Influence Of Al-Si Additions On The Microstructure And Mechanical Properties Of Mg-Li Alloys

Due to the high specific strength,low density,good electromagnetic shielding characteristics,magnesium alloys have attracted many attentions from a large variety of industries like 3C(Computer,Communication and ConsumerElectronics),electron and aerospace.Mg-Li alloy is the lightest structural metallic materials and is also called super-light magnesium alloy.It has a low density,high specific strength and good plastic processing advantages,has been widely used in the aerospace,military,electronic and other fields.The existing Mg-Li alloy can be classified into single ? phase,? + ?diphasic and single ? phase.The single ? phase has poor plasticity and ? + ?diphasic plasticity is improved,but the strength needs to be further improved.Single ?-phase strength is low while its plasticity is high.All Mg-Li alloy have poor heat resistance and corrosion resistance.In this paper,the effects of adding Al-Si eutectic alloy and plastic deformation and heat treatment on the microstructure and mechanical properties of Mg-Li alloy were investigated.In this paper Al-Si eutectic alloy was prepared by magnetic levitation induction heating and argon gas protection heating.Mg-4Li-x(Al-Si),Mg-8Li-x(Al-Si)and Mg-12Li-x(Al-Si)were obtained by mixing with pure magnesium and pure lithium in a mass ratio and maglev smelting.Alloy sheet of1.6 mm thick was obtained by multi-pass hot rolling,and the mechanical properties of the rolled and heat treated sheets were compared.Themicrostructures and mechanical properties of the alloys were characterized by optical microscopy,scanning electron microscopy,transmission electron microscopy and nanoindentation.The organizational structure and strengthening mechanism of Mg-Li alloy with different Al-Si contents were studied,and on the basis of this,the effect of strong plastic deformation on the microstructure and properties of Mg-12Li-3(Al-Si)alloy was further explored by means of equal diameter extrusion.Research indicates:The addition of Al and Al-Si in a single ?-phase Mg-4Li alloy resulted in a significant increase in the strength of the alloy and a decrease in elongation.However,the strengthening effect of Al-Si eutectic alloy on Mg-4Li alloy was higher than that of adding single Al.The tensile strength and elongation of Mg-4Li-3(Al-Si)were 289 MPa and 12.2% respectively,while those of Mg-4Li-3Al were 274 MPa and 11.2% respectively.With further addition of Al-Si,the tensile strength of Mg-4Li-6(Al-Si)alloy is further improved,but the elongation decreases,which is 359 MPa and 10.2% respectively.The mechanism of Mg-4Li-3Al was characterized by EBSD technique,and the strengthening mechanism was mainly twinning and second-phase strengthening.The TEM characterization of Mg-4Li-6(Al-Si)was carried out.The strengthening mechanism was mainly the second phase strengthening and fine grain strengthening,and the second phase strengthening phase was small granular Al3 Li and massive Mg2 Si.Tensile strength of Mg-4Li-3Al,Mg-4Li-3Al,Mg-4Li-3(Al-Si)and Mg-4Li-6(Al-Si)decreased in a smallamount after annealing,but the elongation was improved.The addition of Al-Si to ?+? biphasic Mg-8Li alloy significantly improved the tensile strength of the alloy,but the elongation was reduced.Mg-8Li-5(Al-Si)has the best comprehensive mechanical properties,tensile strength of 240 MPa,elongation of 12.5%.With the increase of Al-Si eutectic,the tensile strength of the alloy continues to increase,but the elongation decreases rapidly.The morphologies of Mg-8Li-x(Al-Si)were characterized by OM and XRD.The results showed that ? phase changed from Al-Si to Mg-Si-Mg-Si-phase.The tensile strength and elongation of the sheet were improved after the homogenization of the ingot,and then the nano-indentation test of Mg-8Li-5(Al-Si)was carried out,and the loading and elastic modulus are higher than that of ? phase.The addition of Al-Si to the single ?-phase Mg-12 Li alloy significantly improved the strength of the alloy while maintaining a high elongation.The precipitated phases in Mg-12Li-x(Al-Si)were characterized by SEM and XRD.The results show that the precipitated strengthening phase is AlLi and Mg2 Si phase.With the increase of the amount of Al-Si eutectic,the precipitated phase of Mg2 Si is dispersed and agglomerated,and the tensile strength of the alloy increases first and then decreases.When the amount of Al-Si eutectic alloy is9%,white particles of brittle phase Mg2 Si in Mg-12Li-9(Al-Si)alloy largely aggregated,alloy strength and plasticity decreased.The tensile strength is 195 MPa,and the elongation is close to 30%.The tensile strength of Mg-12Li-7(Al-Si)is the best.