Research On The Effect Of Al On Microstructure And Properties Of Duplex Mg-Li Alloys

As the lightest metal structure material, Mg-Li alloys possess a series of advantages such as high specific strength, high specific stiffness, excellent electrical and thermal conductivity, good ectromagnetic shielding ability and damping capacity. So Mg-Li alloys have wide application potentials in the fields of automobile, eleetron, comunication and spaceflight industry, et al. However, the application of Mg-Li alloys are still limited because of the low absolute strength, poor stability and inferior creep resistance at elevated temperature. For this reason, addition of alloying elements and thermo-mechanical working for improving the properties of Mg-Li alloys has been studied.In this study, Mg-9Li- xAl alloy were prepared by “mixture method”. The microstructure and mechanical properties of as-cast, heat-treated and hot-extruded alloys were investigated by optical microscopy?OM?, scanning electron microscopy?SEM?, X-ray diffraction?XRD? and mechanical properties testing. The effect of Al on microstructure and mechanical properties of Mg-9Li alloy was studied. The influence of solid solution and hot extruding on microstructure and mechanical properties of Mg-9Li-xAl al oys was analyzed.?1? As-cast Mg-9Li-xAl alloys are mainly composed of ?-Mg and ?-Li phase. When adding 3% of Al, the second phase Mg₁₇Al₁₂ appears in the alloy which are mainly distributed in the boundaries of ? and ? phase. When Al content is 6%, the second phase of alloy transforms Mg₁₇Al₁₂ phase into AlLi and MgAlLi2 phase, and the small block AlLi and granular MgAlLi2 phase distribute dispersively in ? phase, while the lamellar AlLi phase distribute in the ? phase. When the Al content increased to 9%, with the disappearance of MgAlLi2 phase, the second phase of alloy mainly shift to lamellar AlLi phase.?2? The addition of Al can significantly improve the strength of the Mg-9Li alloy. With the increasing of Al content, the strength firstly increase s and then decreases while the elongation decreases. When the content of Al is 6%, the tensile strength and yield strength reach to the max values 186.5MPa and 176.8MPa, which are respectively enhanced by 61.1% and 83.6% compared with that of the alloy without the addition of Al.?3? After the solution treatment, the microstructure of Mg-9Li-xAl alloy are changed as follows: When the Al content is 3%, part of Mg₁₇Al₁₂ phase can be dissolved into matrix. When the Al content is 6%, the block- like AlLi cannot be dissolved into ?-Li, and the MgAlLi2 phase can be wholly dissolved into ?-Li. At the same time, the AlLi phase is precipitated from ?-Mg phase in the form of lamellar; When the Al content is 9%, part of lamellar AlLi phase is dissolved into ? phase and some block- like AlLi phase is precipitated from ? phase. On the whole, the number AlLi phase is reduce in the al oy.?4? After the solution treatment, the hardness of Mg-9Li-xAl alloys have a certain degree of improvement. When the Al content is 6%, the hardness of ? and ? phase after solution treatment reach the maximum values 80.4HV and 90.6HV, compared with cast alloy, respectively, improved by 12.4% and 25.1%. In addition, the effect of solid solution strengthening for ? phase are abviously better than that of ? phase.?5? During the hot-extruding, the grains are refined and the second phase distributes more dispersively. Hence, the comprehensive mechanic al properties of the alloy improved. When the Al content is 3%, the hot-extruded alloy reaches to the best mechanical properties, the tensile strength and the elongation are 209.4MPa and 20.5%, respectively.