Study On The Plastic Deformation Mechanism Of Magnesium Alloy Materials With Viscoplastic Self-consistent Model

Magnesium alloys has been widely used in the automotive industry to realize lightweight and energy saving.At room temperature,a limited number of slip systems are active in the deformation process of magnesium alloys.Twinning is another deformation mechanism,which contributes greatly to plastic deformation.Twinning has been extensively studied.Due to the low symmetry of hexagonal crystal structure(HCP),magnesium alloy has lower ductility compared with steel,aluminum alloy and other metals with cubic crystal structure.The deformation behavior of magnesium alloys,including rolled plates,extruded bars and magnesium alloys doped with rare earth elements,was further studied by using the VPSC model.PTR and TDT twinning models are also used to describe the twin behavior of magnesium alloys.The plastic deformation mechanism under uniaxial compression and tension is studied systematically and the texture evolution 1s analyzed deeply.The main work and achievements are as follows:(1)The deformation behavior of AZ31B magnesium alloy is discussed deeply based on VPSC-TDT model.Under the condition of uniaxial tension,the slip activity of Basal decreases obviously with the increasing strain.The slip activity of Prismatic increases steadily with the increasing strain.The activity of Pyramidal slip is always very low and only acts in small scale plastic strain(0?2%).The activity of Extension Twin increases steadily with the increasing strain,and it decreases slightly with the rotation angle RD to ND.Under uniaxial compression,the volume fraction of the Extension Twin increases until the plastic strain reaches 9%.Generally,the TVFs under different loading paths follow the order:TD>45°>RD.The results indicate that the twinning activity is the main reason tor the negative R value of in-plane compression.The additional hardening caused by the {10-12}twinning boundary has an important influence on the potential hardening coefficient of the Pyramidal slip.When the twin is exhausted,the overall stress will be affected by 6.8%?7.3%.The characteristic anisotropy of Mg alloy sheets,such as tension/compression asymmetry,sinusoidal stress strain curves when twinning is activated,negative R-value,etc.,were well captured by the model.With the aid of simulating the evolution of Mg single crystal,both the HCP crystallographic structure and the orientation distribution of the Mg sheet are responsible to the anisotropy.The polar nature of the deformation twinning leads to the strong tension/compression asymmetry.(2)Mechanical behavior of AZ31B magnesium alloy plates are investigated using different self-consistent mechanisms.For AZ31B sheet,the Affine model has the best predictive capacity and applicability.The second is the Neff model.Although the uniaxial tensile/compression prediction results of the Secant model in RD and TD directions are relatively close to the experimental data,the predicted R-values of the Secant model are quite different from the experimental data.It is so different from the experimental results that the Tangent model predicts,especially in the simulation of ND C stress-strain curve.In the numerical simulation without considering the texture evolution,the predicted R value still varies with the strain.For the predicted results of Affine model,the R-value is approximately constant after 3%strain.However,according to the results simulated by Secant model,the R-value tends to decrease after the strain reaches 4%,and the predicted R-value is too high in the whole process.(3)The polycrystalline viscoplastic model is applied to character the mechanical behavior of ZEK100 magnesium alloy sheet.A characteristic of ZEK100 performance is that the first yield is accompanied by twinking under transverse tensile load(TD_T),which is not seen in AZ31B.The results show that follow-up work hardening behavior of TD samples is stronger compared with RD_T and 45°T.In the plane tension,the yield stress decreases with the change of RD to TD direction.In the plane compression,the yield stress is hardly affected by the loading direction,and the stress-strain curve shows the tipical "S" shape.Compared with the loading along RD direction,ZEK100 sheet has low flow stress under same strain with the loading along TD direction.The flow stress of plane loading follows the order:RD>450>TD.ZEK100 sheet has strong anisotropy in RD-TD plane,low yield strength,high hardening rate,and increased ductility with the change of RD to TD direction.In contrast to the strong asymmetry observed in RD,for TD case,the tensile and compression asyumetry of yield stress is lower.For the uniaxial tension,loading rate ranges from 0.0015-1 to 100s-1.The strength of the deformed teture decreases gradually with the higher laoding rate.After compression deformation,the texture strength increases with the increase of loading rate.(4)The deformation mechanism of AZ31B magnesium alloy was studied using two twinning models.The results predicted by the twinning models of PTR and TDT agree with the experimental data.In the process of tension along the ED direction,the extension twin mechanism plays a small role.The prediction results of the twin models are similar due to the slip-dominated deformation mechanism.For compression deformation,the effects of different twin mechanisms on the stress-strain curve are obviously different.The flow stress predicted by TDT model is slightly higher than the experimental value.The flow stress predicted by PTR model is slightly lower than the experimental value.For the initial texture,the e-axis orientation of most grains is mainly perpendicular to the ED direction.Under larger plastic deformation,the texture results predicted by TDT and PTR are consistent.In the case of small strain,the texture evolution predicted by TDT and PTR is quite different.The TDT model can properly describe the rotation of grain during deformation.However,the texture predicted by the PTR model has no rotation grains with the c-axis rotating to loading direction(ED direction)before the strain of 3.72%.This result is mainly caused that only the predominant twin is considering in each grain for PTR model.