Study On Solidification And Rheological Behavior Of Hcp Structural Mg-Li Alloys

Magnesium alloy has broad application prospects in military,civil and 3C（Computer,Communication,Consume Electronic）fields due to its advantages of high specific strength and specific stiffness.However,its poor plastic deformation propertyat room temperature greatly limits its practical application.The addition of Li can change the axial ratio（c/a）of magnesium alloy and improve the deformation capacity of the alloy.Therefore,the effect of Li content on the solidification behavior and deformation behavior of hcp magnesium alloy has important theoretical value and practical significance.In this thesis,the solidification behavior,microstructure and thermal deformation behavior of a series of Mg-x Li-1Al（x=1,3,5）magnesium lithium alloys were systematically studied by means of thermodynamic calculation software,microstructure characterization,mechanical propertiesdetection,thermal simulation experiments,and other test and analysis methods,which provide theoretical support for subsequent applications and development of hcp structure Mg-Li alloys.The equilibrium solidification and Scheil non-equilibrium solidification paths of Mg-x Li-1Al（x=1,3,5）alloys were calculated by thermodynamic software.It was found that the microstructure of the three alloys during equilibrium solidification and Scheil non-equilibrium solidification wereα-Mg phase at room temperature.The dendrite growth restrain factor（GRF）of Mg-x Li-1Al（x=1,3,5）alloy was calculated by using the binary equilibrium phase diagram,the ternary equilibrium phase diagram and the relationship between temperature and solid fraction during Scheil non-equilibrium solidification.It was found that the GRF values obtained by the three methods were basically the same.GRF value increases with the increase of lithium content,and the average GRF value increases from 5.52 to 14.03.The as-cast Mg-x Li-1Al（x=1,3,5）alloy was obtained by vacuum melting in a protected atmosphere of argon.With the increase of Li content,the grain sizes of the three as-cast alloys are 380μm,295μm and 315μm,respectively.Theoretical calculation shows that there is no linear relationship between GRF value and grain size.Vickers hardness tester was used to test the hardness values of as-cast and homogenized Mg-x Li-1Al（x=1,3,5）alloys.It was found that the hardness values of the three alloys in different states increased with the increase of lithium content.The thermal deformation behavior of Mg-x Li-1Al（x=1,3,5）alloy was investigated by thermal simulation experiments.The deformation conditions were 200-400°C,0.1-0.01s^-1.The constitutive equation of Mg-x Li-1Al（x=1,3,5）alloy was constructed by using the modified Arrhenius function.It was found that the deformation activation energy Q value increased first and then decreased with the increase of lithium content.Among them LA11=116.48k J/mol,LA31=201.47 k J/mol,LA51=198.74k J/mol.The critical condition of dynamic recrystallization was determined by using work hardening rate curve,and the critical condition model of dynamic recrystallization of Mg-x Li-1Al alloy was established by using Sellars equation,and the dynamic recrystallization kinetics model of Mg-1Li-1Al alloy was established by using Avrami equation.By observing the microstructure of Mg-x Li-1Al（x=1,3,5）alloy after hot compression,the hot deformation behavior of the alloy was studied.It is found that the dynamic recrystallization softening mechanism changes from continuous dynamic recrystallization（CDRX）to discontinuous dynamic recrystallization（DDRX）with the increase of deformation temperature or the decrease of strain rate.When the deformation temperature increases from 200°C to 400°C,the twinning type changes from secondary twinning to tensile twinning,and the number of twinning decreases sharply.Under the same deformation conditions,the recrystallization volume fraction of Mg-x Li-1Al（x=1,3,5）alloy is 45.6%,57.3%and 51.3%,respectively.