Preparation And Research On Microstructure And Properties Of Mg-Li-Al-Ca Alloys

With the progress of industrial technology and the effect of global warming,lightweight technology is attracting more and more attention of the researchers.As the lightest structural metallic materials,Mg-Li alloys have lots of advantages such as high specific strength,high specific stiffness,low density and good plasticity.Mg-Li alloys have broad application prospects in the fields of automotive,aerospace,nuclear power,3C products and medical appliances.However,the low strength and poor heat stability of Mg-Li alloy limits its wide applications.Alloying is an important way to improve the mechanical properties and corrosion resistance of magnesium alloys.Li is the best alloying element to reduce the density of magnesium alloys,Al is another important alloying element for Mg-Li alloys and the element Ca can reduce the grain size of Mg-Li alloys.Based on these above,in this study the Mg-6.8Li-3Al-xCa alloys were prepared by “mixture method”.The effect of Ca on the microstructure and mechanical properties of as-cast and as-extruded Mg-6.8Li-3Al alloys was investigated by optical microscopy(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD)and mechanical property testing.The fracture mechanism of the alloys was also investigated.(1)Mg-6.8Li-3Al alloy consist of α-Mg,AlLi,a handful of β-Li and Mg17Al12.The addition of Ca can refine the grain size of alloys and the AlLi,Mg17Al12 were replaced by a new phase Al2 Ca.With the content of Ca increasing,the content of AlLi and Mg17Al12 decrease and finally disappear.(2)The results of mechanical properties of as-cast alloys indicate that: with the content of Ca increasing,the tensile strength and the elongation were increased first,then decreased.A little amount of Al2 Ca which distributes along the grain boundary has the grain boundary strengthening effect on the test alloys.But too much Ca added into the alloys can decrease the amount of Al in matrix which reduce the mechanical properties of Mg-6.8Li-3Al alloys.When the content of Ca is 0.4% the as-cast alloy show the best mechanical properties: the tensile strength and yield strength are 157 MPa and 123 MPa respectively and the elongation reaches to 12.3%.Compared with Mg-6.8Li-3Al alloy,the tensile strength is enhanced by 29.7% and the elongation is enhanced by 61.8%.(3)The dynamic recrystallization occurs during extrusion and the grains are refined,so the grain size of as-extruded alloys is reduced a lot compared with as-cast alloys.The eutectic phase particles are distributed uniformly in the matrix after extrusion.The results of mechanical test indicate the as-extruded alloys perform much better than as-cast alloys in mechanical properties.When the content of Ca is 0.4%,the tensile strength and yield strength of as-extruded alloy are 286 MPa and 249 MPa respectively which increases by 82.2% compared with as-cast alloys.(4)The tensile strength of Mg-6.8Li-3Al-0.8Ca alloy at 150℃ reaches to the peak value 191 MPa which increases by 22% compared with the Mg-6.8Li-3Al alloy.