Mechanical Properties And Microstructure Evolution Of AZ31 Magnesium Alloys Under High Strain Rate

In this thesis commercial AZ31 magnesium alloy in the forms of hot rolled plate with a strong basal texture was chosen as the initial material. In total, seven kinds of specimens for compression were prepared. They were cut from hot rolled plate with their compression axis aligned 0°, 15°,30°,45°,60°,75° and 90° to the normal direction(ND) of the rolled sheet, respectively. Uniaxial compression had been carried out at different strain rates(10-3 and 103 magnitude). The microstructure of sample with specified strain had been quantitatively characterized using optical microscope(OM) and electron backscatter diffraction(EBSD). The high strain rate compression was carried out with split Hopkinson bar(SHPB).Through analyzed the stress-strain curve, we discussed anisotropy and strain hardening of AZ31 magnesium alloy during high strain rate deformation. And through compared with low strain rate deformation, we discussed strain rate sensibility. Main conclusions found are as following:(1) the mechanical properties under high strain rate of AZ31 magnesium alloy with different orientation have strong anisotropy.(2) The stage ? process of strain hardening of 45°, 60°, 75° and 90° specimens is synchronized with twinning process, and they almost have same finish rate.(3) Compared with low strain deformation, 0° orientation has higher yield strength under high strain rate, 30°-90° orientation has almost same yield strength.Using OM and EBSD, we discussed microstructure evolution of twinning behavior and shear band. And through compared with low strain rate deformation, we discussed competitive relation between twinning growth and nucleation. Main conclusions found are as following:(1) in 0° and 15° orientation of AZ31 magnesium alloy, shear bands are formed during high strain rate deformation.(2) To 45°, 60°, 75° and 90° orientation, the variant selection mechanism of {10-12} tension twinning nearly conforms to Schmid factor criterion during high strain rate deformation, but compared with low strain rate deformation, twinning nucleation is largely enhanced.(3) There are a few twins and plenty of slips in the shear band of 0° orientation. Basal slip has the largest SF, so through coordinated with basal slip the twinning can get driving force.