Study On Preparation And Properties Of Superlight-Superplastic Mg-Li Alloy

As the lightest metallic materials, Mg-Li alloy has become one of the hot researchfields of the light alloy. The ultra-light feature makes it have a good application in theaerospace, weapons,3C industry, shipbuilding industry and nuclear energy application.With the deepening application of the Mg-Li alloy, particularly those involving complexcomponents （such as instrument panels, complex bracket, etc.）, the forming processesbecome one of the factors that affect its application. Superplastic forming is an importantmethod to manufacture Mg-Li components. Compared with traditional plastic processing,it has much more advantages. Reciprocating rolling is a common process to preparesuperplastic Mg-Li alloy, but the alloy has serious internal texture after the processing,which will influence the subsequent processing. Conventional extrusion can also refine themicrostructure. Through a certain process, the alloy may also achieve superplasticity.However, the research about this is seldom reported.In this paper, the high temperature properties of Mg-Li alloy with different structure,alloying and deformation processing of Mg-Li alloy were studied firstly, then the Mg-8Lialloy and Mg-8Li-2Zn （LZ82） alloy were extruded with different extrusion ratio, thesuperplastic characteristics of those alloy were investigated, and the cavitations of LZ82alloy under the optimum deformation conditions were also analyzed.Under room temperature, the hardness of Mg-Li alloy decreases with the increase oflithium content until the volume fraction of β phase reaches a certain amount, the Tcurves and the Tcurves of α phase alloy, α+β dual phase alloy, β phase alloy can bedivided into three stages, the trends of each alloy at different stages are dissimilitude. Thealloys with equal volume fraction of α phase and β phase, such as Mg-8Li alloy, aresuitable to use at lower temperatures （less than423K）, as its strength varies little withtemperature. The β phase alloy produces severe necking at high temperatures.Mg-6Li-3.5Al-1Zn-1Ce-0.5Sn alloy still has higher strength （151.53MPa） at473K.Alloying and deformation can improve the microstructure and properties of Mg-Lialloy. The as-cast Mg-8Li alloy is composed of α-Mg and β-Li phases, the microstructureis coarse. After the addition of Zn and Y or Ce, the microstructure was refined, reticularZn₂Ce and granular Mg₆Y phases formed in Mg-8Li-2Zn-0.5Ce and Mg-8Li-2Zn-0.5Yalloy. Alloying process improved the tensile stress obviously, but in Mg-8Li-2Zn-0.5Ce alloy, the reticular Zn₂Ce phases exist at grain boundaries, which decreases the elongationof the alloy. Rolling deformation can refine the microstructure and improve the properties,especially for Mg-8Li-2Zn-0.5Ce alloy. After rolling, the reticular compounds changedinto granular, and distributed in the matrix uniformly, leading to the increasing ofelongation. After extrusion, the microstructures of Mg-8Li alloy were refined, and the αphases and β phases distribute along extrusion direction in fibrous structure. Comparedwith the as-cast alloy, the tensile strength and elongation of Mg-8Li alloy were bothimproved, and extrusion showed more effects on elongation than strength.After one-pass extrusion, the Mg-8Li alloy exhibited an elongation of164.5%at563K and an initial strain rate of5×10^-5s^-1. The Mg-8Li-2Zn alloy exhibited an elongationof228%at563K and an initial strain rate of1.5×10^-4s^-1. The two alloys possessed theproperty of superplasticity. The peak stress decreaseed with the decrease of the initialstrain rate or the increase of temperature during tensile tests.Through two-pass extrusion, high extrusion ratio was achieved, the microstructuresof the LZ82alloy were fairly homogeneous along the extrusion direction, and the grainswere refined to10μm. The two-pass extrusion LZ82alloy exhibited superplasticity with amaximum elongation of758%at563K and an initial strain rate of1.5×10^-4s^-1. The strainrate sensitivity is0.55and the deformation activation energy for flow is estimated as90kJ/mol. After annealing at473K for2h, the average grain size did not changesignificantly, but the degree of equiaxed improved. Many island or peninsula shape βphase tend to aggregation. The two-pass extrusion and annealed LZ82alloy exhibits highstrain rate superplasticity with a maximum elongation of279%at an initial strain rate of1.0102s1and an elongation of151%at an initial strain rate of1.0×10^-1s^-1at623K.The deformation activation energy is89.4kJ/mol. After rolled at553K, the grains of thetwo-pass extrusion LZ82alloy were further refined, but the grain sizes are inhomogeneous.The two-pass extrusion and rolled LZ82alloy exhibits low temperature superplasticitywith a maximum elongation of430%at498K and120%at423K. The strain ratesensitivity was measured as0.55, the deformation activation energy for flow wasestimated as99.24kJ/mol, and after tensile deformation, the morphology of α phase and βphase transferred from flake to the equiaxial shape, the grains occurred spheroidization.The strain rate sensitivity and deformation activation energy or the changes of microstructures all indicate that the dominant superplastic deformation mechanism in thetwo-pass extrusion LZ82alloy is grain boundary slip （GBS） controlled by grain boundarydiffusion.Cavities formed during superplastic deformation. The quantity and size of cavitiesincrease with the decrease of initial strain rate, the increase of temperature or the increaseof true strain. The nucleation of the cavity was generally observed at triangular grainboundary with○shape.The main nucleation mechanism is diffusion mechanism, and thenucleations are continuously. When the true strain is low, the mechanism of cavity growthis controlled by diffusion. When the true strain is high, the mechanism of cavity growth ismainly controlled by plastic deformation, however the diffusion still react. Cavities growand coalesce during deformation, resulting in the materials fracture. The cavities volumefractionf vis select as superplastic deformation damage variable, based on continuumdamage mechanics; the characteristic parementers of the two-pass extrusion LZ81alloyduring superpkastic deformation were measured.