Research On The Microstructure And Mechanical Properties Of The Cast Mg-Li-Zn-Zr Alloy

Research works of this thesis aims at the light magnesium lithium alloys and mainly focuses on the effects of the misch metal and some thermal treatments such as the homogenizing treatment , the hydrogenating treatment etc. on microstructure and mechanical properties of them. Two typical light magnesium lithium alloys, Mg-7.13Li-3.92Zn-lZr and Mg-8.16Li-4.10Zn-2.86Y-0.59Zr-2.14LPC, are investiga -ted systematically for the relations of chemical composition, microstructures and mechanical properties.The typical test alloys of Mg-7.13Li-3.92Zn-lZr and Mg-8.16Li-4.10Zn-2.86Y -0.59Zr-2.14LPC are prepared in vacuum inductance furnace, and their thermal treatments under different conditions were studied. Microstructures and mechanical properties of the test specimens were observed and analyzed under SEM , OM and tensile test. And the effects of the misch metal and some thermal treatments on microstructure and mechanical properties of alloys were analyzed and the mechanisms of variation were discussed.The results were as follows:(1) The high quality magnesium alloys can be prepared in vacuum inductance.(2) The cast Mg-7.13Li-3.92Zn-1Zr alloy is composed of α-Mg, α-Zr and θ' phase(MgLi₂Zn);The shape of θ' likes long acerate and the shape of two heads of it appears sharp corner. After the homogenizing treatment, θ' phase(MgLi₂Zn) turned into θ phase(MgLiZn) which appears short vermiform, and the shape of twoheads changed into obtuse form. In this process there took place passivation phenomenon.(3) The homogenizing treatment can enhance the mechanical properties of Mg-7.13Li-3.92Zn-lZr alloy in evidence. More over, extending the time of the treatment, the tensile strength and elongation percentage of the test specimens turned to lower gradually. In this process there took place coarsening phenomenon. Another, the yield strength of the specimen in 300 °C X48h is the maximum.(4) After the homogenizing treatment of the cast Mg-7.13Li-3.92Zn-lZr alloy, the passivation phenomenon and coarsening phenomenon both influenced the mechanical properties of Mg-7.13Li-3.92Zn-lZr alloy. But comparing the homogenized alloys with the cast alloy, the effects of the passivation phenomenon is more apparent. Besides, comparing the homogenized alloy in different work crafts, the coarsening phenomenon is more and more apparent.(5) The microstructure of the cast Mg-8.16Li-4.10Zn-2.86Y-0.59Zr-2.14LPC alloy appears mesh structure, which composed of the solid solution of LL Zn and RE in Mg and the compound of Mg-Li-Zn-RE. After the homogenizing treatment of the cast Mg-8.16Li-4.10Zn-2.86Y-0.59Zr-2.14LPC, the mesh structure just occurred dissolved partially, and extending the time of the treatment, it still existed. Besides, the homogenizing treatment cannot enhance the mechanical properties of the Mg-8.16Li-4.10Zn-2.86Y-0.59Zr-2.14LPC alloy.(6) After the hydrogenating treatment of the cast Mg-8.16Li-4.10Zn-2.86Y -0.59Zr-2.14LPC alloy, the compound of Mg-Li-Zn-RE has disappeared, which' turned into the discontinuous and exiguous granular structure, and the hydrogenating treatment can enhance the mechanical properties of Mg-8.16Li-4.10Zn-2.86Y-0.59Zr -2.14LPC alloy in evidence. Besides, the optimum condition of the hydrogenating treatment are480"CX48h.