Investigation Of Impact Deformation Mechanism Of Pre-Twinned AZ31 Magnesium Alloy Based On Finite Element Extended Visco-Plastic Self-Consistent Model

As a light alloy material that can replace other materials,magnesium and magnesium alloys have the advantages of high specific strength,and specific stiffness,which are important structural materials in transportation,aerospace and communication electronics.The introduction of twins in magnesium alloys can refine the grain organization.In the meantime,twin boundaries could prevent dislocation slip and provide nucleation positions for dynamic recrystallization.These can effectively improve the static and dynamic yield strength and plasticity of magnesium alloys.Magnesium alloys are inevitably subjected to impact loading in transportation and aerospace applications.In order to explore a method to analyze the deformation mechanism,texture evolution and mechanical response of pre-twinned AZ31magnesium alloy under impact loading conditions,the velocity gradient of impact deformation was obtained by extending the Abaqus/VUMAT subroutine to simulate the macroscopic deformation process of pre-twinned AZ31 magnesium alloy under impact loading conditions.Then visco-plastic self-consistent（VPSC）model coupled historical loading of the velocity gradient was applied to predict the microcosmic deformation process of pre-twinned AZ31 magnesium alloy under impact loading condition.The following conclusions were obtained:（1）The finite element model was extended to study the macroscopic deformation process of pre-twinned AZ31 magnesium alloy under impact loading conditions,and the velocity gradient history loading of impact deformation was obtained.Based on the traditional Johnson-Cook instantonal model,a J-C instantonal model of medium-temperature high-speed impact deformation of pre-twinned AZ31 magnesium alloy was constructed.VUMAT deformation velocity gradient subroutine was written based on Plasto-elasticity theory to extend the Abaqus model,so that it can calculate the velocity gradient under impact loading conditions;the extended Abaqus finite element model can accurately simulate the AZ31 magnesium alloy at medium temperature and high strain rate with the historical loading of incident wave,transmitted wave and velocity gradient,reflecting the dynamic mechanical properties of the material.With the increase of strain rate,the maximum stress of the specimen increases and the final specimen height decreases.With temperature increasing,the maximum stress value decreases.（2）An extended VPSC model was constructed to accurately simulate the macroscopic and microscopic deformation process of pre-twinned AZ31 magnesium alloy under impact loading conditions.It was found that the Abaqus extended VPSC model could better simulate the impact deformation process and represent the pole intensity maximum,pole position and actual texture components.The Abaqus extended VPSC can further optimize VPSC and extend VPSC simulation to medium and the plastic deformation of polycrystals simulation under high temperature impact loading,and provides new ideas for the study of deformation mechanisms in complex plastic processing conditions.（3）The dynamic mechanical response of pre-twinned AZ31 magnesium alloy under impact loading was investigated.The reasons for the stress drop in flow stress were investigated.During medium temperature high speed impact deformation of pre-twinned AZ31 magnesium alloy,twins and deformation shear bands play an important role.The flow stress decreases at the beginning of deformation are due to the appearance of a large number of twins,providing space for the movement of the pinned dislocations.With strain rate increasing,the number of twins increase,causing that stress obviously drop and increment of fluctuation.（4）The deformation mechanism of pre-twinned AZ31 magnesium alloy under impact loading was investigated.During medium temperature high speed impact deformation,basal slip and second-order pyramidal slip are the main mechanisms to coordinate deformation in pre-twinned AZ31 magnesium alloy.When the strain rate is 2150s^-1（impact pressure is 0.3 MPa）,the secondary coordinated deformation mechanism is extension twinning at the beginning of deformation for impact loading at 150℃and 200℃.As the deformation continues,compression twinning replaces extension twinning as the secondary deformation mechanism.At a temperature of 150℃with a strain rate of 4160s^-1（impact pressure is 0.7 MPa）,the compression twin is the secondary deformation mechanism during the whole impact deformation process,which decreases with strain increases.（5）The texture evolution of pre-twinned AZ31 magnesium alloy under impact loading was investigated.The texture evolution law of impact deformation was studied.The formation of double peak texture in pre-twinned AZ31 magnesium alloy at 150℃and strain rates of 2150 s^-1and 4160 s^-1（impact pressure is 0.3 MPa and 0.7 MPa）is due to twinning resulting in large angular deflection.At 200℃and strain rate of 2150 s^-1（impact pressure is0.7 MPa）,extension twinning causes the increment of maximum pole intensity.The main deformation modes of impact deformation are basal slip and second-order pyramidal slip,both of which can rotate the c-axis but in opposite directions.The final textures most grains exhibit close to the center of the（0002）polar,due to the fact that activity of basal slip is higher than that of second-order pyramidal slip.