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Materials Science and Engineering A 527 (2010) 19701974Contents lists available at ScienceDirectMaterials Science and Engineering Ajournal homepage: /locate/mseaEffects of hot rolling processing on microstructures and mechanicalproperties of Mg3%Al1%Zn alloy sheetL. Jina,b, J. Donga,b, R. Wanga,b, L.M. Penga,baNational Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, P.R. ChinabKey State Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, P.R. Chinaa r t i c l ei n f oArticle history:Received 12 April 2009Received in revised form11 November 2009Accepted 20 November 2009Keywords:AZ31 Mg alloyHot rollingGrain refinementTwinningTexturea b s t r a c tThe microstructure evolution and deformation mechanisms of AZ31 alloy sheet during hot rolling wereinvestigated. The initial microstructure in the as-received AZ31 sheet shows basal texture with an aver-age grain size of 37?m. During the subsequent hot rolling processing, dislocation slip occurs in additionto extension, contraction and double twinning, and the twinning mode is dependent on the grain ori-entation, grain size and rolling temperature. Continuous dynamic recovery and recrystallization (CDRR)is the main mechanism of grain refinement for the AZ31 sheet during hot rolling, and the 1011contraction and 1011(1012) double twinning accelerate the refinement process. Contractionand double twinning have positive effect on grain refinement and texture randomization for the AZ31alloy during hot rolling. In this study, higher per-pass reduction of 50% and higher rolling temperature of400C led to more uniform grain structure with more random texture, which resulted in the higher duc-tility than that of the sheet samples rolled at 300C and at per-pass reduction of 30%. Therefore, highertemperatures and larger per-pass reduction rolling are recommended for microstructure optimizationand the mechanical properties improvement of the AZ31 sheet. 2009 Elsevier B.V. All rights reserved.1. IntroductionMagnesium sheet is currently being tested for various appli-cations. However, rolled Mg alloys often exhibit relatively lowstrength and poor formability especially at room temperature dueto the basal or near basal texture usually developed during rolling1. Fine grains and randomized texture are two important require-mentsfortheapplicationofMgalloysheet.TextureevolutioninMgalloys is affected by the interaction between the strain path andthe initial microstructure 2, and the activation of basal slip hasbeen considered as the reason for the basal texture in Mg alloysheet 3. It was also reported that the basal texture originatedfrom the (1012) extension twinning based on the assumptionthattwinningwouldreorientthec-axesparalleltothecompressivestress, supported by experiments 4 and simulations 5. While itis very difficult to change the basal texture of magnesium sheet,such as AZ31 and AZ61 alloys, the intensity of the basal texture canbe weakened and the grain size could be reduced by controllingthe rolling processing if the correlation between the deformationmodes and rolling processing parameters could be established.Corresponding author at: School of Material Sciences and Engineering,Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China.E-mail address: j jinli (L. Jin).Mg is known to deform by slip, twinning and grain bound-ary sliding (GBS). GBS is probably available for the superplasticdeformation or in the nano-materials 6, and it has been observedin fine-grained AZ31 as well 79. The dislocation slip on basalplanes can lead to large plastic deformation, but there are onlytwo independent basal slip systems 10,11, far fewer than thefive independent systems necessary for general deformation 12.Twinning has been considered providing additional deformationin magnesium and 1012?1011? extension twinning and1011?1012? contraction twinning are frequently reportedin Mg and Mg alloys 13. In the case of dislocation slip, prismatic1010?1120?, and pyramidal 1011?1120? systemscan be operated in the AZ31 alloys in addition to slip in basal(0001)?1120? system. For the AZ31 sheet rolled at elevate tem-peratures, non-basal dislocation slip and other twinning modes areeasily activated in addition to basal slip and extension twinning,andthedeformationmodeintheAZ31alloysheetduringhotrollingare strongly dependent on the initial grain structure and the rollingprocessing. In addition, continuous dynamic recovery and recrys-tallization(CDRR)hasbeenreportedasaphenomenonforthegrainrefine and growth in Mg alloys during deformation at elevate tem-peratures 1415, according to the low energy dislocation theory(LEDs) 16. However, the correlation between the rolling process-ing, deformation mechanisms, and the microstructure evolution,is not well understood. Fortunately the electron backscattereddiffraction (EBSD) technique provides the possibility to follow the0921-5093/$ see front matter 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2009.11.047L. Jin et al. / Materials Science and Engineering A 527 (2010) 197019741971Fig. 1. Initial microstructures of AZ31 alloy sheet: (a) IPF map, (b) pole figures and (c) Schmid factor fractions subject to basal slip system.evolution of twins, misorientation, texture and grain structure oftheAZ31sheetduringhotrolling,andtheevidenceforunderstand-ing the effect of deformation mechanisms on the microstructuresevolution. This will open up the possibility of controlling themechanical response by designing microstructure appropriate torolling processing. Therefore, this paper aimed at the microstruc-ture evolution of AZ31 alloy during hot rolling using EBSD, and thecorrelationbetweenthemicrostructureandmechanicalproperties.2. Materials and experimentsThe alloy used in this study was AZ31 alloy. Prior to hotrolling, the alloy was hot-extruded at 573K to a rectangular barwith a cross-section of 110mm10mm. Fig. 1 shows the initialmicrostructure of the AZ31 alloy before hot rolling, including theinverse pole figure (IPF) map, pole figures and Schmid Factor Frac-tions. The results indicate a primarily basal texture with the c-axesnearly perpendicular to the sheet plane and only a few grains withtheir c-axes parallel to the sheet plane and the original extrusiondirection (ED). Fig. 1 (c) shows the Schmid Factor value of majoritygrains is less than 0.3, suggesting that the grain orientation is notfavorable for basal slip.The extruded bar was pre-heated at 573 or 673K for 0.5h,respectively,andthenhot-rolledtothethicknessof3.5and2.5mmin hot rolling mills at a given reduction of 30% or 50% per pass,respectively.Thetemperatureoftherollerswascontrolledatabout473K using internal electric heaters. The total thickness reductionwas about 65% and 75%, respectively. The rolled plates were re-heated between passes to maintain the workability. The rollingdirection was parallel to the extrusion direction of the as-receivedbars. The rolling specimens were water-quenched immediatelyafter each pass. And small samples were taken from the quenchedspecimens for EBSD analysis using a LEOTM1450 Scanning electronmicroscopeoperatedat20kVfittedwithaTSLTMEBSDcamera17.Due to the deformation structure in the EBSD sample, there is lowindexqualityontheareaswithmoredislocationreactionsandfinertwins, but the results still can give a lot of useful information.3. Results and discussionFig.2showsthegrainsizedistributionatvariousper-passreduc-tions and rolling temperatures. The initial extrusion material hascoarse grains ranging from 10 to 160?m and an average grainsize of 37.29?m. Fig. 2(a) shows the grain structure evolution afterrolling 1, 2 and 3 passes at a temperature of 300C and at a per-pass reduction of 30%. After the single-pass rolling, the averagegrain size was greatly reduced with the majority of grains in the1030?m range but a considerable amount of large grains rangingfrom 40 to 85?m. After 2 passes of rolling, the grains were moreuniformly refined with an average grain size of 8.8?m. A furtherrolling pass actually caused a slight increase of grain size to 13?m.Similar results were obtained at the 400C rolling. Fig. 2(b) sum-marizes the average grain sizes of AZ31 sheet after hot rolling atvarious per-pass reductions and rolling temperatures. The resultsshow that at a given rolling pass, the average grain sizes for thesheet rolled at 400C are larger than that of 300C-rolled sheet,and that higher per-pass reduction led to finer grains at the similartotal thickness reduction and at same rolling temperature.Fig. 3 shows the actual tension plots of AZ31 alloy sheet beforeand after hot rolling. The stressstrain curves exhibit that thereare weaker strain hardening behaviors and higher yield stress (YS)and ultimate tensile strength (UTS) in rolled AZ31 alloy sheet thanFig. 2. (a) The distribution of grain sizes of AZ31 alloy sheet after hot rolling 13 pass at 300C with per-pass reduction of 30%, and (b) average grain sizes at various rollingprocessing, in which grains were divided by grain boundaries with misorientation larger than 15.1972L. Jin et al. / Materials Science and Engineering A 527 (2010) 19701974Fig. 3. Stressstrain curve of AZ31 sheet before and after hot rolling.that in as-extrusion sheet. The rolling AZ31 sheets have similar YSand UTS, but have significant difference in their ductility, whichis substantially higher in the sample after hot rolling at highertemperature and larger per-pass reduction.Fig. 4 shows the IPF maps of AZ31 alloy sheet after hot rolling1 pass at 300C and 30% reduction and at 400C with 30% and50% reductions, respectively. Grain boundaries were defined ashigh angle grain boundaries (HAGB) with misorientation of 1590and lower angle grain boundaries (LAGB) with misorientation of215, and HAGB was shown as black lines and LAGB was shownas write lines in the IPF map in Fig. 4. The calculated grain sizesin Fig. 2 are for those grains divided by HAGB. Fig. 4(a) showsthat the microstructure was progressively refined although somecoarse grains remained after the 30% rolling reduction at 300C.The fine grains form typical necklace structures due to the strainaccumulation during rolling which lead to CDRR in these regions.More coarse grains were observed in Fig. 4(b) and (c) after rolling1 pass at 400C with per-pass reduction of 30% and 50%. As shownin Fig. 4(c), a large number of LAGBs in the coarse grain interi-ors, are likely the result of the dislocation slip and interactions,which could develop into HAGB and grain refinement during fur-ther deformation according to the continuous dynamic recoveryand recrystallization (CDRR) model 14,15. This is consistent withthedatainFig.2(b)thatthelargeraveragegrainsizeswereobtainedin the sheet after 1 pass rolling at 400C and at per-pass reductionof 50% but the finest average grains were observed after rolling 2passes at this condition.Fig. 4 also shows some twinning sites during the hot rolling, butit is difficult to identify the twinning modes due to the lower reso-lution.ThemicrostructureasshownintheblackboxinFig.4(c)wasanalyzed by EBSD at a smaller step size of 0.5?m, and the result isshowninFig.5.Fig.5(a)and(b)showstheIPFmapswiththelatticeorientation and grain shape maps with defined twin boundaries. Itcan be seen from the figures that twinning nucleated in grain 1, 2and 3, and the twining mode is 1012?1011? extension twin-ningingrain1,and10111012doubletwinningingrains2and3accordingtothetwinboundarydefinition.Theparentgrains2and3weredividedbythedoubletwins,andthentheinitialgrainswererefinedto3or5finergrainsasaresult.Theresultssuggestthatthe twinning in this process, especially the contraction and doubletwinning,acceleratestheprocessingofgrainrefinement.Thetwin-induced phenomenon could be explained as following. More twinboundaries develop after twinning, which could be the obstaclesfor dislocation slip during straining. Towards the twin boundariesdensity of dislocations and misorientations increase, and at highstrains, HAGBs may develop, and then resulted in the grain refine-ment.AsHumphreyssaid15,dynamicrecrystallizationoriginatesathighangleboundariesregardlessofthedetailsofthemechanismof nucleation. But the grain nucleation and growth here may stillfollow the mechanism of CDRR with the sub-grain formation andthen fine-grain with HAGB.Fig. 6 shows the grain shape maps with defined twin bound-aries of the AZ31 alloy sheet after rolling 3 passes with per-passreduction of 30% at 300C and 400C, and rolling 2 passes withper pass reduction of 50% at 400C, respectively. The grain size islarger in Fig. 6(b) compared to Fig. 6(a) and (c), indicating that thegrains were refined further by higher per-pass reduction rollingand at a lower temperature. The fact that smaller grain size isfound at higher reduction is due to stored energy of deformationbeing higher which leads to higher driving force for nucleation andhence finer grain size. At the lower temperature the softening rateis slower, hence leads to higher work hardening and larger drivingforce for nucleation of grains, and here grain growth is also slow.Inaddition,contractiontwins,doubletwinsandextensiontwinswere observed in the larger grains in Fig. 6, and there is a highervolume fraction of contraction twins and double twins than that ofextension twins, especially there are less extension twins at thehigher rolling temperature. Fig. 6 also shows that the twinningmode is dependent on the grain size. Twinning is more visible inthe parent grains with gain size larger than 20?m, however, fewor no twins were observed in smaller grains as shown in Fig. 6(c).AsshowninFig.1,theinitialgrainorientationsoftheAZ31alloysheet are not favorable for the dislocation slip on the basal plane;in addition, the majority of the grains have their c-axis parallel tothe compressive stress, which is not favorable for the extensiontwinning in those grains. But the basal slip is still the main defor-mationmodeduetothelowestcriticalresolvedshearstress(CRSS),and also the contraction twinning can be possibly activated ascribeto the c-axes of those grains under compression. In the case of afew grains, blue or green grains in Fig. 1(a) with their c-axes paral-Fig. 4. IPF maps of AZ31 alloy sheet after rolling at (a) 300C with reduction of 30%, (b) 400C with reduction of 30% and (c) 400C with reduction of 50%.L. Jin et al. / Materials Science and Engineering A 527 (2010) 197019741973Fig. 5. Microstructures of AZ31 alloy sheet rolled 1 pass at 400C with reduction of 50%, (a) IPF map and the lattice orientation of the parent and twins, (b) Grain shape mapwith defined twin boundaries, extension twin boundaries (86?1210?5) are outlined in red, contraction twin boundaries (56?1210?5) outlined in green and thedouble twin boundaries (38?1210?5) outlined in blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of thearticle.)Fig. 6. Grain shape maps with twin boundaries of AZ31 alloy sheet after rolling: (a) 3 passes at 300C, (b) 3 passes at 400C with per-pass reduction 30%, and (c) 2 passes at400C with per-pass reduction of 50%. In which the twin boundaries were defined as same as that in Fig. 5.lel to sheet plane, 1012 extension twinning is easily occurred.Generally the extension twinning will be reoriented to 86fromthe initial status 13, and, the parent grain will be replaced by theextension twins due to the fast twin growth. In the new twins afterextension twinning, only dislocation slip on basal plane and con-traction twinning are possible due to the new grain orientation.In the case of 1011 contraction twinning, the twins have amisorientation of 56with the parent grains, and the contractionis thinner and longer than extension twins and, thus, is difficultfor twin growth. However, the new grain orientations are morefavorable for extension twinning and dislocation slip on basal slipsystem after contraction twinning. Therefore, the 1012 exten-sion twinning always follows the 1011 contraction twinningduringdeformationinMgalloy,i.e.,the10111012dou-ble twinning. The double twins have misorientation of near 38with the parent grain 13. As a result, there are more double twinsthan contraction twins in the AZ31 sheet after hot rolling as shownin Fig. 6.AsshowninFig.7,thetwinningmodesarealsodependentontherolling temperature. Fig. 7(c) shows the volume fraction of the con-traction and double twins in the AZ31 alloy sheet after hot rolling,and the highest volume fraction of double and contraction twinswas found in the sheet after rolling 3 passes at 400C with the per-reductionof30%,whilethelowestvolumefractioninthesheetafterrolling 3 passes at 300C with same per-pass reduction. The resultssuggest that contraction and double twinning are readily activatedat higher rolling temperatures, which could be ascribe to the lowerCRSS for the contraction and double twinning at higher tempera-ture as similar as the CRSS for non-basal dislocation slip comparingFig. 7. The volume fraction of contraction and double twins in the AZ31 sheet afterhot rolling.1974L. Jin et al. / Materials Science and Engineering A 527 (2010) 19701974Fig. 8. (0001) pole figures of AZ31 alloy sheet after rolling (a) 3 passes at 300C, (b) 3 passes at 300C with per-pass reduction 30% at 400C, and (c) 2 passes at 400C withper-pass reduction of 50%.to Basal dislocation slip 18. On the other hand, the 1011 con-traction and 10111012 double twinning are reorientedfrom the basal poles to 56and 38reference to initial status, andtherefore the activation of the contraction and double twining inthe AZ31 sheet can result in the weakening of the basal texture.Fig. 8 is the (0001) pole figures of AZ31 after hot rolling, showingbroaderdistributionofbasalpoleandweakerbasaltextureinAZ31sheet rolled at 400C compared to the samples rolled at 300C.However, the change in global texture is insignificant due to thelimited volume fraction of twinned material.As mentioned before, fine grains and randomized texture aretwo important requirements for improved mechanical propertiesand the applications of the Mg alloy sheet. In this study, the grainstructure can be effectively refined at lower rolling temperatures,but more extension twinning and dislocation on basal slip systemwere observed in this condition, resulting in higher basal textureintensity. At higher rolling temperatures, more contraction anddoubletwinningcanbeactivatedandthenon-basalslipalsowouldaccommodate larger plastic deformation 18, weakening the basaltexture. Thus, higher rolling temperatures are favorable for texturerandomization,butleadtolargergrainsatgivenper-passreductionhigher temperature rolling resulted. Fortunately, rolling at higherper-pass reduction can offset the large grain size. Therefore, highertemperatures and larger per-pass reductions are recommended forthe AZ31 sheet alloy, to achieve better mechanical properties. Theobserved high ductility in Fig. 3 provides a good example based onabove idea.4. ConclusionsIn summary, it was found that dislocation slip on basal plane isstill the main deformation mode in AZ31 sheet during hot rollingdue to the initial microstructure with basal texture. The 1012extension twinning occurred in grains with their c-axes parallelto sheet plane and the extension twinning reoriented the grainsto 86from their initial state. The 1011 contraction twinningand10111012wereactivatedatlargerthicknessreduc-tion because the c-axes of majority grains were under compressionduring rolling. Twinning is also dependent on initial grain size andthe rolling temperature, twinning is readily observed in the largerparent grains and more contraction and double twinning are acti-vated at higher rolling temperatures. CDRR is the main mechanismof grain refinement for the AZ31 sheet during hot rolling, but thetwinning in this process, especially the contraction and doubletwinning, accelerates the grain refinement process. Basal slip andextension twinning will induce the formation of basal texture; butcontraction and double twinning have positive effect on the grainrefinementandtexturerandomizationfortheAZ31alloyduringhotrolling.In this study at a given rolling reduction, the average grain sizeof AZ31 sheet rolled at 400C was larger than that of the sheetsamples rolled at 300C, and at same rolling temperature higherper-pass reduction led to finer grain size in the sheet materials.And higher per-pass reduction of 50