外文翻译--热轧过程对Mg-3%Al-1%Zn合金薄板微结构和机械性能的影响 英文版

Materials Science and Engineering A 527 (2010) 19701974Contents lists available at ScienceDirectMaterials Science and Engineering Ajournal homepage: /locate/mseaEffects of hot rolling processing on microstructuresproperties of Mg3%Al1%Zn alloy sheetL.a,b, a,b a,b a,babarticleArticleReceivedReceived11AcceptedKeywords:AZ31HotGrainTwinningTextureandandof grain (1positivethisgrainsamplesper-pass1. IntroductionMagnesium sheet is currently being tested for various appli-cations. However, rolled Mg alloys often exhibit relatively lowstrengthto1mentsalloysthebeensheetfromthatstress,issuchbethemodesShanghaiMg 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 basal0921-5093/$doi:and poor formability especially at room temperature duethe basal or near basal texture usually developed during rolling. Fine grains and randomized texture are two important require-for the application of Mg alloy sheet. Texture evolution in Mgis affected by the interaction between the strain path andinitial microstructure 2, and the activation of basal slip hasconsidered as the reason for the basal texture in Mg alloy3. It was also reported that the basal texture originatedthe (1 0 1 2) extension twinning based on the assumptiontwinning would reorient the c-axes parallel to the compressivesupported by experiments 4 and simulations 5. While itvery difficult to change the basal texture of magnesium sheet,as AZ31 and AZ61 alloys, the intensity of the basal texture canweakened and the grain size could be reduced by controllingrolling processing if the correlation between the deformationand rolling processing parameters could be established.Corresponding author at: School of Material Sciences and Engineering,Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China.E-mail address: j (L. Jin).planes 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 10121011 extension twinning and10111012 contraction twinning are frequently reportedin Mg and Mg alloys 13. In the case of dislocation slip, prismatic10101120, and pyramidal 10111120 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,and the deformation mode in the AZ31 alloy sheet during hot rollingare strongly dependent on the initial grain structure and the rollingprocessing. In addition, continuous dynamic recovery and recrys-tallization (CDRR) has been reported as a phenomenon for the grainrefine 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 the see front matter 2009 Elsevier B.V. All rights reserved.10.1016/j.msea.2009.11.047Jin , J. Dong , R. Wang , L.M. PengNational Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University,Key State Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, P.R. Chinainfohistory:12 April 2009in revised formNovember 200920 November 2009Mg alloyrollingrefinementabstractThe microstructure evolutioninvestigated. The initial microstructureage grain size of 37H9262m. Duringto extension, contractionentation, grain size and rollingis the main mechanismcontraction and 1011and double twinning havealloy during hot rolling. In400C led to more uniformtility than that of the sheettemperatures and largerand the mechanical propertiesand mechanicalP.R. Chinadeformation mechanisms of AZ31 alloy sheet during hot rolling werein the as-received AZ31 sheet shows basal texture with an aver-the subsequent hot rolling processing, dislocation slip occurs in additiondouble twinning, and the twinning mode is dependent on the grain ori-temperature. Continuous dynamic recovery and recrystallization (CDRR)refinement for the AZ31 sheet during hot rolling, and the 10110 1 2) double twinning accelerate the refinement process. Contractioneffect on grain refinement and texture randomization for the AZ31study, higher per-pass reduction of 50% and higher rolling temperature ofstructure with more random texture, which resulted in the higher duc-rolled at 300C and at per-pass reduction of 30%. Therefore, higherreduction rolling are recommended for microstructure optimizationimprovement of the AZ31 sheet. 2009 Elsevier B.V. All rights reserved.L. Jin et al. / Materials Science and Engineering A 527 (2010) 19701974 1971Fig. 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 ofthe AZ31 sheet during hot rolling, and the evidence for understand-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-turecorrelation2.rolling,withmicrostructureinversetions.nearlytheirdirectiongrainsfavorablerespectively,inrespectively.473washeateddirectionbars.after each pass. And small samples were taken from the quenchedspecimens for EBSD analysis using a LEOTM1450 Scanning electronmicroscope operated at 20 kV fitted with a TSLTMEBSD camera 17.Due to the deformation structure in the EBSD sample, there is lowindex quality on the areas with more dislocation reactions and finertwins, but the results still can give a lot of useful information.Fcessing,evolution of AZ31 alloy during hot rolling using EBSD, and thebetween the microstructure and mechanical properties.Materials and experimentsThe alloy used in this study was AZ31 alloy. Prior to hotthe alloy was hot-extruded at 573 K to a rectangular bara cross-section of 110 mm 10 mm. Fig. 1 shows the initialof the AZ31 alloy before hot rolling, including thepole figure (IPF) map, pole figures and Schmid Factor Frac-The results indicate a primarily basal texture with the c-axesperpendicular to the sheet plane and only a few grains withc-axes parallel to the sheet plane and the original extrusion(ED). Fig. 1 (c) shows the Schmid Factor value of majorityis less than 0.3, suggesting that the grain orientation is notfor basal slip.The extruded bar was pre-heated at 573 or 673 K for 0.5 h,and then hot-rolled to the thickness of 3.5 and 2.5 mmhot rolling mills at a given reduction of 30% or 50% per pass,The temperature of the rollers was controlled at aboutK using internal electric heaters. The total thickness reductionabout 65% and 75%, respectively. The rolled plates were re-between passes to maintain the workability. The rollingwas parallel to the extrusion direction of the as-receivedThe rolling specimens were water-quenched immediately2. (a) The distribution of grain sizes of AZ31 alloy sheet after hot rolling 13 pass at 300in which grains were divided by grain boundaries with misorientation larger3. Results and discussionFig. 2 shows the grain size distribution at various per-pass reduc-tions and rolling temperatures. The initial extrusion material hascoarse grains ranging from 10 to 160H9262m and an average grainsize of 37.29H9262m. 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 the1030H9262m range but a considerable amount of large grains rangingfrom 40 to 85H9262m. After 2 passes of rolling, the grains were moreuniformly refined with an average grain size of 8.8H9262m. A furtherrolling pass actually caused a slight increase of grain size to 13H9262m.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 thanC with per-pass reduction of 30%, and (b) average grain sizes at various rollingthan 15.1972 L. 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 highertemperature150%highand215asinthatcoarseTheaccumulationMore1inors,whichtherandtheinofpassesitlution.analyzed by EBSD at a smaller step size of 0.5H9262m, and the result isshown in Fig. 5. Fig. 5(a) and (b) shows the IPF maps with the latticeorientation 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 10121011 extension twin-ning in grain 1, and10111012double twinning in grains2 and 3 according to the twin boundary definition. The parent grains2 and 3 were divided by the double twins, and then the initial grainswere refined to 3 or 5 finer grains as a result. The results suggest thatthe twinning in this process, especially the contraction and doubletwinning, accelerates the processing of grain refinement. The twin-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. As Humphreys said 15, dynamic recrystallization originatesat high angle boundaries regardless of the details of the mechanismof 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 withand larger per-pass reduction.Fig. 4 shows the IPF maps of AZ31 alloy sheet after hot rollingpass at 300C and 30% reduction and at 400C with 30% andreductions, respectively. Grain boundaries were defined asangle grain boundaries (HAGB) with misorientation of 1590lower angle grain boundaries (LAGB) with misorientation of, and HAGB was shown as black lines and LAGB was shownwrite lines in the IPF map in Fig. 4. The calculated grain sizesFig. 2 are for those grains divided by HAGB. Fig. 4(a) showsthe microstructure was progressively refined although somegrains remained after the 30% rolling reduction at 300C.fine grains form typical necklace structures due to the strainduring rolling which lead to CDRR in these regions.coarse grains were observed in Fig. 4(b) and (c) after rollingpass at 400C with per-pass reduction of 30% and 50%. As shownFig. 4(c), a large number of LAGBs in the coarse grain interi-are likely the result of the dislocation slip and interactions,could develop into HAGB and grain refinement during fur-deformation according to the continuous dynamic recoveryrecrystallization (CDRR) model 14,15. This is consistent withdata in Fig. 2(b) that the larger average grain sizes were obtainedthe sheet after 1 pass rolling at 400C and at per-pass reduction50% but the finest average grains were observed after rolling 2at this condition.Fig. 4 also shows some twinning sites during the hot rolling, butis difficult to identify the twinning modes due to the lower reso-The microstructure as shown in the black box in Fig. 4(c) wasFig. 4. IPF maps of AZ31 alloy sheet after rolling at (a) 300C with reduction of 30%,per 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.In addition, contraction twins, double twins and extension twinswere 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 20H9262m, however, fewor no twins were observed in smaller grains as shown in Fig. 6(c).As shown in Fig. 1, the initial grain orientations of the AZ31 alloysheet 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-mation mode due to the lowest critical resolved shear stress (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-(b) 400C with reduction of 30% and (c) 400C with reduction of 50%.L. Jin et al. / Materials Science and Engineering A 527 (2010) 19701974 1973Fig. 50%, (a)withdouble referencesarticle.)Fig.400lelGenerallytheextensionextensiontractionInmisorientationisforfavorablesystemsionduringblewiththaninrollingtractionand5. Microstructures of AZ31 alloy sheet rolled 1 pass at 400C with reduction ofdefined twin boundaries, extension twin boundaries (861 2105) are outlinedtwin boundaries (381 2105) outlined in blue. (For interpretation of the6. Grain shape maps with twin boundaries of AZ31 alloy sheet after rolling: (a) 3 passesC with per-pass reduction of 50%. In which the twin boundaries were defined as sameto sheet plane, 1012 extension twinning is easily occurred.the extension twinning will be reoriented to 86frominitial status 13, and, the parent grain will be replaced by thetwins due to the fast twin growth. In the new twins aftertwinning, only dislocation slip on basal plane and con-twinning are possible due to the new grain orientation.the case of 1011 contraction twinning, the twins have aof 56with the parent grains, and the contractionthinner and longer than extension twins and, thus, is difficulttwin growth. However, the new grain orientations are morefor extension twinning and dislocation slip on basal slipafter contraction twinning. Therefore, the 1012 exten-twinning always follows the 1011 contraction twinningdeformation in Mg alloy, i.e., the 10111012 dou-twinning. The double twins have misorientation of near 38the parent grain 13. As a result, there are more double twinscontraction twins in the AZ31 sheet after hot rolling as shownFig. 6.As shown in Fig. 7, the twinning modes are also dependent on thetemperature. Fig. 7(c) shows the volume fraction of the con-and double twins in the AZ31 alloy sheet after hot rolling,the highest volume fraction of double and contraction twinsIPF map and the lattice orientation of the parent and twins, (b) Grain shape mapin red, contraction twin boundaries (561 2105) outlined in green and theto color in this figure legend, the reader is referred to the web version of theat 300C, (b) 3 passes at 400C with per-pass reduction 30%, and (c) 2 passes atas that in Fig. 5.was found in the sheet after rolling 3 passes at 400C with the per-reduction of 30%, while the lowest volume fraction in the sheet afterrolling 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.1974 L. 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 isthe(0001)pole figures of AZ31 after hot rolling, showingbroadersheetHowever,limitedtwoandstructurebutwereintensity.doubleaccommodatetexture.randomization,higherper-passtemperaturestheobservedabove4.stilldueextensiontotoandtionduringtheparentvatedoftwinningtwinning,extensioncontractionrefinement and texture randomization for the AZ31 alloy during hotrolling.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.101112131415161718distribution of basal pole and weaker basal texture in AZ31rolled at 400C compared to the samples rolled at 300C.the change in global texture is insignificant due to thevolume fraction of twinned material.As mentioned before, fine grains and randomized texture areimportant requirements for improved mechanical propertiesthe applications of the Mg alloy sheet. In this study, the graincan be effectively refined at lower rolling temperatures,more extension twinning and dislocation on basal slip systemobserved in this condition, resulting in higher basal textureAt higher rolling temperatures, more contraction andtwinning can be activated and the non-basal slip also wouldlarger plastic deformation 18, weakening the basalThus, higher rolling temperatures are favorable for texturebut lead to larger grains at given per-pass reductiontemperature rolling resulted. Fortunately, rolling at higherreduction can offset the large grain size. Therefore, higherand larger per-pass reductions are recommended forAZ31 sheet alloy, to achieve better mechanical properties. Thehigh ductility in Fig. 3 provides a good example based onidea.ConclusionsIn summary, it was found that dislocation slip on basal plane isthe main deformation mode in AZ31 sheet during hot rollingto the initial microstructure with basal texture. The 1012twinning occurred in grains with their c-axes parallelsheet plane and the extension twinning reoriented the grains86from their initial state. The 1011 contraction twinning10111012were activated at larger thickness reduc-because the c-axes of majority grains were under compressionrolling. Twinning is also dependent on initial grain size androlling temperature, twinning is readily observed in the largergrains and more contraction and double twinning are acti-at higher rolling temperatures. CDRR is the main mechanismgrain refinement for the AZ31 sheet during hot rolling, but thein this process, especially the contraction and doubleaccelerates the grain refinement process. Basal slip andtwinning will induce the formation of basal texture； butand double twinning have positive effect on the grainAnd