The Microstructures And Mechanical Properties Of Mg-Zn-Y Magnesium Alloys

Research works of this thesis aim at high performance Mg-Zn-Y alloys. The microstructures and mechanical properties of the Mg-Zn-Y alloys under the condition of the traditional solidification,rapid solidification and reciprocating extrusion were systematically investigated. Meanwhile, the effects of heat treatment were also studied.The compositions of Mg-Zn-Y alloys are Mg_94.6Zn_4.8Y_0.6,Mg₉₅Zn_4.2Y_0.8,Mg₉₇Zn₂Y₁,Mg_92.5Zn_6.4Y_1.1,Mg_91.6Zn_7.2Y_1.2,Mg₉₇Zn₁Y₂(at.%). The microstructures and phase compositions of the alloys were studies by optical microscope, scanning electronic microscope and X-ray diffraction. The tensile strength and hardness were tested at room temperature.1. As-cast Mg-Zn-Y alloys were firstly prepared and studied for the relations of their chemical composition, microstructures and mechanical properties. The result shows that with the increasing of Y content, the microstructures have refinement. The more Zn and Y content, the better the mechanical properties at ambient temperature. After a series of heat-treating tests, it is sure that the heat treatment can improve the mechanical properties of the alloys. Comparing the mechanical properties of Mg-4.2Zn-0.8Y alloy of T4 and T6 heat treatment with those of the as-cast condition, the tensile strength is improved by 17.2 and 37.7%, respectively, and the yield strength by 88.5 and 107.2%, respectively.2. As-cast Mg-6.4Zn-1.1Y alloys were extruded reciprocating at 350℃by different extrusion passes. The results show the as-extruded microstructures are refined effectively. The grain size is approximately 5μm at the second extrusion. Meanwhile, the mechanical properties are greatly improved, and especially the elongation is improved by about 1701% of that in the as-cast condition. The grains have a similar size, as the number of extrusion passes increases. The mechanical properties of alloys reach an optimum state at the fourth passes. Apparently, the big improvement of the mechanical properties of alloys is greatly attributed to the fine grain size, which is induced by dynamic recrystallization during the hot extrusion process. But, after heat treatment, the mechanical properties of the alloys aren't improved.3. The RS ribbons of Mg-4.8Zn-0.6Y alloy were obtained whose microstructures and properties were investigated. The XRD analysis results indicate that the microstructure of the RS ribbons is composed of the single phaseα-Mg supersaturated solid solution. The maximum microhardness of RS ribbons after heat-treating was gained at 250℃×2h. The microsegregation and its formation for the RS ribbons were investigated. The analysis by the Back-Scattered Electron and Line Scanning indicates that there is almost no microsegregation along the whole section of the RS ribbons. According to the analysis for the SEM microstructures of the RS ribbons consisted of beehive-like grains, the microsegregation is exist. It is presented that the reason of the microsegregation formation for the RS ribbons is not only the difference of solidification speed, but also the contribution of as-cast heredity.