Effect Of Pulsed Current On Microstructure Evolution Of Pure Magnesium During Uniaxial Tensile Process

Due to the advantages of low density,high specific stiffness,high specific strength and good damping properties,magnesium alloys have a very promising application in transportation,aerospace,medical chemical,3C and so on.However,the poor room temperature formability of magnesium alloy hinders the wider application of magnesium alloy in industry.It has been shown that pulsed current treatment can significantly reduce the flow stress and improve the plasticity of magnesium alloys,which can significantly improve the forming efficiency and reduce the cost compared with the conventional forming process,and has a great potential for rapid forming of hard-to-deform metals,but the type of current and the microscopic mechanism have not been thoroughly studied.A large number of previous studies have focused on the microstructure evolution of magnesium alloys under pulsed current,but the interaction of solute atoms,second phases,dislocations and twins in magnesium alloys,and the experimental design cannot completely strip away each influencing factor,resulting in a general lack of academic understanding of the types of currents and non-thermal effects.Therefore,there is an urgent need to reduce the influencing factors of magnesium alloys,compare the difference between DC constant current and DC pulsed current,to deeply investigate the current on microstructure evolution law,for further exploring the microscopic mechanism of flow stress reduction,and providing a theoretical basis for pulse currentassisted forming.Therefore,99.999%(mass fraction)of high-purity magnesium,which excludes the interference of too many influencing factors,was used as the study material in this paper.The uniaxial tensile experiments under different deformation conditions(no current,DC constant current and DC pulsed current)were conducted to investigate the changes of flow stress of pure magnesium.Meanwhile,based on quasi-in-situ EBSD characterization,the microstructure evolution mechanism of pure magnesium under the action of pulsed current was further explored.The main conclusions are as follows:(1)Compared with no current and DC constant current,the application of DC pulsed current during uniaxial tensile significantly reduced the flow stress of pure magnesium.The fracture mode gradually changed from brittle fracture to ductile fracture when the pulsed current was applied.(2)The distribution of twins becomes more uniform after the application of pulsed current.(3)The geometric necessary dislocation density distribution and calculation show that the applied current will make the dislocation movement easier and effectively suppress the increase of geometric necessary dislocation density during the deformation process.The constant current has the most obvious suppression effect on the increase of geometric imperative dislocation density,but the dislocations gather near the grain boundaries,which aggravates the stress concentration near the grain boundaries and easily causes intergranular fracture.The pulsed current will make the distribution of dislocations more uniform,reduce the stress concentration effect and the level of hardening,makes the flow stress lower and increase the plasticity.(4)The twinning behavior is different for the three deformation conditions.(1)With no current,the twins preferentially nucleate and grow in the grains with high basal slip Schmid factor to coordinate the deformation.After the increase of strain,the growing twins hinder the dislocation motion and the stress concentration at the grain and twin boundaries increases,making the highest flow stress of pure Mg sheet.(2)When DC constant current was applied,the twins showed different Schmid behavior from no current one,and no serious dislocation plugging occurred inside the grains,which was at a lower level of hardening.The twins nucleated at the grain boundaries intersected with the grain boundaries and the lower angular grain boundaries gathered near the grain boundaries hindered the dislocation movement and increased the hardening level at the grain boundaries,so it did not significantly reduce the flow stress of pure magnesium.(3)When DC pulsed current was applied,the pulsed current effectively suppressed the dislocation plugging at the twins and grain boundaries.The dislocations were mainly concentrated in the small angle grain boundary positions formed inside the grains,but the distribution was more dispersed and no significant stress concentration occurred.(5)The pulsed current promotes grain rotation.Slip,twinning and grain rotation act together to coordinate deformation.Macroscopically,the material exhibits lower levels of work hardening and flow stress.