Effect Of La And Ca On The Microstructure,Room Temperature Mechanical And Thermal Expansion Properties Of Magnesium Alloys

Because magnesium alloy has the advantages of low density,high specific strength,and excellent electromagnetic shielding performance,it has received more and more attention from researchers in engineering applications.With the rapid development of aerospace,transportation,and electronics industries,electronic packaging materials are required to meet light weight,low expansion performance,high thermal conductivity,low cost and other requirements.By introducing a low expansion coefficient second phase into the magnesium alloy,the light weight advantage of the magnesium alloys can be exerted,which can broaden the application of magnesium alloys in the above fields.In this paper,by introducing Mg₁₂La and Mg₂Ca phases with high melting point and good thermal stability,the effects of La and Ca on the thermal expansion properties of binary magnesium alloys are studied,and the relationship between the microstructure and the room temperature mechanical and thermal expansion properties of magnesium alloys is discussed.First,Mg-x La(x=0,3.2,5.3,8.6,10.2,12.5 wt.%)binary alloys and Mg-y Ca(y=0,4.1,6.7,8.5,10.1,12.4 wt.%)binary alloy have been prepared by melting and casting.And then used X-ray fluorescence spectrometry(XRF)to analyze the composition of the as-cast alloys,Using optical microstructure observation(OM),X-ray diffraction(XRD),scanning electron microscope observation(SEM)and energy dispersive spectrometer(EDS)analyzed the phase composition of the as-cast and extruded alloys.And finally tested the mechanical properties at room temperature and the change of the thermal expansion coefficient with temperature of the alloys.This paper first studied the microstructure,room temperature mechanical properties and the variation curve of the thermal expansion coefficient with temperature of Mg-x La alloys.With the increase of La content,the dendritic arms of the dendritic structure in the as-cast alloys gradually become thinner and smaller,and the volume fraction of Mg₁₂La gradually increases.The addition of La element significantly increased the strength of the as-cast alloys,but at the same time that deteriorates the elongation of the alloys.After the extrusion deformation,the strength of the alloys further increases,which is attributed to the broken of the coarse second phase during the extrusion deformation process,which plays a role of the second phase strengthening.As the temperature increases,the coefficient of thermal expansion(CTE)curves of the as-cast and extruded Mg-La binary alloys first show a linear and rapid rise,and then slowly rise.With the increase of La content,the average CTE of Mg-La binary alloy in the temperature range of 373?673 K shows a gradual downward trend(from 30.3×10^-6 K^-1 to 27.9×10^-6 K^-1).This shows that the introduction of the high melting point Mg₁₂La phase is beneficial to suppress the thermal expansion behavior of the alloys.In the temperature range of 373?573 K,the average CTE of the extruded alloys is lower than that of the as-cast alloys.This is because after extrusion deformation,obvious dynamic recrystallization occurs,accompanied by grain refinement and an increase in grain boundaries.At the same time,the crushing of the second phase increases the contact area between the second phase and the Mg matrix,which is beneficial to inhibit the expansion of the Mg matrix,thereby reducing the CTE of the alloy.Then the effects of Ca addition on the microstructure,room temperature mechanical and thermal expansion properties of the as-cast and extruded Mg-y Ca binary alloys were studied.The microstructure of the as-cast Mg-y Ca alloys showed a typical dendritic shape,and the?-Mg matrix and the intermetallic compound alternately formed a lamellar eutectic structure.The addition of Ca element refines the microstructure of the as-cast Mg-y Ca alloys.With the addition of Ca,the volume fraction of the intermetallic compound Mg₂Ca gradually increases.The introduction of this phase is beneficial to reduce the CTE of the alloy.In the temperature range of 373?573 K,with the increase of Ca content,the average CTE of the as-cast Mg-Ca binary alloys shows a gradual decline(from 29.8×10^-6 K^-1 to 28.7×10^-6 K^-1).The CTE of the extruded Mg-y Ca alloys at 373?573 K is lower than that of the as-cast alloys.The addition of Ca significantly improves the strength of the magnesium alloy,which is attributed to the second phase being broken after extrusion deformation,and the dispersed fine second phase plays a role of strengthening the second phase.