Microstructure Design And Property Modification Of Mg-4Li-1Ca-X Alloys

Mg-4Li-1Ca(LX41)alloy as a new-type ternary magnesium-lithium alloy system possesses the advantages such as low density,excellent mechanical properties and good biocompatibility and has broad application prospects in light-weight structural and biomedical materials fields.However,coarse eutectic Mg₂Ca phase is easily formed in the LX41 alloys prepared by conventional ingot metallurgy and distributed along the grain boundaries,which greatly reduces the ductility,toughness and strength of the alloy and makes subsequent deformation process difficult.So it has important theoretical research significance and engineering practical value to fabricate the high-performance LX41 alloy structural materials by reasonable composition design and process control to refine and distribute uniformly second-phase particles in as-cast alloys and improve the mechanical property and the formability of as-cast alloys,and subsequently by rolling and ECAP processing to refine the microstructure further and distribute the particles homogeneously.In this paper,the effects of minor pure Al and Mg-Al-Ca master alloy on the microstructure and mechanical properties of LX41 alloys have been investigated by conventional gravity casting method.And the influences of Al-Si master alloy additions on the microstructure and mechanical property of Mg-4Li-1Ca-Al alloys are also explored.Solid solution treatment system of Mg-4Li-1Ca(Mg-8Al-18Ca)-0.5Si(Al-12Si)alloy was investigated in detail,and the optimal solution heat treatment process was established.Subsequently,ECAP processing was performed on various Si-containing alloys to investigate the effect of severe plastic deformation on the microstructure transformation and mechanical properties of the alloys.XRD,OM,SEM/EDS were used for phase constituent identification the,microstructure observation and fracture analysis of the as-cast,heat-treated,as-rolled and as-ECAPed alloys.The tensile mechanical properties of the alloys were tested using the Instron universal tensile testing machine.The main conclusions are as follows:(1)With a small amount of pure Al addition into Mg-4Li-1Ca(Mg-30Ca)alloy,the second phase in the as-cast alloy changes from Mg₂Ca to(Mg,Al)₂Ca.There is an obvious increase in the particle number too.While the major secondary phase particles are distributed at the grain boundary as coarse network binary eutectic?-Mg+(Mg,Al)₂Ca.When Ca and Al are added as Mg-8Al-18Ca eutectic master alloy,the number of second phase particles in the as-cast alloy increases dramatically,and?-Mg grains are significantly refined.At the same time,a large number of nano-scale(Mg,Al)₂Ca cluster particles were evenly dispersed in the?-Mg grains.The resultant tensile strength and elongation of the as-cast alloy are 119 MPa and 3.6%,respectively,which are increased by 48.7%and 163.0%;30.7%and 157.1%,respectively,compared with those of the alloys without Al and pure Al addition.Due to the coarse grain size of the matrix and the distribution of the coarse second-phase particles along the grain boundaries,the grains of the as-cast Mg-4Li-1Ca(Mg-30Ca)and Mg-4Li-1Ca(Mg-30Ca)-0.44 Al alloys subject to rolling were elongated to form a typical band-like structure.While the?-Mg matrix grains and the second-phase particles of as-rolled Mg-4Li-1Ca(Mg-8Al-18Ca)alloy are significantly refined and uniformly dispersed,and a more uniform and finer microstructure are formed.The mechanical properties of the alloy are greatly enhanced with the yield strength of 103 MPa,tensile strength of 213 MPa and elongation of 10.3%.(2)For Mg-4Li-1Ca(Mg-8Al-18Ca)-x Si(Al-12Si)(x=0;0.2;0.5;1 wt.%)alloys,the alloy is mainly composed of?-Mg and(Mg,Al)₂Ca phase composition when x is 0 wt.%.When x=0.2 wt.%,a new Al Li phase and Ca Mg Si phase are formed,and the Al Li phase nucleates and grows with(Mg,Al)₂Ca phase as the nucleation core.When x=0.5 wt.%,the clusters of Al Li phase and(Mg,Al)₂Ca phase are aggregated in clusters.When x=1 wt.%,new blocky Mg₂Si phase is formed in?-Mg.Due to the fine particle size,small number and uniform distribution of the second phase particles,Mg-4Li-1Ca(Mg-8Al-18Ca)-0.2Si(Al-12Si)alloy has the highest comprehensive mechanical properties with the yield strength of 86 MPa,tensile strength of 170 MPa and elongation of 3.7%.In alloys with higher Al and Si content,however,a large number of second phases are aggregated at the grain boundaries with coarse network clusters,resulting in a drastic reduction in the ductility,toughness and strength of the alloy.(3)After the solution treatment of Mg-4Li-1Ca(Mg-8Al-18Ca)-0.5Si(Al-12Si)alloy,the coarse network(Mg,Al)₂Ca eutectic phase first decomposes into fine particles first,and then follows the extension of time has occurred again and grew up;the Al Li phase of the lamellar clusters continued to decompose with the increase of the solution temperature and time,and a large amount of solid solution dissolved in?-Mg grains to form nano-solute clusters,while the Ca Mg Si phase structure is stable,does not dissolve nor grow up at all temperatures.Comprehensive analysis of microstructure changes and microhardness results of solution treatment,the best solution treatment process of the alloy was determined to be 400?for 1h.(4)After the alloy is treated by ECAP,the Al Li phase and the(Mg,Al)₂Ca phase are separated.The(Mg,Al)₂Ca phase is distributed in the form of small particles on the?-Mg grain boundary,and the Al Li phase is evenly dispersed in the matrix among them,the alloy matrix is significantly strengthened.The Ca Mg Si phase and Mg₂Si phase did not change during the extrusion process.The extruded Mg-4Li-1Ca(Mg-8Al-18Ca)-1Si(Al-12Si)alloy has the best mechanical properties.In the as-cast state,the network-like second-phase particle clusters formed by the Mg-4Li-1Ca(Mg-8Al-18Ca)-1Si(Al-12Si)alloy at the grain boundaries are broken,refined and uniform during the hot extrusion process,the alloy matrix is significantly refined,and high-density nanoparticles are precipitated so that the yield strength,tensile strength and elongation of the alloy are 150 MPa,293 MPa and 9.6%,respectively,compared with Mg-4Li-1Ca(Mg-8Al-18Ca)-0Si(Al-12Si)alloy increased by 92.3%,87.8%and 96.0%respectively,the mechanical properties have been greatly improved.