Study Of Microstructure And Properties Of Superlight Mg-Li-Al-RE Alloys

Mg-Li alloys are the lightest metal engineering materials. And they possess many virtues such as low density, high specific strength and specific stiffness, good formability, good electro-magnetism screening and shock damping, et al. Mg-Li alloys have wide application potentials in the fields of automobile, electron and spaceflight, et al. However, Mg-Li alloys also have some shortcomings, such as relatively low strength, low elevated temperature strength, low creep resistance and corrosion resistance. This restricts the development and application of Mg-Li alloys. In recent years, light materials are needed more and more urgently with the rapid development of spaceflight in China. Research about Mg-Li alloys with high strength, good corrosion resistance and high-temperature resistance, is very important for the development of spaceflight and automobile.Based on the background of Mg-Li alloys, Mg-Li-Al-RE alloys were prepared in this thesis. And the microstructure, room-temperature and elevated temperature mechanical properties, corrosion resistance were studied. It will provide some reference information for the application development of Mg-Li alloys.In this thesis, the present research status of Mg-Li alloys, the effects of alloying elements on Mg-Li alloys, the corrosion behavior of Mg-Li alloys are studied.α+βdouble phases andβsingle phase Mg-Li-Al-RE alloys were prepared respectively with vacuum melting method. The microstructure and properties were analyzed and characterized with OM, XRD, SEM, tensile tester, micro-hardness tester and electrochemistry workstation.Influence of different RE elements on microstructure, mechanical properties and fracture micrographs ofα+βandβas-cast alloys was studied. Results show that the addition of RE can refine the microstructure of alloys. When the RE content is 0.6-0.9%, mechanical properties of alloys are the best. Because of the high electronegativity difference between RE and Al, compounds containing Al and RE form in matrix alloys. The Al content dissolved in solid matrix, Mg₁₇Al₁₂ and AlLi content all decrease accordingly. This makes the Li content in a phase increase and even makesαphase transfer toβphase. Because of the refine strengthen and second phase strengthen, the strength of alloys increases with the RE content. However, when the compounds amount is too large, the matrix is easy to be cleaved causing the strength decreasing. The compounds containing Al and RE can restrict grain boundary sliding and dislocation movement. This leads to the improvement of corrosion resistance. The decrease of Mg₁₇Al₁₂ amount makes the fracture type transfer from brittle to ductile.The mechanical properties of Mg-14Li-3Al-(0～0.9)MM and LA141-(0～1.2)Nd wrought alloys were also studied. Results show that, during the extrusion at 250℃, the dynamic recrystallization of alloys happens and the grain size of alloys is refined obviously. The refined grains can coordinate the deformation and make the strength and elongation to be improved accordingly. From the results, it can also be known that, the original grain size of alloy has influence on the recrystallization grain size. The large original grain size will lead to large recrystallization grain size. The second particle size and amount also affect the recrystallization grain size. Fine and numerous particles make large resistance for grain boundary movement. This leads to the low grow-rate of recrystallization grains and causing the fine recrystallization grain size. Because of the restriction of second particles against matrix deforming, the crystallization grain size of alloys containing high RE content is not uniform. Because of the refine effects of Nd on Mg₁₇Al₁₂, the fracture of wrought LA141-xNd changes from cleavage to ductile.The electrochemical tests and electrochemical parameters analysis of LA141-(0～2Y) alloys show that the addition of Y will decrease corrosion current density and increase Tafel slope. This demonstrates the improvement of corrosion resistance of alloys. It can also be demonstrated by the micrographs of corrosion surface of alloys. With the increase of Y content, the corrosion film becomes full gradually. And the corrosion resistance of alloys is improved accordingly.