Research On Anisotropy Of AZ31 Magnesium Alloy Under High Strain Rates

Due to the advantages of low density,high specific strength,excellent electromagnetic shielding property,good thermal conductivity and easy recovery,magnesium and its alloys have been widely used in automobile,electronics and aerospace and other fields.However,owing to the limited slip systems,which can be activated at room temperature,magnesium alloys show poor plastic deformation abilities.Furthermore,for wrought magnesium alloys,the strong basal texture formed during extrusion or rolling process results in an obvious anisotropy and tension-compression asymmetry.These characteristics severely restrict the further application and development of wrought magnesium alloys.To date,most studies on the anisotropy behavior of wrought magnesium alloys have mainly focused on the plastic deformation under low and medium strain rates,while the experimental event concerning high strain rate deformation remain insufficient.Therefore,it is vital to conduct a comprehensive investigation to study the plastic behavior and microstructural characteristics of wrought magnesium alloys under high strain rates deformation along different directions.The results will provide important theoretical guidance for fully understanding the anisotropy and asymmetry behaviors of magnesium alloys,exploring the relationship between deformation mechanism selection and anisotropy,and further realizing the improvement of their mechanical properties.In this paper,by means of split Hopkinson pressure bars（SHPB）experiment apparatus,the rolled AZ31 magnesium alloy specimens were impacted along different directions and under the strain rates ranging from 700 to 1600 s^-1at room temperature,and the anisotropic behaviors of those specimens were compared.At the same time,the influences of pre-deformation along RD and multi-directional pre-deformation along RD+TD on the anisotropy,and the detwinning behaviors of rolled AZ31 magnesium alloy were investigated.Twin evolution during high strain rate impact was investigated by using strain restricting rings,and the microstructure characteristics of the impacted specimens were observed by electron back scattered diffraction（EBSD）technology.Furthermore,the SHPB experiment was numerical simulated by pre-processing software Hypermesh and explicit dynamics software Ls-dyna.The results show that the deformation behaviors of the rolled AZ31 magnesium alloy under high strain rates exhibit obvious anisotropy.When the specimen impacted along rolling direction（RD）or transverse direction（TD）,the dominant deformation mechanism is{101 2}tensile twinning.When the specimen impacted along normal direction（ND）,the plastic deformation is mainly dominated by{10 1 1}contraction twinning and pyramidal<c+a>slip which has an higher critical resolved shear stress（CRSS）.High strain rate deformation along ND shows higher strength,more obvious positive strain rate strengthening effect and stronger strain rate sensitivity than RD and TD.Compared with the as-received specimen,both the impact strength along RD and TD are increased for the specimen pre-compressed along RD to the true strain of 4%（4%PRD）.The strength increase along RD is mainly attributed to the texture strengthening,while grain refinement by twin boundaries accounts for the strength increase along TD.For the specimen pre-compressed along RD then TD（4%PRD+2%PTD）,the basal texture is further weakened,more different{101 2}tensile twins variants and secondary{101 2}-{101 2}twins were activated,a higher strength and a lower strain can be observed after impact along RD and TD.However,when the impacting direction is ND,the dominant deformation mechanism for both 4%PRD and 4%PRD+2%PTD specimen are detwinning,the true stress-strain curve shapes are concave-down with decreased strengths.Comparing with 4%PRD specimen,4%PRD+2%PTD specimen presents an higher strength impacted along ND,which is resulted from more complicated tensile twins introduced by multi-directional pre-compression.The SHPB experiment apparatus equipped the strain restricting rings with different thicknesses for obtaining impacted specimens along RD with a true strain of 2%～11%.It can be found that{101 2}tensile twinning dominates the early plastic deformation,the area fraction of{101 2}tensile twins is increased with the increase of true strain.Most{101 2}tensile twins activated in a single parent grain have the highest Schmid factor,which means that these tensile twins follows Schmid law.However,several Non-Schmid{101 2}tensile twins were activated mainly caused by strain accommodation effect of the tensile twins in neighbouring parent grains.When the true strain exceeds 10%,the plastic deformation along RD is mainly dominated by dislocation slip.The observation results of detwinning behaviors for the pre-compressed AZ31magnesium alloy specimen impacted under different strain rates show that with the increasing of impact strain rate along ND,the detwinning degree,average grain size and pole density of{0001}basal texture all increase gradually.At the same time,both area fraction of residual twins and the average width of the tensile twin decrease.When the impact strain rate reaches 1600 s^-1,the tensile twin boundaries introduced by pre-compression completely disappear,and the microstructural characteristics are almost the same as that of the as-received sheet.The specimen exhibits better plasticity and more obvious strain rate sensitivity due to the dominated plastic deformation along ND is detwinning,which has lower activated stress than tensile twin.