Research Of Mechanical Behavior And Its Mechanism Of AZ31B Magnesium Alloy During Rolling And Cyclic Deformation

Due to the high strength-to-weight (rigidity) ratio, electromagnetic shielding performance and easy recycling, magnesium was widely used in airplane, automobile, computer and communication equipment etc, as structural parts. The plastic deformation ability is an important element about the use of magnesium, and the dislocation slip and mechanical twinning are the chief deformation modes at room temperature. The deformation modes are usually affected by deformation ways, loading conditions, textures and their critical resolved shear stress (CRSS). So the researches on the relation between deformation process and mechanism in different deformation ways, textures, and strain rates are significant, and could improve the design and application of magnesium.The evolution of deformed microstructure and corresponding mechanism of AZ31B magnesium alloy were investigated during cold-rolling via multiple passes. The extruded AZ31B alloy plate with fiber texture, after appropriately trimmed, can be cold-rolled to a high reduction of 95.3% with 250mm in length. In the process, before the 3th rolling, twinning plays an important role before total rolling reduction of 20.7%, and the extension twinning is basically responsible for the formation of the basal texture. After the 3th rolling, slip is the predominant deformation mode in rolling, at the same time the contraction twins remained after the previous passes rolling can be deformed, distorted and even fragmented by slip mechanism. The microstructure is refined after the severe cold-rolling with 95.3% reduction, the formation of dislocation boundaries and further more sub-grains/grains may be possible related to multiple slips and dislocation interactions. Multi-pass rolling with lower reduction in per pass and small ratio of the plate thickness to roller radius are favorable for the activation of multiple slips in grains with fiber texture, which is conducive to obtain high ductility.In order to investigate the effect of cyclic frequency on LCF behavior, the extruded AZ31B alloy with fiber texture was prepared for LCF tests at different cyclic frequencies (0.01HZ,0.1HZ,2.0HZ and 5.0HZ) and strain amplitudes (0.75%,1.0%,2.0% and 4.0%) under asymmetric loading. It was found that at the every strain amplitude, the cyclic frequency exerts an intense effect on cyclic hardening rate and LCF resistance. As the frequency increment, the curve of stress amplitude, hardening rate and plastic strain magnitude development were restricted, but the cyclic life was extended.In order to investgate the cyclic deformation process and reveal the relation between the deformation behavior and mechanism, the deformed microstructure, in EBSD situ, was observed under asymmetric loading. At the same time, twinning process was analyzed and the deformation mechanism was simulated by using the self-consistent model (VPSC) about two grains which have different initial orientation. The results show that the twinning-detwinning mechanism plays an important role in cyclic deformation owing to the fiber texture. When the compressive or tensile stress was unloaded during cyclic deformation, some twins become smaller or disappeared. The interconnected combination factors, including the grains orientation distribution, loading ways, critical resolved shear stress (CRSS) all affect the twinning process. The types of the dislocation and proportion can also be affected as twinning by the grains orientation during cycle deformation.The magnesium alloys deformed at higher strain rate exhibit more twins than the lower strain rate which is contrary to the dislocation. During cyclic deformation, twinning-detwinning plays an important role during the cyclic process, and after unloaded and reversely loaded to zero stress there are more remaining extension twins at low frequency (strain rates) compared to at high frequency. When compressed to 2.0% strain at dynamic strain rate 8.0×10-1s-1, in addition to large extension twins, a large number of nanostructured lamella extension twins were found in the matrix. The reason that such nanostructured twins co-exist with large twins is explained in terms of comparison between dislocation and twinning dynamics, i.e., due to the high strain rate the dynamic critical resolved shear stress for slip could be raised and make slip difficult to catch up with deformation. So, the abundant occurrence of the rapid extension twin and nucleation should be needed in widely oriented grains to accommodate strain at the high strain rate.During the deformation, the weights of slip and twinning are strongly sensitive to the cycle frequency, twinning-detwinning predominately contributes to the deformation at high frequency, fewer twins and more dislocations are activated at low frequency. The dislocation slips is corresponding to the trend of cyclic hardening rate. So, at high frequency, the dislocation slips contribute less to the strain-range and less to the total plastic strain magnitude which present the cyclic hardening characteristics. Meanwhile, predominant twining-detwining mode has less effect on cracks initiation and makes the stress amplitude lowered which is resulting in the low cyclic hardening rate and long LCF life.