Study On Mechanical Property Of Magnesium Alloys With Bimodal Grain Size Distribution Using Micromechanic Models

As a reinforcing and toughening structure,heterogeneous materials have obtained good experimental results at the nano to micron scale,which has become a research hotspot in recent years.As a kind of heterogeneous material,the bimodal grain size distribution structure including coarse and fine grains has great potential for application in magnesium alloys.In the present study,polycrystal materials were treated as particle composite materials,in which grains of different sizes were considered to be different types of particles,and grain boundaries were regarded as matrix.Taylor nonlocal strain graindent theory of plasticity was used to modify the influence of particles on the matrix,and then the polycrystal finite elements plane strain model based on microstructure was constructed.The multiphase Eshelby mechanical analysis model was constructed according to Eshelby equivalent inclusion theory.The mechanical properties of polycrystal AZ31 high-toughness magnesium alloy and GW83K high-strength magnesium alloy with bimodal grain size distribution were simulated by using these two models.The effects of distribution,content and shape of coarse and fine grains on the macroscopic mechanical properties of AZ31 magnesium alloy were studied.The mechanical properties of GW83K magnesium alloy corresponding to the grain structure obtained by different annealing processes were studied.The conclusions obtained are as follows:The plan strain model of polycrystal finite element(PFE)of AZ31 magnesium alloy was constructed by using Taylor nonlocal strain gradient theory of plasticity.The three types of grains in the alloy were S grains(d=2.5 ?m),M grains(d=5.2 ?m)and L grains(d=10.6 ?m),and the simulated stress-strain curve is basically consistent with the experimental curve.The difference between the yield strength and the tensile strength is less than 5%,which verifies the reliability of the model.The fracture elongation of polycrystal material was used as the critical strain of crack propagation to evaluate the tendency of crack nucleation and propagation in the strain concentration region.After verifying the validity of the model,the polycrystal finite element plane strain model of AZ31 magnesium alloy was constructed based on the polycrystal phenomenological model of bimodal grain size distribution with different coarse grain distribution by phase field method to study the effect of the coarse grain distribution on toughness of AZ31 magnesium alloy.The simulation results indicate that under the premise that no fracture criterion and perfect interface are in the PFE plane strain model,the distribution of coarse grains has slight effect on the overall mechanical properties of AZ3 1 magnesium alloy.However,it can be seen from the equivalent strain distribution map that the movement of the coarse grains leads to a change of the surrounding grains type,which promotes or hinders the penetration of the shear band.After analyzing each model,it is found that the penetration of the shear band occurs mostly in the coarse and medium grains.High local distortion occurs when crossing triple junction or smaller single fine grains,which increases the tendency of crack nucleation and propagation.When the shear band passes through the larger single/multiple fine grains,the penetration of the shear band is hindered.It can be considered that when the fine grains are wrapped into a coarse grain,the tendency of cracking due to local strain concentration can be effectively alleviated.The mechanical analysis model of AZ31 high-toughness magnesium alloy was constructed by Eshelby equivalent inclusion theory.The simulated stress-strain curve is in good agreement with the experimental curve,verifying the reliability of this model.After verifying the validity of the model,the mechanical response of the whole model and the grains in the corresponding model was studied by changing the content and shape of the three types of grains.The simulation results indicate that the increase of the content of fine grains leads to the decrease of the mismatch strain and the mean disturbance strain of the all types of grains,and increase of the content of coarse grains leads to the opposite trend.The trend of the overall stress-strain curve is contrary to the trend of mismatch strain.It could be seen that the increase of the content of fine grains makes the polycrystal material more stable and coordinated.Compared with the change of the mismatch strain and the stress in three types of grains,the variation of the mean disturbance strain is more obvious.Therefore,the mean disturbance strain is the main influencing factor,and the mismatch strain is the secondary influencing factor in model 1-4.In model 5-8,for the influence on the geometry shape of grains,the analysis shows that with the change of grain morphology from spherical to ellipsoidal,the stress borne by such grains during deformation is gradually distributed to the other two types of grains,which exhibit the decrease of mismatch strain and the stress of grains.In these models,mismatch strain is the main influencing factor.The polycrystal finite element plane strain model of GW83K high-strength magnesium alloy was constructed by using Taylor nonlocal strain gradient theory of plasticity.After verifying the validity of the model,the phase-field method was used to simulate the recrystallized grain growth in the annealing process under real-time conditions.The polycrystal finite element plane strain model was constructed based on the simulated microstructure images.The effects of annealing time and annealing temperature on the mechanical properties of GW83K magnesium alloy were investigated.The simulation results show that the yield strength and tensile strength of GW83K magnesium alloy decrease with the increase of grain size,and the yield strength accords with the Hall-Petch relationship.Analysis of the equivalent strain distribution map indicates that as the grain size increases,the zone of coarse grains could accommodate more dislocations,resulting in the broadening of the shear band.And the finegrained zones lead to the convergence of the shear band,resulting in an increase in local distortion.During the construction of the Eshelby mechanical analysis model of GW83K magnesium alloy,it was found that the influence of interface segregation and second phase precipitation on the matrix was obvious,and the simulation results deviated greatly from the experimental values,which required further correction.Finally,the multi-particles recognition software if optimized and improved.The multiphase recognition module and the 2D images batch processing module are added and improved.The module lays the foundation for subsequent high-throughput simulation calculations from microstructure to performance.In summary,for magnesium alloys with bimodal grain size distribution,the change of different grain properties can effectively avoid stress concentration and strain distribution,and also coordinate the deformation uniformity of different regions inside the material,which is to improve the strength and plasticity of the material and provide a new design idea.