Study On Mechanical Properties And Corrosion Behavior Of As-cast Mg-11Li-xZn Alloys

As the lightest structural metal material at present,magnesium lithium(Mg-Li)alloy has the advantages of high specific strength and stiffness,good seismic performance and good electromagnetic shielding performance.It has been applied in many aerospace vehicles,including the"Zhurong Mars Rover".However,the low absolute strength and poor corrosion resistance of Mg-Li alloy seriously limit its wide application.In addition,the crystal structure of Mg-Li alloy gradually changes from close-packed hexagonal structure(HCP)to body-centered cubic structure(BCC)with the increase of Li content.Compared with the traditional magnesium alloy with close-packed hexagonal structure,the magnesium lithium alloy with body-centered cubic structure has more slip system,which is more conducive to plastic processing and forming,and has great potential in practical industrial production and application.Therefore,taking Mg-11li-xZn(x=0,1,3,6,wt.%)alloy with BCC structure as the research object,this thesis explores its mechanical and corrosion failure mechanism and its relationship with microstructure through microstructure characterization,mechanical property test and corrosion behavior analysis,so as to provide a certain theoretical basis for the development and application of new ultra light magnesium lithium alloy.By comparing the microstructure,mechanical properties and corrosion behavior of as cast Mg-11Li-xZn(x=0,1,3,6)alloy,the results show that after adding Zn element,phase?'(Mg Li₂Zn),?(Mg Li Zn)and?-Mg precipitate in the alloy?-Li matrix,which increases with the increase of Zn element content,and the strength and plasticity of the alloy are improved simultaneously.Among them,the yield strength of Mg-11Li-6Zn alloy is increased from 48 MPa to 173 MPa,the tensile strength is increased from 72 MPa to 184 MPa,and the elongation is increased from 19%to 30%.The increase of alloy strength is mainly due to the fact that the high-density and uniformly distributed?'(Mg Li₂Zn)hinders the dislocation movement,while the?(Mg Li Zn)and?-Mg precipitated along the grain boundary play a positive role in the improvement of alloy plasticity.In terms of corrosion resistance,the corrosionresistanceofMg-11Li-xZnseriesalloysis:Mg-11Li-1Zn>Mg-11Li-3Zn?Mg-11Li>Mg-11Li-6Zn.It can be seen that a small amount of Zn element can improve the corrosion resistance of the alloy,while excessive Zn element plays the opposite role.According to the analysis of the corrosion surface film,it is found that the as-cast Mg-11Li alloy forms a double layer corrosion surface film:the main components of the first layer are Mg(OH)₂and Li₂CO₃,and the second layers are mainly Li OH·H₂O and Mg O.The structure of the film is closely related to the corrosion resistance of the alloy.After adding a small amount of Zn,the corrosion surface film of Mg-11Li-1Zn alloy is thickened,which provides better protection.After adding excessive Zn element,the second phase and matrix in the alloy will produce significant micro galvanic corrosion effect,accelerate the corrosion of the alloy and reduce the corrosion resistance of the alloy.By comparing the microstructure,mechanical properties and corrosion behavior of Mg-11Li-xZn(x=0,1,3,6)alloy before and after heat treatment,the results show that after solution heat treatment,the original?'(Mg Li₂Zn),?(Mg Li Zn)and?-Mg phases in the alloy are dissolved in the matrix,and a large number of new strip?'(Mg Li₂Zn)precipitates in the grain;After aging treatment,there are secondary precipitation of?(Mg Li Zn)and?-Mg phase in the alloy,while strip?'(Mg Li₂Zn)gradually dissolves and large granular?'(Mg Li₂Zn)reappears.For mechanical properties,the strength and plasticity of binary Mg-11Li alloy are slightly improved after solution heat treatment;The mechanical strength of Mg-11Li-6Zn alloy increases but the plasticity decreases,in which the tensile strength increases to 235Mpa and the plasticity decreases to 2.5%;Mg-11Li-1Zn and Mg-11Li-3Zn alloys containing a small amount of Zn elements show a synchronous decrease in strength and plasticity,but still maintain ductile fracture.After solution treatment,the change of mechanical properties of Mg-11Li-xZn alloy is mainly attributed to the dissolution of primary?(Mg Li Zn)and?-Mg phase which regulate toughness,primary?'(Mg Li₂Zn)which regulates strength,and the precipitation of new strip?'(Mg Li₂Zn).When the strengthening effect of the new strip?'(Mg Li₂Zn)phase is dominant,such as the typical as-solubilized Mg-11Li-6Zn alloy,the mechanical strength is greatly improved,but the plasticity decreases sharply;When the dissolution of the original second phase plays a leading role and there are few new?'phases,the strength and plasticity of the alloy decrease.After aging treatment,the plasticity of Mg-11Li-6Zn alloy begins to recover,but the strength decreases,and this trend is more obvious with the increase of aging time.The reason is that?and?-Mg phases in the alloy gradually precipitate during aging treatment,and the plasticity of the alloy gradually recovers,but at the same time,the micro strip?'phase in the alloy matrix dissolves after solid solution treatment,and the secondary granular?'phase re dominates the strengthening effect on the alloy.For corrosion behavior,solution treatment can significantly improve the corrosion resistance of Mg-11Li-6Zn alloy,which is mainly due to the complete dissolution of the primary second phase in the as-cast alloy,avoiding the micro galvanic corrosion effect,so as to significantly improve the corrosion resistance of the as-solubilized alloy.