Influence Of Pre-twinning On High Strain Rate Deformation Behavior Of AZ31 Mg-alloys Sheet

Magnesium and its alloys,as one of the most ideal structural materials for lightweight,have great potential applications in automotive,aerospace and 3C industries.It is well known that magnesium alloy parts are inevitably subjected to severe loading deformation conditions,such as explosion and impact.Therefore,it is important to understand the dynamic mechanical properties and deformation mechanism of magnesium alloys for optimizing the mechanical properties of magnesium alloys.Pre-twinning can be used as a method to improve the formability and strength of wrought magnesium alloys due to the reorientation effect in the extension twin region and its easy activation.The purpose of this study is to investigate the effect of pre-twinning on high strain rate deformation behavior of AZ31 Mg-alloy sheet.Split Hopkinson Pressure Bar（SHPB）was used to test the high strain rate deformation behavior of pre-twined AZ31 Mg-alloy sheet along different directions.The evolution of microstructure and texture of pre-twinning samples before and after pre-compression was analyzed by using MATLAB programming software and Electron Backscatter Diffraction（EBSD）.The high strain rate deformation behavior and deformation mechanism of pre-twinning AZ31 Mg-alloy sheet under different strain rates,deformation temperatures and load directions were analyzed.The loading direction of high strain rate experiment is rolling direction（RD）,transverse direction（TD）and normal direction（ND）of AZ31 Mg-alloy sheet.The strain rate is controlled from 600s^-11 to 2200s^-1,and the deformation temperature is from 20 to 300?C.The experimental results show that the strength along RD and TD of AZ31 Mg-alloy pre-twinning sample increases significantly at room temperature,while ND decreases significantly,and the elongation of the samples hardly changes.With the increase of temperature,the dynamic compressive strength along RD,TD and ND of non-deformed and pre-twinning samples decreases and tends to be consistent,indicating that the pre-twinning effect is becoming smaller and smaller.The strain rate has little effect on the high strain rate deformation behavior of pre-twin samples.The analysis results showed that the increase of strength along RD of primary pre-twinning samples（2.5%TD）is mainly due to the hindrance of twin boundary to dislocations;the increase of strength along TD is due to the continuous growth of pre-twinning and the increase of hard orientation region,which results in the transition of deformation mechanism dominated by extension twins to pyramidal<c+a>slip;and the sharp decrease of strength along ND is mainly due to the initiation of de-twinning.The pre-compression along TD+RD can activate multi-twin variants and secondary extension twin in AZ31 Mg-alloy sheet.The formation of secondary twin and a large number of cross-twin variants can increase the effective grain boundary area,enhance the grain boundary strengthening effect,and increase the strength of RD and TD again.The formation of{1021}-{1021}double twins and a large number of cross-twinning lamellas can effectively inhibit de-twinning behavior and increase the yield strength,when high strain rate compression is applied along ND.The dynamic deformation mechanism of non-deformed and pre-twining samples at high temperature was a combination of slip and twinning,and accompanied by the occurrence of dynamic recrystallization.