Study On High-plasticity Mg-Gd-Mn Alloy And Its Deformation Mechanism

Magnesium has a close-packed hexagonal crystal structure with low symmetry.Not only the number of slip systems is insufficient,but also the critical resolved shear stress(CRSS)required to start different slip systems varies greatly,so only the basal slip can be started at room temperature,and it is difficult to coordinate the strains along c-axis direction,which leads to the poor plasticity.The introduction of rare earth(RE)elements can weaken the textures,promote the plasticity and strength,and improve anisotropy,which is considered to be an effective method to promote formability of magnesium alloys.Among them,Mg-Gd alloy has aroused much interest.In addition,Mn element has the advantages of refining the extruded microstructure and improving the corrosion resistance of magnesium alloys.Therefore,adding minor Mn can further promote the comprehensive properties.In this paper,the Mg-2Gd-x Mn alloys were successfully fabricated,and extruded bars with better properties were obtained by traditional extrusion process.Optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD),electron back-scattered diffraction(EBSD),universal testing machine and electrochemical workstation were applied to test the microstructures,phase compositions,fracture morphologies,mechanical properties and corrosion properties.Insitu tensile/ compression technique together with transmission electron microscope(TEM)double beam diffraction analysis were used to reveal the room temperature plastic deformation behaviors.Effects of different extrusion ratio on microstructures and tensile mechanical properties were also discussed.Adding Mn element to Mg-2Gd alloy significantly refines the grain size of asextruded alloys.Minor Mn results in complete recrystallization of as-extruded Mg-2Gd alloy,and reduce the yield asymmetry.Most of the grains are in soft orientation,which is beneficial to starting the basal slip,and Mg-2Gd-0.5Mn alloy performed excellent room temperature plasticity.The fraction of un-recrystallized grains continues to increase with the increasing Mn content,promoting the strength of the alloy significantly.The lowest elongation can reach 30% although the plasticity decreased with the increase of the strength.In-situ tensile/compression was applied to trace the plastic deformation behaviors of the Mg-2Gd-0.5Mn alloy.The nucleation and growth of the twinning change the grains orientations,the basal planes gradually rotat to be parallel to ED during tensile tests,and the basal planes become perpendicular to ED after compression tests,altogether weakening the RE textures.With the increase of strain,a large number of non-basal slips like prismatic <a> slip and second pyramidal <c + a> slip contribut to the plastic deformation.In addition,the segregation of Gd atoms at the grain boundaries strengthenes the binding forces between grain boundaries,decreasing the tendency of cracks along the grain boundaries.Three extrusion ratios of 10,25 and 79 were selected to prepare Mg-Gd-Mn alloy.As the extrusion ratio decreased,not only the recrystallized grains are refined,but also the proportion of the un-recrystallized microstructure is increased,the texture presents strong basal texture,which led to the obvious increase of the alloy strength.However,the large amount of residual stress inside the microstructure could induce the cracks.and deteriorate the room temperature plasticity of the alloy.