Effect Of Alloying Elements On Microstructure And Mechanical Properties Of Mg-8Li Alloy With Duplex Structure

Mg-Li alloys are well known for their high specific stiffness,and specific strength.They also possess such advantages as good machinability,good magnetic shielding performance and high damp resistance.Mg-Li alloys can be widely used in fields like aerospace exploration,aviation industry,automotive,3C application,and medical aid.While the wide use of Mg-Li alloys is hindered by relatively low strength and low thermodynamic stability.Research on such fields can be beneficial for both theory and application.In this thesis,Mg-8Li alloy is chosen as a basic alloy for its good comprehensive mechanical properties to develop alloys with high performance.Mg-8Li-3Al-2Zn-xNd,Ce-riched RE alloys and Mg-8Li-x（3Zn-Gd）are prepared by steel diecasting.Effects of Nd and RE on microstructure and properties are studied.Influence of extrusion process was studied too.Aging behavior of Mg-8Li-3Al-2Zn-xRE is studied as well.Analysis methods such as SEM,XRD,tensile test,OM are applied.The results show that Mg-8Li-3Al-2Zn is mainly contains two phases of nearly the same ratio,α-Mg andβ-Li..AlLi,MgLi₂Al phase are also detected.The density of is lower than pure Mg,and has a good property with the UTS higher than 220MPa and elongation of 18%.With Nd addition,shape ofα-Mg is refined from d dendritic to ribbon-shaped.Al₂Nd phase is formed while change in mechanical properties is not large.The content of Nd got in experiment may be responsible for it.After hot extrusion,DRX appeared and refined theβ-Li.Phase greatly.Both strength and elongation are enhanced.Mg-8Li-3Al-2Zn-0.5Nd-xRE was prepared to investigate RE influence on alloy.Results showed that with RE addition growing bigger,ratio of Al-RE phase increases andα-Mg is refined.As the tensile test shows,RE deceased alloy strength and improved ductility,which is not usually the case.Elongation of Mg-8Li-3Al-2Zn-0.5Nd-3RE is even bigger than that of Mg-8Li.It can be explained for the refinement ofα-Mg and cost of Al for Al-RE phase formation.Aging study conducted show that the aging softening is not obvious in as-cast Mg-8Li-3Al-2Zn-0.5Nd-xRE alloys.Zn and Gd addition in L8 resulted in formation of I-phase and W-phase,which can be strengthening for alloy.Zn,Gd addition resulted in great refinement inα-Mg.Strength increases as elongation decreases.Solid soluting strengthening and formation of I phase and W phase is responsible.When Zn and Gd are 9 and 3 wt%respectively,strength reaches the highest of 259MPa,while the elongation is as low as 4.7%.Hot extrusion is good for both strength and elongation of the alloys,too.The extruded Mg-8Li-3Zn-1Gd has a elongation higher than Mg-8Li,for which refinement ofα-Mg and DRX inβ-Li phasemay be responsible.After extrusion,strength of Mg-8Li-6Zn-2Gd and Mg-8Li-9Zn-3Gd is decreased compared to that of as-cast alloys.The possible reason for the decrease is the crushed particle secondary phase has poorer interaction with dislocation glide.