Study On The Anisotropic Mechanical Behavior And Deformation Mechanism Of AZ31 Magnesium Alloy Under Wide Range Of Strain Rates

As a lightweight structural material, magnesium alloys show a remarkable potential for application in aviation and automobile industries. However, wide applications of magnesium alloys are hindered by their strong anisotropy at room temperature. In order to overcome this deficiency, macroscale mechanical behaviors and microstructure evolutions of magnesium alloys have been widely investigated by many researchers. Nevertheless, how microstructure evolutions affect the macroscale mechanical behaviors is unclear, and the mesoscale measurements connecting macroscale and microscale deformation is lacking. The exact nature of anisotropic deformation is still under debate. Simultaneous multiscale measurements based on in situ synchrotron x-ray imaging and diffraction are conducted to investigate the anisotropy of magnesium alloys under wide range of strain rates. The loading axis(LA) is either perpendicular or parallel to the c-axis in textured samples, and these two cases are referred to as LA?<c>and LA ?<c>. During defroamtion of magnesium alloy, simultaneous multiscale measurements are obtained in terms of marcoscale stress--strain curves, mesoscale strain fields and microscale diffraction patterns.Electron back-scatter diffraction (EBSD) is conducted to characterize deformation twins.1. During quasi-static compression with a strain rate of 5×10-4 s-1,stress--strain curves, and the evolutions of strain fields and diffraction patterns of magnesium alloys show pronounced anisotropy at room temperature. For the LA?<c>loading, {1012}extension twinning dominates the plastic deformation, stress gradients can be released by twinning to achieve homogeneous strain fields within a short period of time, and reducing strain localizations effectively boosts strain hardening rate. However,dislocation motion dominates the plastic deformation of the LA? <c> sample,dislocations nucleated at defects induce strain concentrations owing to short slide lengths and pileups. Mesoscale inhomogeneous deformation induces reduced strain hardening rate.2. During quasi-static compression at room and elevated temperatures,macroscale mechanical properties, mesoscale strain fields and microscale lattice deformation of magnesium alloys exhibit strong anisotropy, the strain rate is 10-3 s-1.Due to initial texture, {1012} extension twinning is predominant in the LA?<c>sample, while dislocation motion prevails in the LA? <c> loading. With increasing temperature, fewer extension twins are activated in the LA?<c>samples,giving rise to reduced strain homogenization, while {1122}(c+a) pyramidal slip becomes readily activated, leading to more homogeneous deformation for the LA ? <c> loading. Both pyramidal (c + a) dislocation slip and {1012} extension twinning can result in homogenization in plastic deformation by accommodating the deformation along and perpendicular to the crystallographic c-axis. The difference in the strain hardening rates is attributed to that in strain field homogenization for these two loading directions.3. Similar to quasi-static loading, dynamic responses of magnesium alloy show strong anisotropy under split Hopkinson pressure bar loading at a strain rate of 5.5×103 s-1. {1012} extension twinning induces homogenized strain fields and gives rise to rapid increase in strain hardening rate, while dislocation motion leads to inhomogeneous deformation and a decrease in strain hardening rate. Nevertheless,during the early stage of plastic deformation, twinning is dominant in dynamic compression, while dislocation motion prevails in quasi-static loading, manifesting a strain-rate dependence of deformation.4. During planar impact with strain rates from 0.92×105 s-1 to 1.35×105 s-1, the elastic-plastic transformations of LA?<c>and LA?<c>exihibit pronounced anisotropy,while the Hugoniot elastic limit values of both loading are similar, -0.32 GPa. The spall strengths of LA?<c>sample are slightly greather than the LA?<c>loading at a lower impact velocity. With the increase of impact velocity, the spall strengths of both loading increase and the differences between them decrease. When the impact velocity reaches 400 m/s,the spall strengths of LA?<c>and LA?<c>samples are basically the same. EBSD analyses reveal that abundant {1012} extension twins are activated in the LA?<c>sample, while few twins are observed in the LA?<c>sample.