstructures and mechanical properties of extruded and ageing treated Mg-4Zn-1Y alloys were studied through microstructure observation and tensile testing and the strengthening mechanism was revealed to provide evidence for the application of Mg-4Zn-1Y alloy. AZ31 alloy used as medical applications in artery stents and staples were studied as well. The testing results indicated that Mg-Zn-Y alloys shown much potential to be used as medical applications.The experimental results indicated that as the ration of Zn/Y decreased, the microstructures of as-casted Mg-4Zn-xY alloys were more refined whose average grain size was 150μm, 100μm and 50μm respectively and the phase composition was changed from α-Mg, I-phase and W-phase to both α-Mg and W-phase. The structures of second phases were also changed from mix of short rod, net, and spheroidizationto single continuous net structure.After being hot-extruded whose ration was 42.25, the microstructures were greatly refined: Both Mg-4Zn-1Y and Mg-4Zn-2Y were comprised of coarse original grains and large amounts of dynamic recrystallization grains and the grain sizes were 4μm and 2μm respectively while Mg-4Zn-3Y was comprised of uniform and fine equiaxed grains whose size was 6μm. W-phases were twisted and smashed while I-phases were spheroidized which contributed to reducing the effect of separation between I-phases and Mg-matrix. The twin bands, which were formed during extrusion, contributed to the large deformation and hindered the slip of dislocations to improve the mechanical properties. Lots of MgZn2 phases which were acicular in structure participated from the matrix to further improve the mechanical properties of Mg-4Zn-1Y alloy.The crystal structure, stability, density of state, elastic constants and thermal properties of MgZn2 and Mg17Al12 phases were calculated by First Principle Calculation and according to the results:MgZn2 was better to resist deformation when being applied strength, the stiffness was pretty poor but the plasticity was better than that of Mg17Al12.The tensile properties of as-cast Mg-4Zn-xY alloys were increased by a small extent mainly due to the refining effect of Y element and the tensile properties of three alloys were 125.2MPa, 136.6MPa and 140.3MPa, respectively which were improved by 103.59%,86.53% and 82.18% compared with the as-cast alloys. As for Mg-4Zn-1Y alloy, the hardness was further improved as well as the tensile properties after being ageing treated:the hardness was improved from 60.4HB to 64.5HB; yield strength and tensile strength were improved from 180.6MPa to 240.7MPa and 254.9MPa to 330.3MPa, respectively. Elongation was improved from 17.9% to 20.4%. The improvement of the mechanical properties was mainly because of the comprehensive effect of strengthening of extrusion, quasicrystalline, grain refinement and precipitationAfter being multiple-process-rolled, whose whole reduction was 60%, the grain sizes of two metal-sheets were increased from 5μm and 3μm to 30μm and 20μm, respectively because of the high temperature (450 ℃) and insulation After being age-treated, the tensile properties of thicker and thinner sheet metals were 255.8MPa and 278.3MPa, respectively, and the elongations were 15.6% and 16.1%, respectively.Compared with AZ31, Mg-4Zn-1Y has much potential to be used as biodegradable material according to its mechanical properties, biocompatibility and corrosion resisance.