Effect Of Master Alloys On Microstructure And Mechanical Property Of Dual Mg-Li Alloys

As the lightest metal structural material,Mg-Li alloy has the advantages of high specific stiffness,high specific strength and good electromagnetic shielding performance.It has wide application prospects in aerospace,automobile,military and 3C fields.However,Mg-Li alloy has many disadvantages,such as low absolute strength,poor corrosion resistance,low heat resistance and high cost,which greatly limits its wide application.Therefore,the research on composition design,melting preparation and thermomechanical treatment of non-rare earth high-strength and heat-resistant Mg-Li alloys has important theoretical and engineering practical significance for the development and application of new high-performance,low-cost ultra-light Mg-Li alloys.In this paper,different contents of Al-5Ti-1B,Al-12.6Si,Mg-30Ca non-rare earth master alloys were added to dual Mg-Li alloys to change the structure,relative content and alloy elements.Strength,plasticity and toughness of Mg-Li alloy were improved by solid solution in matrix or formation of second phase particles.Mg-Li alloy was prepared by gravity casting.The effect of microstructure transformation and mechanical properties of Mg-Li alloy during as-cast,heat treatment and rolling were investigated.XRD,OM,SEM/EDS and TEM were used to identify the phase composition,microstructure and fracture analysis of as-cast,heat-treated and rolled Mg-Li alloys.The mechanical properties of the alloy were tested by Instron tensile tester.The main conclusions are reached as follows:?1?Minor master alloys,Al-5Ti-1B,Al-12.6Si and Mg-30Ca are added to Mg-9Li dual alloys to explore the micro-structures of?-Mg,?-Li and second phase and their mechanical properties.The effect of adding minor Al-5Ti-1B,Al-12.6Si and Mg-30Ca is significantly remarkable.Al-5Ti-1B can significantly increase the volume fraction of?-Mg phase in Mg-9Li alloy,forming Mg-Li dual phase alloy with coarse?-Mg phase as matrix,and generate a large amount of MgLi₂Al phase and minor TiB₂ fine particles are dispersed in?-Li phase boundary,which increases tensile strength,decreases yield strength slightly and plasticity significantly.Al-12.6Si can refine the?-Mg phase in Mg-9Li alloy remarkably and generate a large amount of MgLi₂Al and Mg₂Si submicron particles,which reduces the relative content of?-Mg,and increases the yield strength and tensile strength of Mg-9Li alloy.However,due to the formation of the brittle hard phase Mg₂Si,its plasticity is the worst.Mg-30Ca has the best effect on the refinement of?-Mg phase in Mg-9Li alloy,forming high-density nano-scale Mg₂Ca particles,which causes the?-Mg phase volume fraction to decrease sharply,the yield strength and tensile strength of the alloy are significantly improved.It also has excellent plasticity and its overall mechanical properties is the best.Ca addition into as-cast Mg-9Li-0.5Si?Al-12.6Si?alloy can effectively refine of?-Mg grains and Mg₂Si particles,and form?Mg,Al?₂Ca phase,which improves the overall mechanical properties of the alloy.With the increasing of Ca content from 0.25 wt.%to 1.0 wt.%,?-Mg tends to be refined first and then coarsened.When Ca content is 0.5 wt.%,the refining effect of?-Mg is the most obvious.Under the combined action of fine grain strengthening and fiber strengthening,the alloy has the best comprehensive mechanical properties.With the increasing of Ca content to1.0 wt.%,?Mg,Al?₂Ca reticular eutectic phase formed,the matrix was split,and the overall properties decreases.Compared with as-cast alloys,the grains of Mg-9Li-0.5Si?Al-12.6Si?-xCa?x=0.25;0.5 wt.%?alloys are refined after solution treatment for 3 hours and rolling.When Ca content is0.25 wt.%,?-Mg is flattened and elongated.Under the combined action of solution strengthening,fine-grained strengthening and fiber strengthening,the tensile strength of the alloy increases obviously.However,due to the aggregation of granular second phases at grain boundaries,stress concentration occurs,resulting in poor plasticity of the alloy.When Ca content is 0.5 wt.%,the volume fraction of?-Mg phase decreases,and the number of granular second phase decreases.The alloy has the best comprehensive mechanical properties.The addition of 0.5 wt.%Al-5Ti-1B into as-cast Mg-8.4Li-3?Al-12.6Si?alloy can effectively refine the grains of?-Mg and Mg₂Si,promote the uniform dispersion of grains,and significantly improve the strength of the alloy.When the content of Al-5Ti-1B increases to 1.0 wt.%,the refinement effect of grains is better,TiB₂ grains increase and disperse evenly,the comprehensive mechanical properties is the best.However,when Al-5Ti-1B content is 1.5 wt.%,TiB₂ particles agglomerate,which reduces the strength and plastic toughness of the alloy.Comparing with as-cast alloy,Mg-8.4Li-3?Al-12.6Si?alloy with different content of x?Al-5Ti-1B??x=0.5,1.0,1.5 wt.%?is refined and strengthened obviously after rolling.However,the elongation decreases due to the stress concentration caused by the accumulation of a large number of second phase particles.With the increasing of Al-5Ti-1B content from 0.5 to 1.0 wt.%,the strength of the alloy increases gradually with the refinement of?-Mg.However,when the content of Al-5Ti-1B master alloy is 1.5 wt.%,the strength of?-Mg alloy decreases;When the content of Al-5Ti-1B master alloy is 1.0wt.%,the refinement effect is the most obvious and the comprehensive mechanical properties is the best.