Materials Science & Engineering A 556 (2012) 253259Contents lists available at SciVerse ScienceDirectMaterials Science & Engineering A0921-50

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E-mjournal homepage: www.elsevier.com/locate/mseaTexture effects on development of shear bands in rolled AZ31 alloyY.B. Chun a,n, C.H.J. Davies b,c

a Nuclear Materials Division, Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yuseong-gu, Daejeon 305-353, South Koreab ARC Centre of Excellence for Design in Light Metals, Monash University, Clayton, Victoria 3800, Australiac Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria 3800, Australiaa r t i c l e i n f o

Article history:

Received 22 December 2011

Received in revised form

31 May 2012

Accepted 15 June 2012Available online 2 July 2012

Keywords:

Magnesium alloys

Texture

Deformation twinning

Shear banding93/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.msea.2012.06.083

esponding author. Tel.: 82 42 868 4571; faxail address: borobang@gmail.com (Y.B. Chun)a b s t r a c t

We investigate the effects of texture on shear banding in highly textured Mg3Al1Zn alloy. The plates

with different textures were rolled to 10% reduction in thickness at various temperatures, and the

microstructure, texture and misorientation distributions within the regions undergoing shear banding

were analyzed with optical microscopy and X-ray diffraction and electron backscattering diffraction

techniques. It is found that texture prior to rolling significantly affects the various features of shear

bands such as the formation frequency of shear bands, the inclined angle of shear bands to the rolling

plane, and orientations of grains in shear bands. We infer that shear bands in AZ31 rolled to a relatively

low strain occur as a result of geometric (texture) softening. Shear banding is most pronounced in the

plate with conventional strong basal texture, whereas the plate oriented favorably for the prism /aSslip shows no sign of shear banding. The inclined angle of shear bands in the plate with basal texture is

around 301, while those in the plates oriented for basal oa4 slip or tension twinning is around 551.The regions of shear bands receive more strain than the region outside the bands, which results in

significant lattice rotation of the grains in the bands, leading to off-basal texture.

& 2012 Elsevier B.V. All rights reserved.1. Introduction

A variety of materials when deformed to a large strain exhibitplastic instability (or plastic flow localization) which appears asjerky flow, Luders bands and shear bands. In ductile materials ashear band is the main manifestation of plastic instability andstrain localization in the shear band often leads to fracture of thematerial. Much effort therefore has been directed towards char-acterizing the structures of shear bands [13] and developingtheories to explain and predict shear banding phenomena [48].This has led to significant progress in understanding the nature ofshear bands formed in cubic-structured metals like Al and Cu.

In magnesium alloys shear bands are readily observed at lowstrains and are considered a typical feature of the deformed micro-structure [9], although limited effort has been made to elucidate thenature of the shear banding. Shear banding is of importanceespecially in Mg alloys with a limited formability at low tempera-tures, as it can be thought of as a deformationmechanism in additionto slip and twinning. Indeed, Couling et al. [10] suggest that shearbanding can be a mechanism responsible for superior cold-rollabilityof Mg alloys containing thorium or Misch metal. However, recentwork on MgY alloy attributes improved rollability to promotion ofnon-basal slip modes rather than shear banding [11]. In addition, theorientations in shear bands are known to be favorable for easyll rights reserved.

: 82 42 868 8549..

deformation modes like basal /aS slip [1214], which suggests thatshear banding is a possible source of texture modification.

It is well known that texture affects the mechanical properties ofMg alloys (e.g., among others, [1518]) by its influence on the activedeformation modes. In the same manner, the formation of micro-structural features including shear band are expected to rely on theinitial texture of material. In this study, we analyze shear bandsformed in AZ31 plates with different textures to see how textureand active deformation modes affect the formation of shear bands.2. Experimental procedures

The material investigated in the present work is commerciallyavailable Mg3Al1Zn (AZ31) plate. The 76 mm thick plate wasreceived in the hot-rolled and thermally stabilized condition. Theplate exhibited strong basal texture such that the c-axes of mostof the grains were aligned within 401 from the normal direction(ND) of the plate (Fig. 1, top left image). The microstructure of theplate consists of equiaxed grains with an average grain size of30 mm. Smaller plates measuring 60 mm in length, 40 mm inwidth and 4 mm in thickness with four different orientationswere cut from the as-received plate and these plates are termedthe NB, TB, RB and N45RB plates based on the alignment of the caxes prior to rolling (Fig. 1, left-hand column of the pole figures).Thus, for example, the TB sample was cut from the original plateso that the basal poles were aligned with the transverse directionof the smaller plate; the N45RB was cut so that the basal poles

www.elsevier.com/locate/mseawww.elsevier.com/locate/mseadx.doi.org/10.1016/j.msea.2012.06.083dx.doi.org/10.1016/j.msea.2012.06.083dx.doi.org/10.1016/j.msea.2012.06.083mailto:borobang@gmail.comdx.doi.org/10.1016/j.msea.2012.06.083dx.doi.org/10.1016/j.msea.2012.06.083

Fig. 1. Texture evolution, represented by the (0002) pole figure, of the AZ31 plates with four different starting textures during 10% rolling at various temperatures.

Y.B. Chun, C.H.J. Davies / Materials Science & Engineering A 556 (2012) 253259254were aligned at 451 to the normal direction in the rollingdirection. The plates were rolled at 25 1C, 150 1C or 250 1C to10% reduction in thickness in a single pass, which was followedby water quenching. All the plates were rolled at a rolling speed of14.8 m/min in a two-high mill with 350 mm diameter rolls.

To quantify textures of the rolled plates, four incomplete polefigures (1010, (0002), 1011 and 1012) were measured from theplate surface using a GBC-MMA X-ray diffractometer (XRD). Thebasal and prism pole figures shown in the present work are thosereconstructed from the orientation distribution function (ODF)based on the spherical harmonics method [19].

Microstructures of the rolled plates were analyzed using bothoptical microscopy and the electron backscatter diffraction (EBSD)technique. Samples for optical microscopy were mechanically groundand polished, and etched using a solution consisting of picric acid(16.8 g), acetic acid (40ml), water (40 ml) and ethanol (280ml). AnEBSD analysis was undertaken using a JEOL 7001F field-emission-gunscanning-electron microscope equipped with Nordlys II detector andHKL Technology Channel 5.0 acquisition system. Only samples rolledat 250 1C were examined by EBSD. Specimens were cut from therolled plate in the RDND plane and were mechanically ground andthen electro-polished in a solution consisting of 200ml nitric acid and800ml methanol at 25 1C under an applied voltage of 5 V.3. Results

3.1. Texture

The texture after 10% rolling depends strongly on the initialtexture (Fig. 1). Texture of the NB plates was little affected by10% cold-rolling, whereas remarkable changes in texture wereobserved in the TB, RB and N45RB plates. For the TB plates asubstantial portion of (0002) poles which had been in the TD wasreoriented towards the ND, but with the maximum intensityremaining aligned to the TD. The maximum intensity at the TDincreased slightly with increasing rolling temperature. The RB plateexhibited the most radical change such that the majority of (0002)poles, which had been aligned with the RD, were completelyreoriented towards the ND. The increasing rolling temperatureresulted in slight spread of (0002) poles towards the RD. TheN45RB plates also underwent texture change by 10% cold rolling.The location of the peak intensity in the basal pole figure wasmoved slightly towards the ND and some portion of the (0002)poles on the upper hemisphere were reoriented to a position 301away from the ND towards the RD on the lower hemisphere.

3.2. Optical metallography

Shear bands were found in some but not all cases (Fig. 2), thecharacteristics of which are dependent on the texture prior torolling. The NB plates show the most profuse shear bands andthese bands are inclined at 7301 to the rolling plane. Thedensity of shear bands slightly decreases with increasing rollingtemperatures. The RB plates include a few shear bands which areirregularly spaced throughout the specimen. The inclined anglesof these bands are 750601. The N45RB plates also contain shearbands which are inclined towards only one direction that isclose to the basal planes of the majority of grains and the inclinedangle is 50551. Shear bands are not observed in the TB platesirrespective of the rolling temperature.

At a higher magnification we see that the shear bands in the NBplate comprise thin twins which are densely populated along thebands (Fig. 3a). The TB plate includes some twins which are

Fig. 2. Effect of starting texture on microstructure of 10% rolled plates at various temperatures. Note that the NB plate shows profuse shear bands whereas the TB plate isfree of the bands.

Fig. 3. Optical micrographs of the 10% cold rolled AZ31 plates: (a) the NB, (b) TB, (c) RB and (d) N45RB plates.

Y.B. Chun, C.H.J. Davies / Materials Science & Engineering A 556 (2012) 253259 255homogeneously distributed throughout the microstructure (Fig. 3b).The RB and N45RB plates exhibit profuse twinning and the shearbands in these plates show relatively fine-grained structures (Fig. 3cand d). The plates rolled to 10% at 150 1C (Fig. 4) show similartexture dependencies of microstructure development to those foundin the cold-rolled plates. However, rolling at 150 1C significantlyincreases the densities of thin twins in all plates compared withthose in their cold-rolled counterparts.3.3. EBSD analysis of samples rolled at 250 1C

As shown in Figs. 58, microstructures are represented byband contrast maps which are superimposed with grains bound-aries, and the (0002) pole figures from the shear band and thenon-shear-banded region (i.e., the region outside the shear band)are presented separately. Within the shear bands of the NB plate(Fig. 5a) we see fine grains which are light in contrast and free

Fig. 4. Optical micrographs of the AZ31 plates rolled to 10% at 150 1C: (a) the NB, (b) TB, (c) RB and (d) N45RB plates.

Fig. 5. EBSD analysis of the NB plate rolled to 10% at 250 1C: (a) band contrast mapsuperimposed with grain boundaries, and the (0002) pole figures for (b) the non-

shear-banded region and (c) shear band (surrounded by yellow dashed line). In (a),

the red and black lines correspond respectively to boundaries with misorientation

angle less than 151 and high-angle boundaries with misorientation angle higherthan 151. White arrows indicate double twins. In (c), C0 and CSB indicate themajority of the c-axes prior to rolling and the c-axes of grains in shear band. (For

interpretation of the references to color in this figure legend, the reader is referred

to the web version of this article.)

Y.B. Chun, C.H.J. Davies / Materials Science & Engineering A 556 (2012) 253259256of low-angle boundaries, suggesting dynamic recrystallization.Some thin twins (indicated by white arrows) are observed in thenon-shear-banded region and these are found to be double twins(i.e., secondary 1012 tension twin in the primary 1011 compres-sion twin [20]). The local texture of the non-shear-banded region(Fig. 5b) is similar to the texture prior to rolling. The shear band(Fig. 5c), however, shows an off-basal texture such that the c-axesof the grains in the band are tilted by 201 from the ND to the RD.In the TB plate (Fig. 6a) some thick 1012 tension twins areobserved in the absence of shear bands. The (0002) pole figurefrom the tension twin reveals the expected reorientation of thec-axes in the TD toward the ND (Fig. 6c).

The shear band in the RB plate contains relatively high densityof low-angle boundaries, compared with the non-shear-bandedregion (Fig. 7a), suggesting that the shear bands received rela-tively large strain and most grains are still in the deformed state.The majority of grains in the non-shear-banded region have theirc-axes in the ND (Fig. 7b), which suggests extensive tensiontwinning in this plate. The c-axes of grains in shear band, on theother hand, are tilted by 451 from the ND towards the RD (Fig. 7c),and thus the c-axes of grains are nearly perpendicular to the planeof shear band (the white line in the (0002) pole figure).

The shear band in the N45RB plate also shows higher densityof low-angle boundaries than the non-shear-banded region(Fig. 8a). The (0002) pole figure from the non-shear-bandedregion (Fig. 8b) suggests that 10% rolling results in rotation ofthe peak intensity position towards the ND by 101. The (0002)pole figure from the shear band is basically similar to that fromthe non-shear-banded region but includes an additional poleintensity at 201 (in the counterclockwise direction) from theND towards the RD (Fig. 8c). The additional (0002) intensity iscaused by tension twinning, which therefore suggests a higherpropensity of tension twinning in the shear band.

Turning to misorientation angle distributions (Fig. 9) thedistributions of misorientation angle from shear band and thenon-shear-banded region in the NB plate (Fig. 9a) are similar toeach other, showing peak frequency at 301 which is typical forhexagonal close packed metals with strong texture. The misor-ientation angle distribution in the TB plate (Fig. 9b) shows two

Fig. 6. EBSD analysis of the TB plate rolled to 10% at 250 1C: (a) band contrast mapsuperimposed with grain boundaries, and the (0002) pole figures for (b) grains

deformed by slip and (c) grains that underwent tension twinning. In (a), the red

and black lines correspond respectively to boundaries with misorientation angle

less than 151 and high-angle boundaries with misorientation angle higher than151. Note that no shear band is observed in the TB plate. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of

this article.)

Fig. 7. EBSD analysis of the RB plate rolled to 10% at 250 1C: (a) band contrast mapsuperimposed with grain boundaries, and the (0002) pole figures for (b) the non-

shear-banded region and (c) shear band (surrounded by yellow dashed line). In (a),

the red and black lines correspond respectively to boundaries with misorientation

angle less than 151 and high-angle boundaries with misorientation angle higherthan 151. In (c), C0 and CSB indicate the majority of the c-axes prior to rolling andthe c-axes of grains in shear band. (For interpretation of the references to color in

this figure legend, the reader is referred to the web version of this article.)

Y.B. Chun, C.H.J. Davies / Materials Science & Engineering A 556 (2012) 253259 257peaks at 301 and 861. The latter peak corresponds to the 1012tension twin boundaries. The non-shear-banded region in the RBplate (Fig. 9c) exhibits two peaks at 601 and 861. The formerpeak corresponds to the boundaries between the two differentvariants of the f1012g tension twins. The distribution of misor-ientation angle in shear band on the other hand does not showany noticeable peak, although the distribution frequencies at301, 601 and 861 are slightly higher. In the N45RB plate(Fig. 9d) both the shear band and the non-shear-banded regionshow a peak at 861, the peak frequency of which is slightlyhigher in shear band.4. Discussion

During deformation strain tends to concentrate in the regionswhere resistance to continuous flow is first lost, which in case ofrolling often leads to shear banding. Accordingly, the formation ofshear bands is interpreted in terms of strain-induced softeningwhich results from various sources such as lattice rotationtowards more favorable orientation for deformation (i.e.,geometric softening), dynamic recrystallization, and localizedincrease in temperature (i.e., thermal softening). We can dismissthe latter two causes for the following reasons. Because therolling strain is so small (10% reduction in thickness whichcorresponds to an equivalent strain of 0.12) and magnesiumhas a relatively high thermal conductivity thermal softening isunlikely to be related to the shear bands observed in the presentwork. Dynamic recrystallization is also excluded from considera-tion, as the phenomenon is extensively observed only in the NBplate rolled at 250 1C. Geometric softening is the probablemechanism responsible for shear banding as the characteristicsof shear bands observed depend on the initial texture. In whatfollows, the effects of textures on the formation mechanism andcharacteristics of the shear bands in AZ31 plates are explored.

The 10% rolled NB plate exhibits profuse shear bands (Fig. 2),while a limited number of shear bands were observed in the RBand N45RB plates. Extensive shear banding in the NB plate maybe related to a lack of easy deformation modes. The NB platetexture is oriented unfavorably for basal /aS slip, prism /aS slipand tension twinning. It is therefore likely that the plate showsresistance to homogeneous deformation during rolling, and thiswill lead to local stress build-up which can activate compressiontwinning, which is often followed by tension twinning resultingin double twins. As the double twins are known to be favorablyoriented for basal /aS slip [20,21], local regions populated by thistype of twins can provide a source of geometrical softening,leading to profuse shear banding in this plate. On the other hand,the RB and N45RB plates are respectively oriented favorably fortension twinning and basal /aS slip. Therefore, these platesexhibit more uniform deformation with activation of each ofthese deformation modes, and thus have less shear bands thanthe NB plate. The absence of shear bands in the TB plate issomewhat surprising because the plate is oriented favorably for

Fig. 8. EBSD analysis of the N45RB plate rolled to 10% at 250 1C: (a) band contrastmap superimposed with grain boundaries, and the (0002) pole figures for (b) the

non-shear-banded region and (c) shear band (surrounded by yellow dashed line). In

(a), the red and black lines correspond respectively to boundaries with misorienta-

tion angle less than 151 and high-angle boundaries with misorientation angle higherthan 151. In (c), C0 and CSB indicate the majority of the c-axes prior to rolling and thec-axes of grains in shear band. CSB indicating the upper left direction is the c-axes

reoriented by tension twinning. (For interpretation of the references to color in this

figure legend, the reader is referred to the web version of this article.)

Fig. 9. Distributions of misorientation-angle of the AZ31 plates rolled

Y.B. Chun, C.H.J. Davies / Materials Science & Engineering A 556 (2012) 253259258prism /aS slip which is the second easiest slip mode in Mg.Indeed, the TB plate exhibits resistance to shear banding even atmuch higher strains. The optical micrograph of the TB sample thatwas warm-rolled to 44% at 150 1C (Fig. 10) reveals that there areno pronounced shear bands although the edge cracks are initiatedat the strain. It is shown in previous work [22] that the TB plateexhibits an excellent rollability at 150 1C, which is attributed topromotion of prism /aS slip at the temperature. In addition, agood portion of grains in the TB plate is deformed by tensiontwinning (Figs. 3b, 4b and 6a). Accordingly, it is conjectured thatco-activation of prism /aS slip and tension twinning in the TBplate enables homogeneous deformation under rolling, leading tosuppression of shear banding and an excellent plasticity. Thisresult suggests that activation of prism /aS slip is effective inaccommodating strain, although the mechanism behind this isnot clearly understood [23].to 10% at 250 1C: (a) the NB, (b) TB, (c) RB and (d) N45RB plates.

Fig. 10. Optical micrograph of the TB plate rolled to 44% at 150 1C. Note that thereis no pronounced shear banding in the microstructure although the edge cracks

are initiated at the rolling reduction.

Y.B. Chun, C.H.J. Davies / Materials Science & Engineering A 556 (2012) 253259 259Others find that recrystallized grains in shear bands inmagnesium alloys show an off-basal texture in which the c-axesof grains are away from the ND towards the RD [12,13]. In thepresent work, the NB plate shows a similar trend, with a 201 tiltof the c-axes towards the RD (Fig. 5). It is possible that the largeshear strain concentrated in the shear band results in latticerotation of the grains about the TD, resulting in an off-basaltexture towards the RD. Sun et al. [13] report that the tilt of the c-axes in shear bands increases with increasing strain and this maysuggest that the larger shear strain at higher imposed strainsresults in lattice rotation to the larger extents. On the other hand,the c-axes of grains in shear band in the RB plate is nearlyperpendicular to the plane of shear band and is 401 away from theRD to ND. While the rotation of matrix is caused by tensiontwinning, the orientation in the shear band is unlikely to berelated to tension twinning as the frequency of misorientations at861 for the shear band is quite low when compared with that formatrix (Fig. 9c). It is possible that prior to rolling some grains hadtheir c-axes orientated between the ND and RD. As these grainswould have been aligned favorably for basal /aS slip, geometricsoftening can take place in the local regions populated by suchgrains and as a consequence the imposed deformation is concen-trated in the regions, resulting in shear bands. The shear bands inthe N45RB plate have similar orientations to the non-shear-banded region (Fig. 8). Both regions are readily deformed bybasal /aS slip as most of the grains in the plate are orientedfavorably for the slip mode. A relatively large strain in shearbands, however, promotes more tension twinning, as shown byreorientation of c-axes (Fig. 8c) and increased frequency at 861 inthe distribution of misorientation-angle (Fig. 9d).

EBSD micrographs (Figs. 58) reveal that the amount of strainaccommodated by shear banding is also affected by texture.Densely populated fine grains (grains with light contrast inFig. 5a) suggest that dynamic recrystallization takes place inshear bands of the NB plate. This in turn indicates that duringrolling shear bands received considerable strain which is higherthan the critical strain required for an initiation of dynamicrecrystallization. On the other hand, minimal signs of dynamicrecrystallization are found in the shear bands formed in the otherplates, although shear band regions in the RB and N45RB platesshow a relatively high density of low-angle boundaries comparedwith the non-shear-banded regions (Figs. 7a and 8a). This impliesthat although the shear band regions in the latter plates receiveda relatively large strain this is, however, not sufficient to inducewidespread dynamic recrystallization.5. Conclusions

As was anticipated the starting texture had a strong effect onthe characteristics of shear bands such as the formation frequency,shear band angle and orientation in the shear bands. However,beyond this generalization the different textures allow us to drawseveral conclusions that are not immediately obvious. Mostimportantly, we can infer that shear bands in AZ31 rolled to arelatively low strain occur as a result of geometric (texture)softening. Furthermore:1. Shear banding is most pronounced in the plate with conven-tional strong basal texture. The amount of shear strain loca-lized in shear bands is also higher for the plate with the harderorientation. The inclined angle of shear bands in the plate withbasal texture is around 301, while those in the plates orientedfor basal /aS slip or tension twinning is around 551.2. The plate oriented for prism /aS slip is free from shear bandsafter 10% rolling, suggesting that activation of prism /aS slipis effective in accommodating strain.3. Orientations within shear bands are quite different from thoseoutside the bands, which is attributed to lattice rotationcaused by large shear strain localized in shear bands. Thisstrain localization leads to an off-basal texture.Acknowledgments

The authors gratefully acknowledge the Australian ResearchCouncil for funding of the Centre of Excellence for Design in LightMetals, and the Monash Centre for Electron Microscopy for theprovision of EBSD facilities. Thanks to Ms Juan Zhao for assistancewith additional EBSD figures.

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Texture effects on development of shear bands in rolled AZ31 alloyIntroductionExperimental proceduresResultsTextureOptical metallographyEBSD analysis of samples rolled at 250degC

DiscussionConclusionsAcknowledgmentsReferences