Twinning,Dynamic Recrystallization And Their Influence On The Flow Stress Behavior Of AZ31 Magnesium Alloy During Hot Deformation

Magnesium alloys with high strength and specific-strength,good damping property,electromagnetic shielding and free cutting property are extensive used in the field of modern transportation and information technology etc.However,application of magnesium alloys is inhibited for their limited slip system.Therefore,deformation mechanism in magnesium alloy is required exhaustive research.In order to study flow softening,deformation twinning and dynamic recrystallization,compression tests were carried out on as-extruded AZ31 magnesium alloy samples at temperatures of 350?,400? and 450? and strain rates of 0.03 and 0.3s-1 with various strains of 0.1,0.2,0.5,0.7 and 1.Following conclusions were obtained:Electron Back-Scattered Diffraction(EBSD)and HKL-EBSD soft ware were applied to determine Euler angle,position of grains and twins in(0002)pole figure and to identify the type of twinning.The results indicate that extension twinning decreases with temperature increasing and strain rate decreasing.Mirror-transformation matrix was used to analyze the twin variants.Schmid factors for six twin variants in matrix grains were investigated.It is found that the initiation of twinning does not only depend on the Schmid factor.At intermediate and low temperature,the activation of twinning is related to Exy component.At high temperature,Ezx determines the initiation of twining.It can be concluded that the nucleation position of twinning depends on the accommodation strains induced by twinning and the main deformation mechanism of corresponding neighboring grain by combining accommodation strains with Schmid factor.Bulge nucleation takes place during the hot compression of magnesium alloy AZ31,indicating that discontinues dynamic recrystallization occurs.Extention twinning-inducing dynamic recrystallization was also detected.Basis on the microstructure evolution,the volume fractions recrystallized were determined.It is shown that the volume fractions recrystallized increases with increasing temperature and strain.According to the orientaion map,tiny recrystallized grains nucleate on twinning boundary and inside twins.Parallel twin lamellaes are developed in the grains and separate the parent grains into several sections.As the initiation of dynamic recrystallization,the new grains consume the parent grains gradually.The twins also quickly lose their original morphology.With temperature decreasing and strain rate increasing,the amount of twinning increases providing more nucleation positions,and followed by grain refinement.X-Ray technology(XRD)was applied to detect the evolution of texture and the orientation distribution figure(ODF).The main texture components were calculated using ODF.The result shows that(0001)[0001]and(01-10)[0001]basal texture instead of(01-10)[0001]fiber texture gradually become main texture during hot deformation.The flow curves were analyzed using the double differentiation method and double minima were detected on all the curves.The critical strain for twinning and dynamic recrystallization(DRX)were identified.This result indicates that twinning is prescursor of DRX.The contour lines describing the Schmid factors for basal-slip,prismatic slip,2nd-order pyramidal slip and extension twinning in(0002)pole figure were calculated and superimposed on(0002)pole figure at various strains.Basal slip is the dominant deformation mechanism during the whole deformation proc.essing.Extension twinning and 2nd-order pyramidal slip are good secondary deformation modes(after basal glide)at early stage of deformation.On increasing the deformation,prismatic glide replaces extension twinning as a possible deformation mode in addition to basal glide.According to the relationship between critical resolved shear stress/average Schmid factor(CRSS/m)of basal slip and strain,CRSS/m of basal slip decreases dramatically during the first 20%of straining and approaches to final value.Based on dynamic recrystallization,the evolution of texture,Schmid factor and EBSD results,it can be concluded that most of the flow softening is attributable to texture change and a minor portion is associated with the volume fraction recrystallized.The schematic diagram for the flow stress was established.