Study On Mechanical Properties And Corrosion Behavior Of Mg-14Li Based Alloys With BCC Structure

As ultra-light metallic materials,magnesium-lithium(Mg-Li)alloys have a wide range of application prospects in aerospace,military and medical fields due to specific strength,easy formability,good low-temperature impact resistance and excellent electromagnetic shielding performance.However,the shortcomings such as low absolute strength,plastic instability,poor thermal stability and corrosion resistance limit Mg-Li alloys development and application.With the increase of lithium content,the crystal structure of Li-Mg alloys gradually change from a close-packed hexagonal structure(HCP)to a body-centered cubic structure(BCC).Compared with the traditional Mg alloys with a HCP structure,the BCC Mg-Li alloys have more slip systems,which is more conducive to plastic processing,and has huge application potential in actual industrial production.Therefore,in this paper,we took BCC structure Mg-14Li(wt.%)based alloys as the research object,and explored its mechanical and corrosion failure mechanism via microstructure characterization,mechanical property test,and corrosion behavior analysis to provide a certain theoretical basis for the research and application of ultra-light Mg-Li alloys.Through investigation and comparing the microstructure,mechanical properties and corrosion behavior of as-cast Mg-14Li and Mg-14Li-8Zn alloys,we found that the microstructure was consisted of?-Li and?-Li/Mg7Zn3 eutectic phase after adding Zinc element.The comprehensive mechanical properties of Mg-14Li-8Zn alloy were significantly improved that the yield strength increased from 57.6MPa to 148.8MPa,the tensile strength increased from 89.6MPa to 173.1MPa,and the elongation increased from 17.2%to 22.6%,which was mainly due to fine grain strengthening,solid solution strengthening and precipitation strengthening.However,in 0.1M/L Na Cl solution,the corrosion resistance of Mg-14Li-8Zn alloy was greatly reduced.The two alloys had the same composition of corrosion surface film mainly including Mg O,Mg(OH)₂,Li₂CO₃and Li OH.Therefore,the main reason for the low corrosion resistance of the Mg-14Li-8Zn alloy was that the corrosion micro-electric couple formed by the?-Li/Mg₇Zn₃eutectic phase and the?-Li phase accelerates the anodic dissolution of the?-Li phase matrix.Through investigation and comparing the microstructure,mechanical properties and corrosion behavior of the hot-rolled Mg-14Li alloy and Mg-14Li-8Zn alloy,it is found that when the reduction ratio is 37.5%,the Mg-14Li alloy has obvious(110)rolling texture.When the reduction rate is increased to 75%,the dynamic recrystallization degree of Mg-14Li alloy increases,resulting in a decrease in the strength of the(110)rolling texture and an increase in the strength of the(200)texture,which causes obvious mechanical anisotropy.Compared with Mg-14Li alloy,the addition of zinc element reduces the texture effect of Mg-14Li-8Zn alloy(reduction rate37.5%)and improves its mechanical properties.When the reduction rate is 75%,the dynamic recrystallization effect of the Mg-14Li-8Zn alloy is significant,the texture strength is further reduced,the elongation is increased to about 1.7 times,and the mechanical anisotropy is not obvious.In addition,the corrosion resistance of Mg-14Li alloy decreases as the reduction rate increases.This is because more grain boundaries and grains with different orientations increase the number of defects in the alloy corrosion surface film,which leads to an increase in pitting corrosion sources and accelerating Corrosion.However,the corrosion resistance of the Mg-14Li-8Zn alloy increases with the increase in the reduction rate.This is due to the large amount of plastic deformation that promotes dynamic recrystallization,and the?-Li/Mg7Zn eutectic phase is severely fragmented and Dispersed in the matrix,the micro-galvanic corrosion effect of the eutectic phase and the?-Li phase is weakened,and the corrosion resistance is improved.Through investigation and comparing the microstructure,mechanical properties and corrosion behavior of Mg-14Li-8Zn alloy before and after heat treatment,it is found that after solution treatment(T4,400?/4h),the?-Li/Mg7Zn eutectic phase is completely dissolved,and the alloy is a single?-Li phase,and after aging treatment(T6,T4+200?/6h),LiMgZn phase precipitated from?-Li matrix phase and distributed in rod shape along grain boundary and spherical shape in crystal.Due to the dynamic strain aging effect,both the T4 and T6 alloys exhibit obvious sawtooth rheology when they are stretched and deformed at room temperature.Compared with the as-cast alloy,the mechanical properties of the solid-solution alloy are reduced due to the growth of the grain size,and the mechanical properties of the aged alloy are slightly improved compared with the solid-solution alloy due to the dispersion strengthening of the precipitated LiMgZn phase.The mechanical properties are still lower than the as-cast alloy.In addition,for corrosion resistance,as-cast alloys with continuously distributed eutectic phases have the worst corrosion resistance,while the corrosion resistance of solid solution and aged alloys has been greatly improved,and the structure of solid solution alloys is uniform and corrosion resistant.The performance is the best,followed by the aged alloy containing the dispersed LiMgZn phase.