Research On The Effect Of La/Ce On Microst-ructure And Properties Of Mg-Li-Al Alloys

Mg-Li alloys as a minimum density of metal structural materials, has become thepreferred material for lightweight.Mg-Li alloys has many advantages, such as lowdensity(1.35~1.65g/cm3), high stiffness-to-weight ratio, superior strength-to-weightratio, good deformation ability, good damping ability, and high energetic particlepenetration resistance. Therefore, the use of Mg-Li alloys in electric appliances,aerospace, military engineering and automobiles is drawing much attention from theenvironmental viewpoint. Nevertheless, the application of the Mg-Li alloys is restrictedbecause of their low strength, poor corrosion resistance and poor heat stability. For thisreason, addition of alloying elements and thermo-mechanical working for improving theproperties of Mg-Li alloys has been studied.In this thesis, Mg-RE(La/Ce) master alloy was prepared by“mixture method”, thenMg-RE(La/Ce) was added into Mg-Li-Al alloys and the influence of La/Ce on themicrostructure and mechanical properties of Mg-9Li-3Al alloys was investigatedby optical microscopy (OM),scanning electron microscopy(SEM), energy dispersivespectrometry(EDS), X-ray diffraction(XRD)and mechanical properties testing.Meanwhile, the thermal deformation behavior of alloys was studied by thermal/mechanical simulation experiment, and formulated the hot extrusion deformationprocess of the alloys, then systematically studied the effect of hot extruded process onthe microstructure and mechanical properties of the two-phase Mg-Li alloys. This thesisalso first proposed and explored the remelting process of Mg-Li alloys, in order to openup new ideas to improve the microstructure and mechanical properties of alloys.The results show that:①Mg-9Li-3Al alloy is consist of α-Mg、β-Li、Mg17Al12and a little AlLi phase,with the La/Ce mixed rare earth added, Mg17Al12and AlLi phase existed in alloy wererepealed by new Al3La and Al2Ce phase.②La/Ce mixed rare earth significantly refined the grains of the as-cast Mg-9Li-3Al alloy, and the addition of La/Ce can effectively enhance the mechanical propertiesof the alloys, the tensile strength and the yielding strength increase first and thendecrease, the elongation decreases slowly.When the La/Ce addition is0.5%, theas-cast Mg-9Li-3Al-xRE(La/Ce) alloy reaches to the maximum tensile strength159.7MPa and yielding strength131.2MPa. ③The hot compression deformation process of the as-cast Mg-9Li-3Al-2.0RE(La/Ce) alloy shows that the flow stress becomes higher with increase of strain rates atconstant temperature, and lower with increase of deformation temperature. It is clearthat the higher the temperature, the more obvious the softening effect of dynamicrecovery. Considering the effects of temperature and strain rate on the hot deformationbehavior, and ultimately determine the hot extrusion temperature of250℃.④The dynamic recrystallization occurs during the hot-extruding of Mg-9Li-3Al-xRE(La/Ce) alloys and the grains are significantly refined, the second phase Al3Laand Al2Ce distribute dispersively in the alloy matrix form the block and rod structuresreunited gradually transform into part of the rod and particle structures. the results ofmechanical properties test indicates that with the increase of La/Ce mixed rare earthcontent, the strength of extruded alloys increases first and then decreases, but theelongation showing a tendency of decrease first and then increase. And the tensilestrength and the yielding strength of extruded Mg-9Li-3Al-0.5RE(La/Ce) alloyrespectively reaches to its peak value234.7Mpa and213.5Mpa, but a minimumelongation of13.9%. Compared to Mg-9Li-3Al-0.5RE(La/Ce) alloy, extruded Mg-9Li-3Al-2.0RE (La/Ce), although the strength of the alloy decreased, but theelongation increased to23.7%, and performances good mechanical properties.⑤The comparison result of the microstructure and mechanical properties betweenas-cast alloys and hot-extruded alloys indicates that after extrusion, the microstructureof the alloys is obviously refined. And the mechanical properties of hot-extruded alloysare much better than the as-cast alloys, after extrusion, the strength and ductility ofalloys are significantly improved.⑥Remelting and then extrusion of extruded Mg-9Li-3Al-2.0RE (La/Ce) alloycan significantly improve the tensile strength and yield strength, and less impact on theductility, which provides a new process ideas for improving the strength of the alloys.