Reasearch On The Mechanical And Corrosion Behavior Of ZA21 Magnesium Alloy Bar Under Load-Corrosion

Magnesium alloys have broad application prospects due to their low density,high specific strength and rigidity,but their poor service performance still restricts the large-scale application.The service performance is often manifested as mechanical properties,corrosion performance,and trength attenuation after corrosion.The service performance is greatly affected by the microstructure auch as grain size and texture,and service conditions such as load and corrosion.Studying the mechanical and corrosion behavior of materials under load and corrosion is conducive to the development of new materials,the formulation of service consitions and service life.Therefore,in this thesis,a bimodal and uniform ZA21 magnesium alloy bar with texture characteristic of c-axes perpendicular to extrusion direction was prepared,and its tensile-compressive asymmetry and compression mechanical anisotropy under different load directions were systematically studied.The corrosion behavior and strength decay behavior in 3.5 wt.%NaCl solution were explored,and the main conclusions are as follow:The ZA21 magnesium alloy bar with bimodal structure and uniform structure exhibit obvious differences in mechanical properties and tension-compression asymmetry under tensile and compressive loads along the axial direction of the bar.Due to the grain boundary strengthening and the sensitivity of the extension twins to grain size,the bimodal structure obtains a higher strength than uniform structure,with a tensile yield strength of 206.42 MPa and a compressive yield strength of 140.28 MPa.Prismatic slip and extension twins dominate deformation under tension,while basal slip and extension twins dominate deformation under compression,and the higher proportion of twins under compression resulting in higher yield in tension and lower yield in compression,which resulting in the existance of tensile-compressive asymmetry.Meanwhile,the suppression of extension twinning by fine grains in bimodal structure causes the lower twins proportion difference under tension and compression,and the basal slip under tension and prismatic slip under compression in bimodal structure assist deformation respectively,which weakens the tensile yield strength and enhances the compressive yield strength,thus weakens the tension-compression asymmetry of bimodal structure to some extent.The compression and tensile yield strength ratios of bimodal structure and uniform structure are 0.68 and 0.56,respectively.The ZA21 magnesium alloy bar with bimodal structure and uniform structure exhibit obvious different stress-strain and mechanical properties anisotropy under axial and radial compression.The angles between the c-axis of the grains and loading direction are basically in the range of 60?90° under axial compression and 0-90° under radial compression.The dominant deformation mechanisms are extension twinning and basal slip.However,the Schmidt factor of basal slip under axial compression is small,thus requires relatively high stress for activating,resulting in higher strength.As a result,the anisotropy of mechanical properties exists.What's more,the higher proportion of extension twins of uniform structure under axial compressioni leads to weaker compressive strength,resulting in the weakening of the anisotropy.The selection of extension twin variants basically follows Schmid's law,and the nucleatation of part non-Schmid twins are induced by low angle grain boundaries adjacent grains or inside the grains.As the deformation increases,the angle between the c-axis and the loading direction decreases,and the difference between the maximum and minimum Schmidt factors increases,resulting in the increasing of the tendency of the twin variants to follow Schmidt's law.In addition,the activated twins grow larger and merge with the parent grain.Furthermore,compression causes the c-axis of the grains to rotate toward the loading direction.Under the combined action of the nearly 90° sudden change in the c-axis of the grains caused by twinning,the longitudinal sections of the fractured specimens under axial and radial compression are all prismatic orientation.The bimodal structure and uniform structure exhibiting only prismatic orientation in the transversal section and the coexistence of basal and prismatic orientations in the longitudinal section show obvious different corrosion behavior in 3.5 wt.%NaCl solution.The corrosion rate first increased and then rapidly decreased and remained stable during the 16 days continuous immersion.The corrosion rate followed the rule of the transversal section of uniform structure<the longitudinal section of uniform structure<the transversal section of bimodal structure<the longitudinal section of bimodal structure.The corrosion rate in each stage of corrosion is controlled by electrochemical performance.In corrosion nucleation stage,coarse grains and prismatic oriented grains preferentially corrode nucleate due to their higher electrochemical activity.In the early and late stages of corrosion,the electrochemical uniformity controls the corrosion rate through microgalvanic corrosion and corrosion product film.The uniform structure with uniform grain size and the transverse section with only prismatic orientation exhibit better electrochemical uniformity,which obtain weacker the micro-galvanic corrosion strength and a smooth and protective corrosion product film,and perform better corrosion resistance.Pitting corrosion and intergranular corrosion are the main corrosion types,and significantly affected by the grain size.Micro-galvanic corrosion accelerates the nucleation and expansion of pitting corrosion in bimodal structure.The smaller the grain size,the wider the range of intergranular corrosion,but intergranular corrosion is hardly affected by grain orientation.The bimodal structure and uniform structure ZA21 magnesium alloy bar after corrosion in 3.5 wt.%NaCl solution were compressed along the axial and radial directions,and obvious differences in the corrosion residual strength and its attenuation are occurred.The reduction of effective force area,the reduction of interatomic bonding force caused,and the stress concentration caused by pitting corrosion and intergranular corrosion are the fundamental reasons for the reduction of corrosion residual strength.The competition between the initial strength and corrosion resistance caused by the grain size distribution results in high strength but low corrosion resistance in bimodal structure and excellent corrosion resistance but low strength in uniform structure under the same loading direction,and finally causes the corrosion residual strength of the two to be at the same level.Based on the corrosion residual strength attenuation negative exponential model controlled by pitting,introducing the influence of intergranular corrosion on corrosion residual strength,optimizing the model,the correlation coefficient after optimization is as high as 95%.In addition,the fracture mechanism before and after corrosion is still smooth brittle fracture;but the main crack of the fracture passes through the pitting and part of the grain boundary after corrosion.There are a large number of secondary cracks inside the pit and near the main crack,showing typical transgranular fracture and intergranular fracture.