AZ31镁合金退火过程中的织构演变 - 图文-

1、Texture evolution during annealing of magnesium AZ31alloyM.T.P e rez-Prado*,O.A.RuanoDepartment of Physical Metallurgy,Centro Nacional de Investigaciones Metal u rgicas(CENIM,CSIC Avda.Gregorio del Amo,8,28040Madrid,SpainReceived22August2001;accepted16October2001AbstractThe microstructural evolution

2、 of an AZ31magnesium alloy sheet with annealing is analyzed by texture analysis.A through-thickness texture gradient has been found in the as-received material.Upon annealing,normal grain growth occurs in the mid-layer and secondary recrystallization starts in the outer surfaces,leading to a homogen

3、eous11 20 texture throughout the thickness.Ó2002Acta Materialia Inc.Published by Elsevier Science Ltd.All rights reserved. Keywords:Recrystallization;Texture;Microstructure;AZ311.IntroductionMagnesium alloys are emerging as potentially good candidates for numerous applications,espe-cially in th

4、e automotive industry.Their good prop-erties,such as low density and high specic strength,make them promising replacements for other heavier materials like,for instance,steel,cast iron,and even aluminum1.However,the disad-vantage of Mg alloys is that they only exhibit limited ductility due to their

5、hcp structure.There-fore,signicant development eorts are needed in order to widen the applicability of these materials.In particular,several studies have shown the potential of Mg alloys for superplasticforming 29that can be conducted,in specic cases,at high strain rates and low temperatures7.One of

6、 the key prerequisites for a microstructure capable of superplasticdeformation is ane grain size,that should remain stable during deformation.In con-trast,signicant grain growth at high temperatures (0.8T m0.9T moccurs often in Mg alloys5,6.Texture analysis has proved to be a successful tool to inve

7、stigate the microstructural evolution of superplasticalloys upon annealing and deforma-tion1015,and thus a very useful aid to optimize their processing.However,relatively few studies have applied this tool to characterize potentially superplasticMg alloys.n this work texture anal-ysis will be used t

8、o study the microstructural evo-lution of an AZ31alloy during annealing at high temperature.2.Experimental procedureThe AZ31magnesium alloy subject of this study was received as-extruded in the form of a sheet, 5mm in thickness.The alloy composition is the following:3%Al,1%Zn,0.2%Mn and Mg(bal.Scrip

9、ta Materialia46(2002 149155 *Corresponding author.Fax:+39-91-534-7425.E-mail address:tppradocenim.csic.es(M.T.P e rez-Prado.1359-6462/02/$-see front matterÓ2002Acta Materialia Inc.Published by Elsevier Science Ltd.All rights reserved.PII:S1359-6462(0101212-XAnnealing treatments were performed i

10、n a ra-diation furnace at 450and 520°C during 30min,3h,and 17h with the aim of studying the evolution of microstructure and texture of this alloy at high temperature.Microstructural examination was performed by optical microscopy.Surface preparation consisted of grinding with progressively ner

11、SiC papers and mechanical polishing with diamond paste of par-ticle sizes ranging between 6and 1l m.Final pol-ishing was performed with a solution of colloidal silica.The samples were kept in ethanol between the dierent polishing steps to avoid rapid oxida-tion.Revealing the grain structure was achi

12、eved by subsequent etching at room temperature during 30s in a solution of ethyleneglycol (60%,acetic acid (20%,water (19%,and nitricac id (1%.Porosity could not be avoided.Average grain sizes (d ave were calculated from the optical micrographs by the linear intercept method.X-ray texture analysis w

13、as performed using the Schulz reection method in a Siemens D5000dif-fractometer furnished with a close eulerian cradle.Texture is represented by means of specic cal-culated pole gures.Calculation of the full pole gures was done by means of the SIEMENS DIFFRAC/AT software,using the measured 10 10,000

14、2,11 20,10 12,and 10 11pole gures.The latter pole gures were mea-sured in the reection mode,with the angle Khi ranging from 0°to 85°.Pole gures were ana-lyzed with the aid of the software Carine Crystal-lography.3.ResultsFig.1shows the microstructure of the as-received material.Optical mic

15、rographs revealing the grain structure both in the regions close to the surfaces as well as in the mid-layer of the sheet are presented.It can be seen that a grain size gradient along the through-thickness direction is present as a consequence of the thermomechanical process-ing.The grain size is cl

16、early smaller in the regions close to the surfaces (d ave ¼20l mthan in the mid-layer (d ave ¼32l m.Grains possess an equiaxed shape in both regions.Annealing at the two temperatures investigated (450and 520°Cresulted in normal grain growth in the mid-layer.Fig.2shows the microstructu

17、re of the mid-layer after heat treatments at 450°C during 3h (Fig.2aand 17h (Fig.2band at 520°C during 3h (Fig.2c.Grain size increases with annealing time (for isothermal treatmentsand with annealing temperature (for isochronal treat-ments.Values of the grain size corresponding to all the

18、annealing treatments performed are shown in Table 1.In contrast,in the regions close to the surfaces,secondary recrystallization or abnormal grain growth starts already after a heat treatment of 30min at 450°C.Specic grains grow excessively,consuming the smaller recrystallized grains 16.Fig.3a

19、illustrates the microstructure of one of the regions close to the surface of this AZ31alloy annealed at 450°C for 3h.Evidence of twinning can be appreciated in the growing grain.Second-ary recrystallization in the surface regions takesFig.1.Microstructure of the as-received material:(aregion cl

20、ose to the surface,(bmid-layer,ED is the extrusion direction,TD is thetransverse direction,and ND is the normal direction.150M.T.P e rez-Prado,O.A.Ruano /Scripta Materialia 46(2002149155place upon annealing under all conditions investi-gated.The size of the abnormally growing grains increases with a

21、nnealing time (for isothermal heat treatmentsand with annealing temperature (for isochronal heat treatments.After a very severe heat treatment (520°C for 17honly few grains could be detected in the sample examined,as shown in Fig.3b.The size of these grains in all cases ex-ceeded 1mm.Texture me

22、asurements revealed the presence of a through-thickness texture gradient in the as-received material.Fig.4illustrates the texture of this AZ31alloy in the mid-layer (Fig.4acand in the regions close to the surfaces (Fig.4dfby means of the (0002,(10 10and (11 20pole gures.The presence of a strong conc

23、entration of poles in the center of each pole gure indicates that in the mid-layer there are grains with the 0002(basal,10 10(prismaticand 11 20(pris-maticplanes parallel to the sheet plane (RP.The basal texture component predominates,as can be inferred by examining the intensity contours.The presen

24、ce of quasi-circular contours surrounding the central poles suggests that there is more than one direction parallel to the extrusion direction in all cases,although complete bers are not formed.In particular,the most important texture com-ponents are 0002h 1450i and 0002h 1210i .However,in the regio

25、ns close to the surface of the sheet grains are oriented predominantly with onlyFig.2.Optical micrographs showing the microstructure of the AZ31alloy (mid-layerannealed at (a450°C and 3h,(b450°C and 17h and (c520°C and 3h.ED is the extrusion direction.Table 1Grain size values in the m

26、id-layer corresponding to annealing treatments at 450and 520°C Grain size (l m30min 3h 17h 450°C 363765520°C4443aaSee Section 3,paragraph 3.Fig.3.Microstructure of the AZ31alloy (outer surfacesannealed (aat 450°C for 3h and (b520°C for 17h.The extrusion direction is the hori

27、zontal.The transverse direction is the vertical.M.T.P e rez-Prado,O.A.Ruano /Scripta Materialia 46(2002149155151the planes 11 20(prismaticand 0002(basalparallel to the RP (Fig.4df.A close look at the intensity levels reveals that grains with 11 20planes parallel to the RP predominate.The amount of g

28、rains with the prismaticplanes 10 10parallel to the RP is signicantly reduced (Fig.4ein comparison with the mid-layer (Fig.4b.Again,the presence of circular intensity contours sur-rounding the central poles (see,for example,Fig.4dindicates that there are several directions aligned with the extrusion

29、 direction.In particular,the most important components are 11 20h 10 10i and f 1 120gh 3140i .As a consequence of annealing,the 11 20(prismatictexture component (planes 11 20parallel to the sheet planeis strengthened in the mid-layer.Fig.5acillustrates this eec t for the annealing performed at 520&#

30、176;C for 3h.As was shown in Fig.2,grain growth takes place under these annealing conditions.The strengthening of the 11 20component suggests that grains with the 11 20prismaticplane parallel to the sheet plane grow faster than those with other orienta-tions.It can also be seen in Fig.5that the 0002

31、(basaland the 10 10(prismaticcomponentsare retained.This suggests that the grains orientedwith the 11 20plane parallel to the sheet plane grow mainly at the expense of randomly oriented grains.The texture evolution in regions close to the surface of the rolling sheet after a 3h annealing at 520°

32、;C is shown in Fig.5df.A dramaticc hange can be noticed under these conditions.Grains with 11 20planes parallel to the RP predominate,whereas those orientations with basal and pris-matic10 10planes parallel to the RP are absent.As shown in Fig.3a,in these regions secondary recrystallization takes pl

33、ace upon annealing.The texture data suggests that the grains that tend to grow abnormally are those oriented with planes 11 20parallel to the RP.The texture of this AZ31alloy after a severe annealing at 520°C/19h is shown in Fig.6.As can be seen in Fig.3b,secondary recrystallization is well dev

34、eloped at this point.The texture was mea-sured in dierent regions with the aim of capturing orientation information about more than just one grain.It can be seen in Fig.6that a single compo-nent predominates,namely the 11 201 100.Thus,this is a stable orientation for abnormally growing grains in thi

35、s AZ31 alloy.Fig.4.Direct pole gures showing the texture of the as-received AZ31alloy.152M.T.P e rez-Prado,O.A.Ruano /Scripta Materialia 46(20021491554.DiscussionA through-thickness texture gradient is observed in the as-received AZ31alloy.Several explanations have been put forward to understand the

36、se texture heterogeneities,that are,for instance,rather com-mon in rolled sheets 17.For example,through-thickness texture gradients have been attributed to the temperature gradient present during hot rolling due to the lower temperature of the rolls,or to the presence of a marked shear component in

37、the outer surfaces.The same arguments can be applied to this case assuming a temperature gradient origi-nated by the lower temperature of the extrusion die.Texture gradients can have a marked eect on the mechanical properties and therefore they have to be taken into account in models aiming to ex-pl

38、ain the deformation behavior of materials.Pre-vious studies 11,for example,have shown that dierent deformation mechanisms predominateFig.5.Direct pole gures showing the texture of the AZ31alloy annealed at 520°C for 3h.The vertical axis is parallel to the extrusiondirection and the horizontal a

39、xis is parallel to the transverse direction.Fig.6.Direct pole gures showing the texture of the AZ31alloy annealed at 520°C for 19h.The vertical axis is parallel to the extrusion direction and the horizontal axis is parallel to the transverse direction.M.T.P e rez-Prado,O.A.Ruano /Scripta Materi

40、alia 46(2002149155153154 M.T. Prez-Prado, O.A. Ruano / Scripta Materialia 46 (2002 149155 e during superplastic deformation in the outer surfaces and in the mid-layer of an 8090 aluminum alloy that had a through-thickness texture gradient in the as-received condition. Although texture studies in as-

41、extruded sheets have not been published, several investigations have described rolling textures for hexagonal metals 18,19. According to these studies, hot rolling in Mg alloys gives rise to a basal texture, with 0 0 0 1 planes parallel to the sheet plane. Rao and Prasad 19 report the predominance o

42、f a 0 0 0 22 1 0 component in hot rolled Mg of 1 99.87% purity. Cold rolling would, in turn, give rise to rotations of the basal plane normal about 10° away from the sheet plane towards the rolling direction. In order to rationalize the rolling texture of Mg these authors used the model of Caln

43、an and Clews 20, in which the rolling process can be rationalized as a compression perpendicular to the sheet plane and a tension in the rolling direction. In simple slip, this model predicts that the compression will rotate the active slip plane such that its normal moves toward the stress axis. As

44、 easy slip in Mg occurs on basal planes, the rolling process would orient basal planes so that they become parallel to the sheet plane. In the AZ31 alloy of this study, however, the texture both in the outer surface and in the midlayer is formed by several components: in the outer surfaces, basal pl

45、anes and 1 1 0 prismatic 2 planes are predominantly parallel to the sheet plane; in the mid-layer, basal planes, 1 0 0 and 1 1 1 0 prismatic planes are predominantly par2 allel to the sheet plane. Using the same argument as that of Rao and Prasad described above, the presence of several texture comp

46、onents suggests the activation of basal and prismatic slip systems during the sheet extrusion. Additionally, the presence of a texture gradient may indicate that dierent slip systems are activated in the outer surfaces and in the mid-layer. Mg is more prone to slip on non-basal planes than other hex

47、agonal metals like Zn or Cd, since the axial ratio is close to the ideal value 1.633 21. Reed-Hill and Robertson 22 have found prismatic slip 1 0 0h1 1 0i at 298 and 77 K using Mg crystals 1 2 oriented to suppress basal slip. Moreover, particular local congurations of orientations may favor the acti

48、vation of specic slip systems. Further work is in progress to clarify the origin of the dierent texture components present in the asreceived AZ31 material. Only few studies deal with recrystallization textures in hexagonal metals. In the review by Philippe 18, recrystallization is said to give rise

49、to basal textures, in which grains have the 0 0 0 2 plane parallel to the sheet plane. Here, however, upon moderate annealing, growth of grains with prismatic planes 1 1 0 at the expense of ran2 domly oriented grains takes place in the mid-layer. In the outer surfaces, secondary recrystallization oc

50、curs and grains with prismatic planes 1 1 0 2 tend to grow abnormally. A predominantly basal texture is not observed neither in the outer surfaces nor in the mid-layer after annealing. In a recent paper by Solas et al. 23, a model for the simulation of deformation and recrystallization textures in h

51、cp metals is presented and it is applied to Zn polycrystals. The experimental results and the predictions of the model are consistent when nucleation takes place in the grains with the highest stored plastic energy. According to this model, the growth of 1 1 0 grains may indicate that these 2 orient

52、ations, that are favorable for accommodating prismatic slip during processing, would store more plastic deformation than others. The ease for prismatic slip may be due to high extrusion temperatures. In the outer surfaces, where a larger amount of shear is present, the presence of a larger amount of

53、 stored plastic deformation would constitute the driving force for secondary recrystallization. After severe annealings a homogeneous texture is produced throughout the thickness, with f1 1 0g grains growing abnormally from the outer 2 surfaces toward the mid-layer. The disappearance of the texture

54、heterogeneity after secondary recrystallization has been also reported by Mishra et al. 24 in Fe3%Si. 5. Conclusions (1 A through-thickness texture gradient exists in the as-received AZ31 material. In the outer surfaces, basal and 1 1 0 prismatic components 2 M.T. Prez-Prado, O.A. Ruano / Scripta Ma

55、terialia 46 (2002 149155 e 155 predominate; in the mid-layer, basal, 1 0 0 and 1 1 1 0 prismatic components predominate. 2 (2 Upon moderate annealing normal grain growth takes place in the mid-layer: grains with 1 1 0 prismatic planes parallel to the sheet 2 plane grow at the expense of randomly ori

56、ented grains. In the outer surfaces the onset of secondary recrystallization takes place: grains with 1 1 0 2 prismatic planes parallel to the sheet plane grow abnormally. Upon severe annealing the process of secondary recrystallization proceeds until a homogeneous texture is achieved throughout the thickness. (3 The predominance of a prismatic 1 1 0 2 recrystallization texture has not been reported before in hexagonal materials. Acknowledgements A postdoctoral grant from CAM (MadridSpain as well as a CICYT