| Magnesium lithium alloy is the metal structure material with the lowest density discovered so far,and has the characteristics of high specific strength and high specific stiffness,which has a huge development potential in the fields of national defense,aerospace,nuclear industry,electronic 3C products and so on.The absolute strength of single ?-phase magnesium-lithium alloy is relatively high,but its plastic forming ability is low.This shortcoming greatly limits its application.The Severe Plastic Deformation Processing(SPD)process can refine the material structure,improve the texture,and finally obtain a material with high strength and high plasticity.The Canning Compression process can provide threedimensional compressive stress,inhibit the initiation of cracks during processing,and improve the plastic forming ability of the material at medium temperature or even room temperature.In the present study,the single-? phase Mg-5Li-3Al-2Zn(wt.%)alloy was subjected to compression experiments at room and medium temperatures.X-ray diffraction(XRD),electron backscattering diffraction(EBSD),transmission electron microscopy(TEM)and high resolution electron microscopy(HRTEM)were used to investigate the effects of solution and encapsulated compression on the microstructure and properties of the alloy.The deformation structure and texture evolution process of Mg-5Li-3Al-2Zn Mg-Li alloy during compression deformation at room temperature and medium temperature were systematically analyzed.The size,morphology,quantity and distribution of Al Li second phase in deformation were also studied.And also explored the interaction mechanism between the second phase and twins in the matrix,compared the deformation microstructure differences between room temperature compression and medium temperature compression,revealed the plastic deformation behavior and deformation mechanism of Mg-Li alloy under different temperatures,enriched the theoretical basis of plastic forming of Mg-Li alloy,and had guiding significance for future industrial production.The main research conclusions of the present study are as follows:(1)Solution temperature and solution time have significant effects on the microstructure of the materials.After solution treatment at 325? for 2-4h,the microstructure interface is clear,but a large amount of Al Li second phase is still distributed along the grain boundary in lamellar form.When the temperature increased to 350?,it was found that the distribution of the second phase of filamentous Al Li in grain and grain boundary decreased significantly after solution for 2h,and with the extension of time,whisker-like Al Li phase precipitated again after solution for 4h.By comprehensive comparison,350?-2h was selected as the best solution process parameter.(2)With the increase of the canning thickness,the three direction compressive stress significantly increases,which can inhibit the crack initiation and significantly improve the compression forming ability of the material.Combined with the compression deformation behavior,the optimal wall thickness is determined to be 4mm.The maximum deformation of the casing is 31% at room temperature,and 53% at 240?.Compared with the normal deformation of 17% at room temperature,the deformation of the casing can be increased by2-3 times.(3)Through the package set of compressed at room temperature under different deformation were 6%,16%,26%,the microstructure of research,found that with the increase of deformation,deformation twin morphology and quantity are big change: the early stage of the small deformation mainly lenticular parallel,cross {10(?)2} tension twin,when the deformation increases to 16%,a small amount of {10(?)1}-{10(?)2} compression-extension pairs of twin,and more slip is activated,both coordinated magnesium lithium alloy coating plastic deformation at room temperature.The selection of twin variants of the tensile twin obeys Schmid's law,and the probability of activation of multiple twin variants is increased,which is conducive to the initial random texture transformation to the grain C-axis orientation of CD compression direction.Under the joint action of deformation microstructure and texture,the hardness of the material increases from 72 HV in solid solution state to 96 HV.(4)The maximum deformation of Mg-5Li-3Al-2Zn alloy reaches 53% when 240?.The microstructure and deformation texture of samples with compression deformations of 13%,26%,40% and 53% were studied.Compared with room temperature,more fine strips and blocks of Al Li second phase were precipitated by medium temperature deformation.In addition,with the increase of temperature,the critical shear stress of non-basal slip <c+a>decreases,which makes it greatly activated during compression.A large number of interactions among the second phase,dislocation and twin structure in the sample coordinate the medium temperature compression deformation,which significantly improves the medium temperature forming ability of the sample.After the hardness test of the sample after compression deformation at medium temperature,it was found that the microhardness first increased to the maximum value of 104 HV with the increase of compression deformation,whereafter decreased and then increased,which was different from the hardness change rule of room temperature deformation.The interaction of rich second phase,twinning and dislocation at the early stage of compression resulted in work hardening and significantly increased hardness of the material.However,when the deformation reaches26%,the shear band is formed.At the intersection of twinning structure,there is a large stress concentration,which leads to dynamic recrystallization nucleation.Therefore,the dynamic softening of the material is caused,and the hardness value decreases.In the subsequent deformation process,the deformation strengthening plays a dominant role,and then the hardness is improved. |
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