Study Of Deformation Behavior Of Rolled AZ31 Magnesium Alloy At Room Temperature

Magnesium and its alloy usually have strong texture,and due to the appearance of 2}1{10 tensile twinning,this type of twinning is always responsible for the low yield stress,asymmetry between tension and compression and anisotropy of alloy.In the current study,a rolled AZ31 magnesium alloy is employed to analyze the effect of 2}1{10 tensile twinning on evolutions of microstructure,texture and mechanical properties.Experiments of tracking evolution of crystal orientation at different strain conditions were conducted using SEM,XRD,EBSD and TEM techniques.To analyze the deformation characteristic,several samples were cut from initial materials along different direction and then performed uniaxial compression and tension tests at one or several successive strains.Based on calculations of twin variants and its Schmid factors,the effect of different variants on deformation behavior was also explained.When tension along RD,the yield stress of the re-compression along RD is enhanced due to the grain refinement by 2}1{10 tensile twins.The reinforcement effect of tensile twins on the yield stress of re-compression is mainly reflected at the early stage of deformation.Tensile twins were introduced at an initial strain of 1.3% during compression along the RD and a gradual twin growth with increasing cumulative strain along the same loading direction was observed.The occurrence of twin depends on the initial texture.If a grain has a c-axis that is oriented close to ND,then twins in that grain may be activated.On the other hand,if a grain has a c-axis that is oriented away from ND,slip in that grain may accommodate deformation,and twins are not nucleated in this grain.Furthermore,twins nucleation would prefer at the low misorientation grain boundaries.The growth of twins mainly rely on two modes: the thickening of the lamellae for existing twin and new twins is nucleated at grain boundary.With increasing compressive strain,the twins begin to coalescence and eventually encompassed the whole grain.The evolution of microtexture shows that the basal texture becomes weak due to the propagation and coalescence of tensile twins.The twin variant selections that occurred were analyzed in terms of the macroscopic Schmid factor.The analysis indicates that the activation of specific twin variants depending on the strain path,and the twin variant selections were governed by the Schmid factor.When compression along RD,most twins appear parallel each other and they belong to one twin variant.When tension along ND,several twin were observed and they interact each other and these twins have similar Schmid factor values.The nucleation of different twin variants will further affect the evolution of texture.For compression along RD,the initial texture will gradually re-orient to twinning texture which parallel to the loading direction,which the distributions of twinning texture when tension along ND will spread in the region of RD-TD.There are different mechanical properties appeared due to generation of different twin variants at different loading conditions.When compression along RD,in the matrix regions,the Schmid factor values of prismatic slip system are highest,and the distribution of basal slip system in the range of 0 ~ 0.5,while in the twined regions,the Schmid factors values of prismatic become lower,and the Schmid factor values of basal slip also in the above range.When tension along ND,in the matrix regions,the Schmid factor of basal slip system have higher values,but it become decrease in the twined regions.Furthermore,these differences in twinning features between two strain paths resulted in different texture characteristics and affected the activation of slip modes.A method based on quasi-in-situ EBSD and SEM was utilized to examine the microstructure and texture of the specimens deformed in compression along ND.The coexistence of tensile twin and pyramidal slip modes has been proved.The crystallographic domains favoring every type of twinning and gliding have been exactly expounded and described through inverse pole figures.Tensile twins were often present in post deformed sample but capricious.Contraction twins were not observed during deformation.From the analysis of deformed regions,the Schmid factor appears as a relevant parameter to prognosticate the nature of the slip systems(basal,prismatic and pyramidal)if they are activated.When tension along RD,the crystallographic orientation domains favoring twinning and slip systems have accurately described and explained.Tensile twins were present,but the number is small and they appeared to be relatively stable.These twins resulted from the accommodation of compression along tensile direction based on the analyses of twin variants and their SF values.Prismatic <a> slip was the dominant deformation mechanism under tension and it contributed to a large elongation.With experimental examination,the twinning evolution in rolled AZ31 magnesium alloy during multidirectional deformation was proposed.For TD compression,2}1{10 tensile twinning was the most commonly observed deformation mode in the rolled sheet after compressive strain of up to 2.5%.Twin variants were observed in several grains,and these variants have a large impact on the following deformation due to their orientation distribution.For RD recompression,four types of twin modes were observed.For remaining parent grains,one of the twinning behaviors can be observed: nucleation and growth of new {10-12} tensile twinning.For existing twin bands generated during previous compression along TD,the rest of the three types of twinning behaviors were occurred.The first kind is the new 2}1{10 twin bands appeared in existing twin bands,and this type of twinning behavior usually called 2}1{10-2}1{10 double twinning.And the second is disappear or fade away of primary twins(here,these twins are generally named de-twinning,and this de-twinning behaviors appear the thin or even disappear of the primary twin lamellar).The final type is the growth of existing twin band.