Effects Of Alloying Elements On Microstructure And Mechanical Properties Of Ultra-light Mg-14Li-xZn/Al Alloys With Single β-Li Structure

Mg-Li alloy is the lightest metal engineering material,which possesses broad applications in the domains of aviation industries and military affairs.LA141（Mg-14Li-1Al）is super-light alloy among the commercial Mg-Li alloys.But its wide uses are limited by existing problems involving low strength and worse age hardening stability.In this thesis,superlight Mg-14Li is chosen as the based alloy.It has great significance to improve strength of the Mg-14Li based alloy with no obvious increase of density.Results are as follows:First,effects of Zn,Y,Gd elements on microstructural evolutions and mechanical properties of Mg-14Li based alloy are investigated.Results show that Mg-14Li-xZn-yY/Gd alloys are characterized by predominant singleβ-Li phase.W-phase and MgLiZn phase appear after additions of Zn and Y elements which are enriched in those precipitates.While additions of Zn and Gd introduce the existence of MgGd₃,GdMg,Mg₂Zn₁₁ and Zn phases,and most of Gd elements are dissolved into the matrix.After hot extrusion,dynamic recrystallization（DRX）significantly refinesβ-Li equiaxed grains,and broken precipitated phases are distributed along the extrusion direction.But research finds that strengthening effects of Zn,Y or Zn,Gd additions are limited.Theory analysis shows that the as-cast Mg-14Li-6Zn-2Gd alloy possesses higher comprehensive properties with yield strength（YS=116.5MPa）,ultimate tensile strength（UTS=125MPa）and elongation（EL=26.1%）under the great help of grain refinement strengthening,second phsae strengthening and solid solution strengthening.But its density is over to 1.42 g/cm³.Based on the situation of limit strengthening by Zn,Y or Zn,Gd addition,influences of Al,Y addition on microstructure and mechanical properties of Mg-14Li based alloy are studied systematically.Results show that Mg-14Li-xAl alloys mainly consist ofβ-Li and MgLiAl₂ phases.The increased Al addition greatly improves the strength of the Mg-14Li based alloy because of solid solution of Al elements,accompanied by a significant reduction in ductility.Fracture model is found to illustrate the transform of rupture type in different alloys.Research finds that addition of Y element leads to the introduction of homogeneous distribution of Al₂Y particles in the matrix of Mg-14Li-3Al alloy and grain refinement,which play a positive role in strength enhancement.The as-cast Mg-14Li-3Al-0.5Y alloy has the better combination of strength and EL（YS:187.5MPa,UTS:205.5MP,EL:3.1%）.After hot extrusion,β-Li phases obtain a great further refinement by DRX,and alloys are strengthened with worse EL.Based on Mg-14Li-3Al-0.5Y alloy with high strength and low density,after addition of Ca element,semicontinuous netlike Al₂Ca phases appear along grain boundaries,which consume the dissolved Al atoms.β-Li grains has a significant refinement,too.Elongation of Mg-14Li-3Al-0.5Y alloy improves to 7.0%,while strength decreases after 0.5%Ca addition.Similarly,obvious grain refinement ofβ-Li occurs with the addition of Mn contents leading to homogeneous distribution of precipitated Mn phase and Al₂Mn₃ phase,which makes a better enhancement in EL.As a result,the superlight Mg-14Li-3Al-0.5Y-0.2Mn alloy is developed.Its density is 1.36 g/cm³.The YS,UTS and EL are 144.0MPa,175.7MPa and 11.0%,respectively.