Researches On Preparation, Microstructure And Mechanical Properties Of High Strength And Superplastic Mg-Li Alloy

Mg-Li alloy has the lowest density in structural metallic materials possessing highspecific strength and good plasticity, which shows a substantial application in the areas ofaerospace, automobile, the3C electronics, etc. However, the strength of Mg-Li alloy is poor(σb≤200MPa), which limit the further application of Mg-Li alloy. To resolve the problemabove, a new design Mg-10.73Li-4.49Al-0.52Y alloy was investigated in this paper. Bysevere plastic deformation and heat treatment, etc., the Mg-Li alloy with high strength orsuperplasticity was achieved.Firstly, the comprehensive mechanical properties of Mg-10.73Li-4.49Al-0.52Y wereimproved through equal channel angle processing (ECAP), cold rolling and short timeannealing. The initial grains of the as-cast alloy are coarse and β-phase occupies the mainposition, while α-phase distributes inside the β-phase or along the boundary. Meanwhile, thereare lots of precipitate phases Al2Y and AlLi in the grain. Prominent effect of dynamicrecovery (DR) impedes the process of dynamic recrystallization (DRX) during deformation.While after a short time annealing, the alloy processed by12pra (ECAP, cold rolling andannealing) is completely recrystallized with grain size refined to27μm. The ultimate strengthand elongation of the as-cast alloy are131.1MPa and47.1%, respectively. Due to the effectof DR and dislocation-coordinated deformation, the elongation of the alloy processed by12pr(ECAP and cold rolling) reaches90.5%. Subsequent short time annealing treatment leads tothe increase of strength (ultimate strength of237.6MPa for12pra) and a significant decreasein elongation, which can be explained by the dislocation source-limited hardening and grainboundary strengthening mechanism.Secondly, for preparing high strength Mg-Li alloy, solid-solution treatment and largestrain rolling (LSR) for2passes with total reduction of75%were conducted. Aftersolid-solution treatment, the α-phase and AlLi phase disappear. With the increase ofdeformation temperature, θ-phase and AlLi precipitated in turns. When the deformationtemperature reaches623K, typical ‘necklace’ shape of microstructure appears due to theeffect of continuous dynamic recrystallization (CDRX), meanwhile the phase composition ofthe alloy is the same with the solid-soluted one. The Mg-Li alloy rolled at623K with75% reduction shows excellent ultimate tensile strength of327.7MPa, which is mainly due to themechanisms of solid-solution strengthening and fine-grain strengthening.Lastly, tensile testing at elevated temperature was conducted at473~623K with initialstrain rate of5×10-4~1×10-2s-1on Mg-Li alloy, which was deformed by ECAP with8passesBCroute at373K. An average coarse grain size of155μm is obtained after ECAP process.Results indicate the Mg-Li alloy exhibits good superplasticity with a maximum elongation of512%at523K with5×10-4s-1, and even with1×10-2s-1, the elongations exceed150%at473~623K. The value of the strain rate sensitivity is0.53and the activation energy is90kJ/mol, indicating that the dominant deformation mechanism for this alloy is grain boundarysliding controlled by a combination of grain boundary diffusion and lattice diffusion. Besides,the nucleation, growth, and coalescence of cavities are the main failure mechanism duringsuperplastic deformation.