Study On Hot Deformation Behavior And Mechanism Of Mg-(4-5)Li-4Zn-(1-2)Y Alloy

Magnesium-lithium（Mg-Li）alloys are currently the lightest metal structural material with high specific strength and specific stiffness,excellent electromagnetic shielding performance,good damping and shock absorption performance,which have good application prospects in green energy saving.However,the Mg-Li alloys obtained by casting have the disadvantages of low absolute strength,poor corrosion resistance,and poor thermal stability,which severely limit the development of the Mg-Li alloys.Alloying can significantly improve the performance of Mg-Li alloys,and hot deformation and heat treatment are also considered as commonly used strengthening methods for Mg-Li alloys,which currently become the main research direction of Mg-Li alloys performance improvement.However,there are still few researches on the effects of the composition and heat treatment process on hot deformation behaviors of Mg-Li alloys.Therefore,studying the influence mechanism of composition and heat treatment can provide a theoretical basis for the hot working strengthening of Mg-Li alloy.In this paper,two kinds of Mg-Li-Zn-Y（LZW）alloys,namely LZW441 and LZW542alloys,are prepared by vacuum melting and casting.Subsequently,solution treatment and homogenization treatment were carried out,and the above two alloys were hot-deformed at different temperatures and strain rates using a Gleeble thermal-mechanical testing machine.The microstructure of the alloys in different states（cast state,homogenized state,solid solution state and hot compression state）were observed by metallographic microscope（OM）,scanning electron microscope（SEM）and transmission electron microscope（TEM）.Besides,X-ray diffractometer（XRD）was used to analyze the phases of alloys in different states.The universal material testing machine and microhardness tester were used to test the mechanical properties of the alloys at room temperature.Then the microstructure evolution characteristics and the effects of precipitation and distribution of the second phases on hot deformation behaviors were analyzed by the above analysis and test methods.The establishment of constitutive models revealed the hot deformation mechanism of different alloys.With the help of the hot processing maps,the hot processing parameters of the alloys were optimized to provide a theoretical basis for hot deformation researches of the Mg-Li alloys containing rare earth elements.First,the microstructure of the pre-deformed LZW alloy was studied.Results show that As-cast LZW alloys had the dendritic structure,and the second phases in as-cast LZW441alloy were W-Mg₃Y₂Zn₃ and Mg Zn₂,while the second phases in as-cast LZW542 alloy were W phase and Mg₂₄Y₅,which caused that as-cast LZW542 alloy had the higher strength and hardness and lower plasticity.Grains coarsening did not occur after homogenization treatment at 400℃for 2 h,while occurred apparently in as-cast LZW542 alloy after solution treatment at 520℃for longer than 4 h.There was no significant change in the type of the second phase before and after heat treatment.After homogenization treatment,a large amount of submicron Mg₂₄Y₅ precipitates precipitated in grains,but after the solution treatment,the network W phase dissolved and spheroidized.The microhardness of the alloys slightly increased after homogenization treatment but slightly decreased after solution treatment.Secondly,the hot deformation behavior of the LZW alloys was studied.Results show that the flow curves of as-cast LZW441 alloy can be divided into three stages of work hardening,dynamic softening and steady flow under most deformation conditions;the flow curves of as-cast LZW542 alloy showed no dynamic softening stage of stress drop under most deformation conditions;the flow curves of as-cast LZW542 alloy showed no steady flow stage under most deformation conditions.The second phase with a larger size in the as-cast LZW542 alloy led to a higher hot deformation activation energy.The main rate controlling mechanism of the LZW alloy was the combined effect of solute atom dragging and dislocation climbing.More solute atoms in the LZW542 alloy,submicron second phase precipitated during homogenization treatment and nanoparticles dynamically precipitated during high-temperature deformation cause a drag effect similar to that of the solute atoms,resulting in a decrease of activation energy and stress component.Third,the hot processing maps of LZW alloys were studied and the hot processing parameters were optimized.Results show that the peak efficiency regions of as-cast LZW441alloy,as-cast LZW542 alloy and as-homogenized LZW542 alloy appeared around450℃/0.001 s^-1,450℃/0.001 s^-1 and 450℃/1 s^-1,respectively;the flow instability regions of the above alloys appeared in 250～370℃/0.02～1 s^-1,250～330℃/0.025～1 s^-1 and both250～350℃/0.025～1 s^-1 and 360～425℃/0.001～0.005 s^-1,respectively;the optimal hot working parameters of the above alloys were 450℃/0.1 s^-1,450℃/1 s^-1 and 450℃/0.1 s^-1,respectively.The as-cast LZW alloys had better hot workability,the grain sizes after hot deformation of as-cast alloys were similar,and the hot workability of as-homogenized LZW542 alloy was relatively poor.Finally,the microstructure evolution of the LZW alloys during hot deformation was studied.Results show that dynamic recrystallization（DRX）in LZW alloys hardly occurred at low temperature and the volume fraction and size of DRXed grains increased with increasing temperature and decreasing strain rate.Twinning occurred in LZW542 alloys at low temperature and high strain rate.During high temperature deformation,complete DRX occurred in as-cast LZW441 alloy at all strain rates,and dynamic precipitation of the nano-W phase and Mg₂₄Y₅ phase occurs at 450℃/0.1 s^-1,resulting in grain refinement.Dynamic recovery（DRV）occurred in as-cast LZW542 alloy at 400℃/0.001 s^-1 due to the inhibition of DRX nucleation by dynamic precipitation,while at 450℃completely DRX occurred,and the DRXed grain size decreased with increasing strain rate.Incomplete DRX occurred in as-homogenized LZW542 alloy at high temperature and low strain rate,and and the local nucleation and growth of DRXed grains caused by second phases resulted in the inhomogeneous grain size.