Study On The Mechanical Behavior And Deformation Mechanism Of Mg-3Al-1Zn Alloy Containing Multiple Texture Components

In this thesis commercial AZ31 magnesium alloy in the forms of hot rolled plate with a strong basal texture and extruded bar with a strong fiber texture was choosen as the starting material.A variety kinds of specimens with different texture distribution were prepared.The microstructure,deformation mechanism and texture evolution in the process of plastic deformation and heat treatment was investigated systematically by compound use of electron backscatter diffraction(EBSD),X-ray diffraction(XRD)and the method of slip trace analysis.In the present study,the relationship between the volume fraction of soft oriented grains and mechanical properties,and its deformation mechanisms were studied in the quantitative viewpoint.The strengthening effect of bimodal texture on slip and twinning,the thermal stability of bimodal texture during heat treatment,and annealing hardening behavior were investigated in detail.In addition,combined with crystal plasticity simulation,strain path dependence of {10(?)2} twinning in a rolled Mg-3Al-1Zn alloy is analyzed.The results in this thesis show as follows:? The mechanical behavior and deformation mechanism of bimodal textured materials(soft and hard oriented grains distributed with different proportions)were studied from the perspective of quantitative for the first time.In the initial deformation,the yield strength drop remarkablely when a small amount {10(?)2} twins were produced,and when the volume fraction of soft-oriented grains exceed a certain proportion,there were no use to improve the yield strength even a few grains with hard orientation existed.The yield strength was deviate from the theoretical value of ROM(rule of mixture)inordinately.When the volume fraction of soft oriented grains below 50%,the deviation increased gradually,and the deviation decreased gradually when the volume fraction higher than 50%.Compared to monolithic texture,more strains at the initial stage of deformation is accommodated by soft oriented grains,and activating more twin variants.Even the twin variant with lower Schmid factor(SF)can be activated by the role of strain accommodation.? For rolled plate of Magnesium alloy AZ31,bimodal textures can be prepared by compression along thanserve direction(TD)to different strains sach as 2.5%,3.8%,5.4%,7.0%,and followed by annealing at 250°C for three hours.Through texture tailoring,the quantative relationship between bimodal texture and tension-compression yield asymmetry was established.Good asymmetry can be achieved when two texture component are distributed in a proper proportion.?(0002)distribution might pose a great influence on the yield strength of slip and twinning predominant deformation after compound calculation of activation stress difference of slip/twinning(?Stress)and geometrical compatibility factor(m?)between neighboring grains.Compared to the monolithic texture(MT),<0002>//ND,the bimodal texture(BT),<0002>//ND together with <0002>?ND would generate a higher compressive yield strength,irrespective of a similar tensile yield strength in the two textures.The different compressive yield strengths in the two texture mainly comes from their different boundary obstacle effects against {10(?)2} twinning as the presence of a large number of neighboring grains with a high angle between their c-axes in BT texture.? Both the <0002>?ED texture and the <0002>//ED texture components in extruded Magnesium AZ31 show good thermal stability either at 300°C or 450°C,and such thermal stability was not influenced by the volume fraction of each texture.? A new annealing hardening mechanism was found in pre-twinned Mg-3Al-1Zn alloy.Annealing hardening phenomenon(20-25 MPa)during compression along certain direction appears in all the tested annealing temperature(200 °C,250 °C and 350 °C)for whatever 0.5 h or 3 h annealing time.Annealing can remove the pre-existing twins,leading to a transition from a detwinning predominant deformation with low activated stress to a basal slip predominant one during deformation.This change of deformation mechanism accounts for the annealing hardening phenomenon.? Comparative study on strain path dependence of {10(?)2} twinning using two typical twinning models(EVPSC-PTR and EVPSC-TDT)were carried out.Compared with the PTR modal,the TDT model permits better curve fitting and texture prediction.The EVPSC-PTR model can not fit the TD-c and ND-t flow stresses simultaneously,whereas the EVPSC-TDT can.To better fit both the tension(ND-t)and compression(TD-c)curves,strong latent-hardening for the critical resolved shear stress(CRSS),for {10(?)2} twinning by other twinning system(htt),is necessary.