Effect Of Prefabricated Extension Twinning On High Strain Rate Deformation Performance And Microstructure Of AZ31mg-alloys Sheet

As one of the lightest structural materials,magnesium alloys have been widely used in automobile,aerospace and other industries due to their low density,high specific strength,specific stiffness,good damping performance and other advantages.Magnesium alloy will inevitably subject to an impact of high strain rate loading if it was made of a component of vehicle.The mechanical properties and deformation ability under high strain rates of magnesium alloys should be paid more attention to ensuring their response to the severe application environment.Due to the orientation effect,easy to be activated and the effective reduction of grain size,the prefabricated extension twins can be used as an important method to improve the formability and strength of deformed magnesium alloys.The purpose of this thesis is to study the effect of preformed tensile twins on the deformation properties and microstructure of Mg-3Al-1Zn?AZ31?magnesium alloy sheet at high strain rates.The pre-extension twins were introduced by quasi-static compression experiment with the deformation amount of 2.5% and 5% along rolling direction?RD?.The Split Hopkinson Pressure Bar?SHPB?was used to test the high strain rates compression behavior of the pre-extension twin samples,and the evolution of the microstructure after high strain rates deformation was observed by Optical Microscope?OM?and Electron Backscatter Diffraction?EBSD?.The Schmid factor?SF?of different deformation modes was analyzed by using self-made software.The high strain rates compression experiments were carried out along normal direction?ND?,rolling direction?RD?and transverse direction?TD?at the strain rates ranging from 900s^-11 to 1800s^-1,and the deformation temperature was at room temperature to 350?.The experimental results showed that the increasing strength of pre-extension twinning samples?2.5%RD?along RD is mainly due to the hindrance of twin boundary to dislocations,the increasing strength along TD is due to the pre-twinning boundaries and the increasing portion of hard orientation region,which is resulting in the transition of deformation mechanism dominated by extension twins to pyramidal slip.The sharp decreasing of the compression strength along ND is mainly due to the initiation of de-twinning and slip provided by pre-twinning boundaries.The 5%RD sample can activate more {10???2} extension twins in AZ31 Mg-alloy sheets.The formation of a large number of {10???2}extension twinning boundaries can enhance the effective grain boundary area,further enhance the grain boundary strengthening effect,and increase the strength of RD and TD samples.The microstructure after high-speed deformation at room temperature is related to the texture after pre-extension twinning.There is no extension twin in three kinds of samples at 1500s^-1.The formation of extension twinning boundaries can effectively inhibit de-twinning behavior and increase the yield strength,when high strain rates compression is applied along ND.The dynamic deformation mechanism of non-deformed and pre-twining samples at high temperature is a combination of dynamic recrystallization,slip and twinning.After high-speed deformation at 250?,the microstructure is mainly dynamic recrystallized grains,and the recrystallized grains grow at 350?.