Dynamic Mechanical Behavior And Micromechanism Of Mg-7Gd-5Y-1.2Nd-0.5Zr Magnesium Alloy Under High Strain Rate

With the advantages of low density,high specific strength,high specific stiffness,vibration and noise reduction,rich reserves and so on,magnesium alloys have a broad development prospect in the field of aerospace.However,the traditional commercial grade magnesium alloys generally have some problems,such as low strength and toughness,obvious anisotropy of mechanical properties and so on,which are difficult to meet the lightweight requirement of aircraft.Mg-7Gd-5Y-1.2Nd-0.5Zr(EW75)alloy is an important lightweight material for new aircraft.Among them,dynamic deformation performance is an important index to evaluate its service safety.Therefore,this paper mainly studies the deformation mechanisms of EW75 alloy at different temperature gradients and strain rates,and carries out Johnson-Cook(JC)constitutive equation and cumulative damage equation fitting according to the experimental data to simulate the whole process of high strain rate deformation of EW75 alloy under service conditions,which could provide necessary data and design basis for the launch of rocket and spacecraft.In this paper,the dynamic compression experiments of EW75 magnesium alloy in extrusion direction(ED),transverse direction(TD),and normal direction(ND)are carried out at the strain rate of 1000?2300s^-1 and the temperature of 20?300?by using the split Hopkinson pressure bar experimental device.Based on the experimental results,it is found that all the true stress-strain curves of the alloy are"C-type",showing continuous yield without obvious yield point,and there is no obvious anisotropic phenomenon.The"C-type"curve is a comprehensive reflection of the normal strain rate strengthening effect,strain strengthening effect,and strain softening effect dominated by different micro mechanisms.The microstructure of EW75magnesium alloy in its original state and after deformation is observed.It is found that the(0002)basal texture along TD direction is weak for EW75 magnesium alloy samples with rare earth addition and multi-directional forging deformation.The deformation is dominated by non-basal slip mechanism and participated by many different micro mechanisms.The main deformation mechanisms of ED and ND direction specimens after dynamic compression are pyramidal<a>slip and prismatic slip,and the auxiliary deformation mechanisms are pyramidal<c+a>slip,basal slip and{10(?)2}tension twinning,dynamic recrystallization(DRX),and grain boundary sliding.The pyramidal<c+a>slip is the main deformation mechanism of TD direction specimen after dynamic compression,basal slip and{10(?)2}tension twinning,DRX,and grain boundary sliding are the auxiliary deformation mechanisms.Among them,the proportion of DRX mechanism increases with the increase of temperature.But even when the temperature reaches300?,the grain size is still uneven and does not reach complete recrystallization.After multi-directional forging,the microstructure of EW75 magnesium alloy is mainly composed of equiaxed fine grain zone with the average grain size of 4.45?m and fine grain zone with the average grain size of 0.5?m.The dynamic deformation microstructure evolution along ED direction under the strain rate of 2300s^-1and different strain is studied by applying a pressure ring.It is found that at the initial stage of deformation,although the tensile stress along TD direction is beneficial to the initiation of{10 (?)2}tension twinning,the non-basal slip mechanisms in the ultrafine grain band start first,and the multi mechanism modes of prismatic slip,pyramidal<a>slip,and pyramidal<c+a>slip are involved in the deformation.When the deformation exceeds a certain critical strain value(7%),various micro deformation mechanisms in the equiaxed fine grain region start successively.The{10(?)2}tension twinning,basal slip,and pyramidal<c+a>slip start first,followed by continuous dynamic recovery and DRX.At the same strain(7%),with the increase of strain rate,the occurrence probability of various non-basal slip in the ultrafine band and{10(?)2}tension twinning,basal slip,prismatic slip,and pyramidal<c+a>slip in equiaxed fine grain zone increases,and the increase of deformation heat can also enhance the trend of dynamic recovery and DRX.Based on DRX softening term accompanied by strain decay in high strain rate deformation of EW75 alloy,and according to the compression true stress-strain curve under quasi-static and high strain rate,JC constitutive equation and cumulative damage equation including strain softening attenuation term are established,and the parameters are fitted.It is found that the fitting results of the modified model based on the constitutive equation are in better agreement with the measured results of the true stress-strain curves of high-speed impact compression under different temperatures and strain rates.Based on the modified and fitted JC constitutive equation and cumulative damage equation of EW75 magnesium alloy,LS-DANY commercial software is used to simulate the deformation process of cap specimen under local shear stress in ED,TD,and ND directions.It is found that the deformation basically occurs in the shear stress concentration region.It is found that when the strain reaches a certain extent,the crack initiates in the local deformation zone and expands with the increase of strain.The measured results are in good agreement with the numerical simulation results.