Investigation Of Twinning Mechanism In AZ31B Magnesium Alloy During Plastic Deformation And Its Influence On Mechanical Behaviors

Owing to advantages of high specific strength,high specific rigidity and good electromagnetic shielding performance,wrought magnesium alloys have received so much attention to their broad applications in the fields of aerospace,automobile and electronic communication.However,magnesium alloys have hexagonal close-packed crystal(HCP)crystallographic structure,which induce the limited number of independent slip systems for homogeneous plastic deformation at room temperature,so they usually present a strong anisotropy behavior and the insufficient plastic performance to restrict the processing and applications.To investigate the inherent deformation mechanisms and laws and their influence on mechanical behaviors of the magnesium alloys is of significance for improving the plastic performance and service performance of these alloys.In addition to dislocation slip,mechanical twinning is one of the deformation mechanisms and plays an important role in the plastic processing and application of wrought magnesium alloys.Twinning mechanism during deformation in AZ31B magnesium alloy was addressed in this paper.A series of works have been conducted to investigate the mechanism and its effect on the mechanical behaviors.Based on the analysis of crystallographic orientation and symmetry,a reliable calculation program for identifying twin variants was developed firstly.Then,the character and principle of primary twinning type selection and twin variant selection in the process of tensile plastic deformation were investigated,respectively.Furthermore,the characteristics of deformation mechanisms under different strains were analyzed in detail by using in situ Scanning electron microscope-Electron backscatter diffraction(SEM-EBSD)observation technique.Finally,Bauschinger effect and low cycle fatigue behavior were studied.The main results are as follows:1.Based on the analysis of the characteristics of the primary twins and their variants during tensile deformation along the extruded direction of the sample,it was showed that refer to tensile axis,the orientation range of the grains deformed by {10-11} contraction twining in the inverse pole figure is higher than that of {10-12} extension twining.Average Schmid factor for the activated {10-11} contraction twin variants is higher than that of activated {10-12} extension twin variants.With the increasing of tensile strain,the number of both two type twins increase.While the number of {10-11} contraction twin increases more rapidly than that of {10-12} extension twins.This phenomenon is related to the fact that the Schmid factor distributed in grains for ?10-1 1} contraction twinning is averagely much higher than that of {10-12} extension twins,resulting in a rapid increment of resolved shear stress(RSS).Although the selection of {10-12} extension twin variants is highly correlated to Schmid factor rank,the Schmid factor law is not the only criterion to identify the twin variant selection.The non-Schmid factor behavior of {10-11} contraction twinning is more evident than that of {10-12} extension twinning.2.The results of in situ SEM-EBSD observation showed that basal<a>slip with the lowest CRSS is activated earlier and acts as the dominant slip mechanism during the tensile plastic deformation.It was suggested that prismatic<a>and pyramidal<c+a>slip could be activated in grains with high Schmid factor under high loading stress,or could be activated as strain accommodation mechanisms for twin variant selection.Kernel average misorientation(KAM)varies within individual grains,indicating inhomogeneity of the plastic deformation induced by slips.Slips are activated first in small grains.Local plastic strain induced by slips is mainly concentrated at the grain boundaries.Both {10-12} extension twinning and {10-11?contraction twinning are activated and developed with increasing stress/strain.Due to lower CRSS,{10-12} extension twinning is activated earlier than {10-11} contraction twinning.Based on the analysis of strain accommodation theory,it was found that the proposed local Schmid factor(LSF)and strain accommodation factor(SAF)could be used as criteria for the prediction of twin variant selection.This is in good agreement with the experimental results.The accommodation factor is generally the lowest for the activated twin variants.3.In order to investigate the Bauschinger-like effect during unloading process and the Bauschinger effect during reversal loading process of plastic deformation,the extruded AZ31B alloy was loaded with different pre-strains and strain rates.The results showed that Bauschinger-like and Bauschinger effects are significantly different in tension-unloading-compression(T-U-C)mode from them in compression-unloading-tension(C-U-T)mode.For T-U-C mode,the Bauschinger-like effect during unloading is strain rate sensitive under high pre-strains(16%and 20%);however,the strain rate sensitivity is not obvious under the C-U-T mode.For T-U-C mode,the strain rate sensitivity of Bauschinger effect is high when low pre-strain is applied(8%and 12%)and becomes more evident with the increasing of strain rate.For C-U-T mode,the Bauschinger effect becomes indistinct with the increase of pre-strain,but the strain rate has no effect on it.Based on the deformed structure observation,it is found that the two kinds of Buaschinger effect are related to the activity of detwinning.4.The asymmetric stress-controlled low-cycle fatigue behavior of the alloy was investigated with different loading stresses and frequencies,respectively.It was found that the peak stress has a significant effect on the plastic strain amplitude,the plastic strain amplitude increases with the increase of peak stress.During cyclic deformation,a large number of dislocations and twins are activated simultaneously,and the interaction between them]eads to cyclic hardening and fatigue life reducing significantly.The effect of loading frequency on the plastic strain in the cyclic steady period is miscellaneous:in the same cycle,the lower the loading frequency is,the more deviation of hysteresis loop position is,and the higher the ratcheting strain rate is.Ratcheting strain rate decreases with reduction of peak stress.Based on the energy density model,fatigue life was analyzed and it was suggested that the change of plastic strain energy density dominates in the total change of strain energy density,and the effect of loading frequency on the fatigue life is realized via the energy dissipation in cyclic plastic deformation.