Effects Of Zn And Sn Elements On Microstructure And Mechanical Properties Of β/α+β Mg-li Alloys

Magnesium-lithium alloy is the lightest structural metal material in the world,with excellent electromagnetic shielding and damping properties.It has become one of the most ideal and promising structural materials in the fields of aerospace,automotive,3C electronics,medical equipment,and more.However,low strength is an important factor currently restricting the large-scale application of magnesium-lithium alloys.Additionally,due to the short development time,the research on the design and deformation processing of magnesium-lithium alloys is not sufficient,and many alloy elements and processing methods have not been applied to the development of new magnesium-lithium alloys.By designing the alloy composition and combining different rolling processes,mechanisms such as precipitation strengthening and work hardening can be introduced to enhance the strength of magnesium-lithium alloys.This is of great significance for the development of new high-strength magnesium-lithium alloys.In this thesis,more than 6 wt.%of Zn was added intoβ-phase Mg-Li alloy the microstructure and mechanical properties of the alloys were investigated.When the Zn content was 9 wt.%,Zn was dissolved in the grain in the form of nano-B2 phase.The B2 phase has a good semi-coherent relationship with the matrix,which increased the compressive yield strength of Mg-13Li-9Zn to 300 MPa through coherent strengthening,modulus strengthening and order strengthening.When the Zn content increased to 15 wt.%,nanoscale B2 phase transformed into coarse L1₂-type Mg Li₂Zn phase,which reduced the compressive yield strength of alloy to 205 MPa.When Zn further increased to 22 wt.%,Mg Li₂Zn phase transformed into disordered Mg Li Zn phase with further increased size,and compressive yield strength of Mg-13Li-22Zn alloy was 225 MPa.Due to weakened binding energy at grain boundaries caused by coarse second phases,as-cast Mg-13Li-9Zn,Mg-13Li-15Zn and Mg-13Li-22Zn alloys all exhibited brittle intergranular during tensile testing.The quenching treatment of Mg-13Li-15Zn and Mg-13Li-22Zn produced a dispersed nano-scale decomposition zones in the matrix.The size of this zone decreased as the Zn content increases.The two alloys exhibited a hardness of 115 HV and 125 HV,respectively,due to the combined effect of solid solution strengthening and decomposition strengthening.Both alloys exhibited natural aging hardening behavior,with the Mg-13Li-15Zn alloy reaching its peak aging value after being placed at room temperature for 175 hours.The increase in Zn content accelerated the aging kinetics,resulting in a peak aging time of 140 hours for the Mg-13Li-22Zn alloy.Additionally,its peak-aged hardness was 1.1 times higher than that of Mg-13Li-15Zn’s peak-aged hardness.The natural aging hardening behavior of both alloys is attributed to the precipitation and growth of the nano B2 phases.After the Mg-13Li-9Zn alloy underwent multi-pass hot rolling（M-HR）,complete dynamic recrystallization happened with a grain size of approximately 4μm.This increased the yield strength to 280 MPa,and the fracture transformed from the brittle intergranular mode into a ductile mode with an elongation percentage of 22%.In order to shorten the rolling time,single-pass large strain rolling（S-LSR）was performed,which further refined the recrystallized grain size to 400 nm and suppressed the excessive growth of B2 phase.As a result,the yield strength increased to 308 MPa.However,this caused a decrease in elongation percentage to 12%,and the fracture mode became a ductile-brittle mixed fracture.After further quenching and annealing treatment of the as-cast Mg-13Li-9Zn alloy,the second phase melted back into the matrix,resulting in an over-saturated solid solution structure with pure grain boundaries and an improvement in alloy ductility.Multi-pass rolling with small deformation（M-SDR）at room temperature caused the re-dynamic precipitation of nano B2phase and nanoα-Mg grains in the matrix,which together increased its strength to 380 MPa.The fracture mode was a ductile-brittle mixed fracture.In order to change the morphology of columnar grains of as-castβ-phase and avoid stress concentration,Mg-13Li alloys were further alloyed with Sn element which has low solid solubility inβ-Li.The results show that the addition of Sn to as-castβ-phase Mg-Li alloys can cause a transition from columnar to equiaxed grains,and the grain size decreases as the Sn content increases.The volume fraction of eutectic phase increases with the increase of Sn content.This can promote discontinuous dynamic recrystallization during deformation.The ultimate tensile strength（UTS）of as-cast Mg-13Li-（2,4,6）Sn are 109 MPa,114MPa and 120MPa,respectively,and the fracture modes are ductile fracture.Then by reducing the Li content to 8 wt.%,the introduction of hardα-Mg further improved the performance of the alloy,and explored the influence of Sn and Zn elements on the microstructure and properties of Mg-8Li alloy.The Sn element can increase the volume fraction ofα-Mg,thereby improving the strength.The UTS of as-cast Mg-8Li-（2,4）Sn is 116 MPa and137 MPa,respectively.Zn element can refineα-Mg and increase its volume fraction.The UTS of as-cast Mg-8Li-（4,10）Zn are 133 MPa and 154 MPa,respectively.After the Mg-8Li-4Zn alloy was rolled,a large number of twins with a size of 0.8μm were introduced into theα-Mg,which increased the UTS of the alloy to 265 MPa.The Sn element further refines the twins to240 nm and promotes the nucleation of the B2 phase inβ-Li,so the UTS of the as-rolled Mg-8Li-4Zn-4Sn alloy is further increased to 312 MPa.Zn element has the effect ofβ-phase stabilizing element on quenched Mg-8Li-9Zn.After quenching,the concentration of Zn element inβ-Li increased from 5.5 wt.%to 11.9 wt.%,while the concentration of Zn element inα-Mg remained at 6.5 wt.%.Due to the difference in Zn element concentration in the two phases,the volume fraction ofβ-Li in the as-quenched alloy increases to 70%.The microstructure ofβ-Li in as-quenched Mg-8Li-9Zn is consistent with that of as-cast Mg-13Li-9Zn,both of which have a large number of dispersed nano-B2 phases,so the compressive YS of as-quenched Mg-8Li-9Zn alloy is as high as 400 MPa.