Research On Microstructure And Mechanical Properties Of The Magnesium Alloy Produced By Cyclic Expansion Extrusion With An Asymmetrical Extrusion Cavity

Magnesium（Mg）and its alloys,which have low density,high specific strength,well damping performance,excellent electromagnetic shielding performance and easy recyclability etc.,are the lightest metal structural materials and have broad application prospects in national defense,aerospace,automobile and 3C communication.However,due to their poor strength and low ductility at room temperature,the development and application of Mg alloys are still limited.Thus,the research on improving the strengthening and toughening of Mg alloys are of great importance to promote the development of Mg alloys and Mg industries.Severe plastic deformation is a practical and effective technology to prepare Mg alloys with high strength,which can not only achieve grain refinement,but also lead to the homogeneous of second phases,optimize the basal texture and improve the comprehensive mechanical properties.For developing Mg alloys with high strength and toughness,we proposed a novel severe plastic deformation method entitled cyclic expansion extrusion with an asymmetrical extrusion cavity（CEE-AEC）to fabricate the thick plate Mg alloys of 50 mm×100 mm×220 mm（Length×Width×Height）with fine grain structure and excellent properties.Introducing the shear strain by attaching an asymmetrical extrusion cavity is the core advantage of this technology.The CEE-AEC process was applied to commercial alloys Mg-Al-Zn and rare earth Mg alloys Mg-Gd-Y-Zn-Zr.The effects of processing parameters and alloy composition on microstructure,texture and mechanical properties were investigated systematically.Furthermore,microstructural evolution and strengthening mechanism were analyzed in depth,and the main contributions of grain boundary strengthening,solid solution strengthening,precipitation strengthening and texture optimization to the properties of different Mg alloys were discussed.The detailed research contents and results are as followed:（1）The elevated deformation behavior of as-received Mg-13Gd-4Y-2Zn-0.5Zr alloy was investigated using Gleeble-1500 thermal simulator via hot compression test.The results showed that the flow stress was mainly influenced by deformation temperature and strain rate.The peak flow stress increased with the decrease of deformation temperature at a given strain rate.Furthermore,the peak flow stress increased with the increase of strain rate at a given temperature.The thermal activation energy of the as-received alloy was calculated to be208.2kJ/mol,and the constitutive equation was established to be:ε?=2.2303×10¹³[sinh（0.01604524σ）]^4.79323924exp(-₈²._314T^08.2).The processing maps describing the variation of power dissipation efficiency showed the influence of different strain on the flow instability region was small and two safe processing regions were obtained:420⁴45℃/0.01⁰.05 s^-1 and 475⁴90℃/0.05⁰.1 s^-1.（2）The compression stress-strain curves of Mg-13Gd-4Y-2Zn-0.5Zr was introduced into Deform-3D finite element simulation software to build the alloy model,and the simulation of different die parameters and different processing routes were carried out.The results showed that the metal flow difference was occurred and the shear deformation was acted on Mg alloy due to the introduction of asymmetric cavity.Furthermore,the metal flow velocity difference increased with the increase of cavity height difference.The distribution of stress and strain were heterogeneous,and the relationship of different area was as follows:strain in asymmetrical cavity zone>strain in non-asymmetrical cavity zone>strain in center zone.The alloy processed on Route B showed a more uniform and higher stress/strain than Route A.（3）AZ31B alloy was prepared by CEE-AEC through two different processes:isothermal deformation and processing under decreasing temperature.In terms of microstructure,the grain structure showed a significant refinement to 5.8±0.2μm after 3 passes of CEE-AEC,forming a homogeneous grain structure.Meanwhile,the basal texture was gradually weakened and inclined,leading to an increase in Schmid factor.Due to the activation energy was increased,dynamic recrystallization and grain boundary migration were more likely to achieve,the average grain size increased with the increase of deformation temperature.On the contrary,the maximum texture density decreased with the increase of deformation temperature.The process of CEE-AEC under decreasing temperature can effectively restrain grain growth and fine grain structure was obtained.In terms of mechanical properties,on one hand,the mechanical properties showed a remarkable increase with the increase of passes and the yield strength,ultimate tensile strength and fracture elongation improved by 77%,41%and 91%,respectively.On the other hand,with the increase of deformation temperature,the tensile strength showed a tendency of decrease,while the fracture elongation increased.The process of CEE-AEC under decreasing temperature was a more effective way to obtain the high comprehensive performance of Mg alloy.There was a dynamic competitive equilibrium between grain boundary strengthening and texture modification.（4）The decreasing temperature from 480℃by a drop of 20℃every step was selected to carry out the CEE-AEC process to prepare Mg-13Gd-4Y-2Zn-0.5Zr alloy.Microstructure results showed a dramatical grain refinement was achieved of 1.4±0.3μm which was mainly led by continuous dynamic recrystallization（CDRX）and discontinuous dynamic recrystallization（DDRX）as well as the particle-stimulated nucleation（PSN）induced by interdendritic long-period stacking ordered（LPSO）phases.With increasing CEE-AEC passes,the intensities of basal texture were gradually weakened,and the basal plane tended to incline to transverse and normal directions with different degrees of different passes,leading to a remarkable increase in Schmid factor for the activation of basal slip system.The samples after3 passes of CEE-AEC exhibited the best comprehensive mechanical properties with tensile yield strength ultimate tensile strength and fracture elongation improved 49%,40%and 500%,respectively.Different processing routes showed different effects on grain structure.Route B was proven to be a more effective way to fabricated the alloy with finer microstructure and more dispersed texture.The initial fiber texture was disintegrated and a new texture with the different extent of inclination or spread of basal poles was developed.The main contribution of the strength of Mg-13Gd-4Y-2Zn-0.5Zr alloy fabricated by CEE-AEC was grain refinement and solid solution strengthening.