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Journal of Alloys and Compounds 487 (2009) 309–313

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

journa l homepage: www.e lsev ier .com/ locate / ja l l com

exture evolution in cold-rolled AZ31 magnesium alloy duringlectropulsing treatment

ei Guan a,b, Guoyi Tang a,∗, Yanbin Jiang a, Paul K. Chu b

Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, ChinaDepartment of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

r t i c l e i n f o

rticle history:eceived 27 April 2009eceived in revised form 19 July 2009

a b s t r a c t

The microstructural evolution in cold rolled AZ31 magnesium alloy during electropulsing treatment (EPT)is analyzed by texture analysis. The recrystallization mechanism during electropulsing is found to dependon the previous amount of reduction. The recrystallized grains give rise to the tilted basal texture by a

ccepted 20 July 2009vailable online 28 July 2009

eywords:lectropulsingecrystallization

rotation of 45◦–60◦ from the rolling direction (RD) towards the normal direction (ND). The mechanism ofthe microstuctural evolution during electropulsing is discussed from the point of view of grain boundarymotion.

© 2009 Elsevier B.V. All rights reserved.

extureZ31 magnesium alloy

. Introduction

It is well known that magnesium sheet alloys exhibit poorormability in conventional forming operation at ambient temper-ture [1]. The problem is commonly attributed to the hexagonallose packed (hcp) structure that consequently limits slipping atemperature below 498 K. Hence, in order to avoid cracking andbtain the desirable mechanical properties, Mg alloys such as AZ31Mg–3%Al–1%Zn) must be rolled at elevated temperature therebyuffering from adverse effects arising from heat treatments. Con-entional heat treatment in a furnace requires a long time resultingn grain coarsening as well as reduction in strength and ductilityf the wrought magnesium products. There is thus a pressing need

s to develop a new heat treatment protocol for magnesium alloyheets or strips.

Electropulsing treatment (EPT) is an effective method for grainefinement through recrystallization [2–5]. Conrad et al. [6–8]ound that EPT could enhance the nucleation rate of recrys-allization and refine the grain size when it was implementeduring annealing of cold worked copper, and the phenomenon wasxplained by the accelerated mobility of dislocations induced by

PT. Recently, Xu et al. [3] obtained ultrafine recrystallized grains incold-rolled AZ31 strip using EPT, and the outcome was ascribed

o the greater vacancy mobility induced by electropulsing due tocceleration of the dislocation propagation and annihilation. Du

∗ Corresponding author. Tel.: +86 755 26036752; fax: +86 755 26036752.E-mail address: tanggy@mail.tsinghua.edu.cn (G. Tang).

925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2009.07.114

[9] also found that recrystallized grains could be formed in ECAPedAZ31 using EPT, and the reason was that the thermal compressivestress could accelerate the motion of the dislocations. Although finerecrystallized grains can be produced in cold-rolled magnesiumalloys by EPT, the EPT mechanism is still unclear.

Texture analysis is a viable tool to investigate the microstruc-tural evolution in magnesium alloys [10–13]. However, not muchwork has been performed to apply this tool to characterize theeffects of EPT on the microstructural evolution in cold-rolled AZ31magnesium alloy. Here, texture analysis is utilized to study themicrostructural evolution of cold-rolled AZ31 magnesium alloy andthe EPT mechanism is also discussed.

2. Experimental details

AZ31 magnesium alloy (Mg–3 wt pct Al–1 wt pct Zn) sampleswere extruded to form strips 2.96 mm wide and 1.5 mm thick. Afterannealing at 573 K for 1 h, the extruded strips were cold-rolled in asingle pass down to 1.35, 1.17 and 1.03 mm, corresponding to com-pressive deformation rates of 10%, 22%, and 31%, respectively. Therolling direction (RD) was parallel to the extrusion one and coldrolling was performed without side cracking.

The EPT process is schematically illustrated in Fig. 1. The AZ31strip was treated by multiple electropulsing in which the strip

moved at a speed of 2 m/min through a distance of 225 mm betweenthe two electrodes. It took about 7 s to move the strip from the anodeto cathode. A custom made electropulsing generator was utilizedto discharge the positive multiple pulses. The pressure betweenthe electrodes and strip was just sufficient to keep good electrical

310 L. Guan et al. / Journal of Alloys and Co

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Fig. 1. Schematic view of the EPT process.

ontact without causing deformation of the strip. Electropulsesith various frequencies and durations (70 to 80 ms) were applied

o the strip. Parameters including frequency, root-mean-squareurrent (RMS), amplitude, and duration of current pulses wereonitored by a Hall Effect sensor connected to an oscilloscope. The

emperature of the AZ31 strip near the cathode was measured usingRaytek MX2 infrared thermoscope. The important EPT parametersre listed in Table 1.

Prior to optical examination, the samples were sectioned, coldounted, and polished with 6 and 1 �m diamond paste. Thereere then etched in acetic picral (5 ml acetic acid, 6 g picric acid,

5 ml ethanol, and 5 ml water) for 5 s. The average grain size (dave)as calculated from the optical micrographs by the linear interceptethod.

The X-ray texture analysis was conducted on the surface of theold-rolled and EPT AZ31 samples. Four incomplete pole figuresup to = 70◦), namely (0 0 0 2), (101̄0), (101̄1), and (101̄2), werecquired from each sample by X-ray diffraction in the back reflec-ion mode with Cu K� radiation. The pole figures show that onlyhe (0 0 0 2) basal plane texture has been developed and so only the0 0 0 2) pole figures are shown in this paper.

. Results

Fig. 2 shows the microstructure of the cold-rolled AZ31 sam-les with 10% and 31% rolling reduction and the corresponding0 0 0 2) pole figures. The optical micrographs reveal the typicaltructure of prolonged crystals along the rolling direction mixedith a large number of shear bands and twinning resulting from

he intensified strains. After low rolling reduction (10%) as showny Fig. 2(a), the cores of the grains are relatively un-deformed andsignificant amount of plastic strain is accommodated in the grain

oundary regions. Therefore, the recrystallized grains caused byhe difference in the dislocation density across the original grainoundaries can be found by strain induced grain boundary migra-ion (SIBM). Further reduction to 31% increases the twinning ashown in Fig. 2(b) and the intersection of the deformation twins

able 1xperimental conditions of AZ31 strip under EPT.

PT sample no. Rolling reduction (%) Frequency (Hz)

10 10910 151.210 182.522 127.322 154.722 176.331 107.931 14231 168.5

ote: Jm is the amplitude of current density of electropulsing, and Je represents the RMS v

mpounds 487 (2009) 309–313

becomes severe. Some recrystallized grains form at the pre-existedgrain boundaries. Accordingly, it can be inferred that subgains firstdevelop in the vicinity of the serrated grain boundaries, and as colddeformation proceeds, the subgrain structure develops throughoutthe entire grain volume.

The cold-rolled samples show a grouping of the c-axis aroundthe strip ND, as shown by Fig. 2(c) and (d). This fiber texture is com-mon in cold-rolled magnesium alloys [14]. As the rolling reductionincreases, the intensity of the basal-type texture diminishes from5.7 to 5.3. It may be because the recrystallized grains decorate someshear bands and twins [15]. It has been reported that in the AZ31alloy, recrystallization occurs at room temperature at the sites ofheavy deformation such as shear bands and twins [15]. These tinyrecrystallized grains possess small misorientation randomly andthis may contribute to this phenomenon [15,16].

With regard to low rolling reduction (10%) at 423 K, incom-plete recrystallization occurs mainly along the grain boundary. Thebulging part of a pre-existing grain boundary sets off the nucleationof previous grain boundaries. This implies that the strain inducedgrain boundary migration (SIBM) mechanism plays an importantrole, as indicated by Fig. 3(a). As shown in the corresponding polefinger in Fig. 3(c), the tilted basal plane orientation by a rotation of45◦–60◦ around RD towards ND is apparent.

As the EPT temperature is increased to 523K, normal graingrowth occurs as shown by Fig. 3(b). The distribution of the grainsize is inhomogeneous (dave = 25 �m). The corresponding pole fin-ger in Fig. 3(d) reveals that the extent of tilting becomes lowerthan that shown in Fig. 3(c) and the intensity of basal-type tex-ture becomes higher. The enhanced basal plane texture may stemfrom the grain growth via the SIBM mechanism. Consequently, theorientation coherence between the recrystallized grains and matrixgrains becomes stronger.

Compared to low rolling reduction, the macro-texture evolu-tion at high rolling reduction is opposite. As the EPT temperatureincreases, the extent of tilting becomes obvious as shown in Fig. 4(c)and (d), and the intensity of the basal-type texture becomes smaller.The reason for the phenomenon is believed to be the differentnucleation mechanism during EPT under different rolling reduc-tion. As shown in Fig. 4(a), the predominant nucleation sites arelocated in the shear bands and twins. This is different from the SIBMmechanism shown in Fig. 3(a). As shown in Fig. 4(b), when the tem-perature increases, nucleation results from the mutual intersection,termed of “twin recrystallization”, and refined grains averagingabout ∼9 �m are observed for complete recrystallization. As recrys-tallization proceeds, the orientation coherence of recrystallizedgrains with the matrix grains diminishes rapidly, and the localrandomized texture gives rise to the weakened basal texture. Con-

cerning the two samples shown in Figs. 3(b) and 4(d), the “tilted”basal texture corresponds predominantly to the ultrafine recrys-tallized grains located at the original grain boundary by the SIBMmechanism and in the shear bands and twins by twin recrystalliza-tion mechanism.

Jm (A/mm2) Je (A/mm2) Temperature (K)

309 18.34 423309 21.54 473311 23.89 523318 20.81 423321 22.26 473330 24.24 523375 22.74 423375 25.30 473380 28.27 523

alue of current density during EPT and is related to the Joule heating effect.

L. Guan et al. / Journal of Alloys and Compounds 487 (2009) 309–313 311

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ig. 2. Microstructures of the cold-rolled AZ31strips with rolling reduction: (a) 10%he arrow direction.

. Discussion

According to our experimental results, two principal microstruc-ural changes take place with increasing rolling reduction atifferent EPT temperature. During low rolling reduction (10%), theecrystallized grains are formed along the grain boundaries via theIBM mechanism after EPT. With respect to high rolling reduction31%), twin recrystallization forms in fine grain structure. The grainize decreases as the mechanism transforms from SIBM to twinecrystallization during EPT.

In addition to the microstructure changes, the texture evolu-ion changes correspondingly. In low rolling reduction (10%), theilted basal texture in Fig. 3(c) represents the recrystallized grainsorming along the grain boundaries. In comparison, in high rollingeduction (31%), the fine recrystallized grains form along the shearands and within twins corresponding predominantly to the tiltedasal texture. Accordingly, as recrysatallization progresses, the localandomized texture gives rise to the weakened and tilted basalexture.

It is well known that the crystalline defect distribution aftertraining is general heterogeneous and the electrical resistivity isensitive to the microstructural details of the materials such asrain boundaries, dislocations, vacancies, twins, etc. Because of thexistence of these defects, an inhomogeneous physical field is gen-rated in the metal. The resistivity is higher in an area with high

efects. When electropulsing is conducted through a metal speci-en, thermal and athermal effects [17,18] are stronger due to the big

egional resistivity and the strong detour of the current in the areaith defects. This is termed the “selective effect” of electropulsing.ccording to previous reports, the thermal effect may be attributed

1%, and their cooresponding (0 0 0 2) pole figures (c and d). The RD is aligned with

to Joule heating as a result of the pulsed current and local com-pressive stress gradient [19] by the inhomogeneous physical field,whereas the athermal effects may be explained by theories of elec-tron wind [20].

The dynamics of recrystallization depends on the nucleation rateand growth rate of the recrystallized nuclei. In polycrystalline mag-nesium alloy after high rolling reduction, due to low stack fault inmagnesium alloy, recrystallization nucleation during annealing canbe defined as the motion of low angle boundary migration (LABM)[21] and recrystallization as the motion of high angle boundary(HABM) [22]. In the EPT case, the distinct acceleration of lowand high angle boundary migration results from the acceleratedinterchange of the vacancies and single atoms [3,23]. This in turnincreases the nucleation rate and decreases the growth rate of themajority of recrystallized grains.

The boundaries move in response to a driving pressure (P) whichgenerally arises from stored dislocations or from the energy of theboundaries in the materials. In most cases (see Ref. [19]), the veloc-ity of the moving boundary (v) is given by the following equation:

v =MP, (1)

where M, the boundary mobility, is usually assumed to vary withtemperature according to the following relationship:

M =M0exp[−Q ]

, (2)

RT

where M0 is a constant, T is the absolute temperature, Q is the acti-vation energy for boundary migration, and R is the gas constant.We believe that the combined thermal and athermal effects aris-ing from electropulsing are responsibile for the enhancement of

312 L. Guan et al. / Journal of Alloys and Compounds 487 (2009) 309–313

Fig. 3. Optical micrographs showing the microstructure of the EPT1 and EPT3 samples (10% rolling reduction) at (a) 423 K and (b) 523 K, respectively, and their cooresponding(0 0 0 2) pole figures (c and d). The RD is aligned with the arrow direction.

Fig. 4. Optical micrographs showing the microstructure of the EPT7 and EPT9 samples (31% rolling reduction) at (a) 423 K and (b) 523 K, respectively, and their cooresponding(0 0 0 2) pole figures (c and d). The RD is aligned with the arrow direction.

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rain boundaries migration. In conventional recrystallization in aomogeneous alloy, the driving pressure P is usually given by:

= PV + PR, (3)

here PV is the volume energy and PR is the grain boundary energy.n EPT, the thermal and athermal effects can offer additional drivingressure:

P = Pth + Path, (4)

here Pth is the local thermal compressive stress given byth = 2a�SgradT/ϕ [�S is the difference in entropy between grainoundary and crystal (approx. equal to entropy of melting), gradT

s the temperature gradient, 2a is the thickness of the grain bound-ry, and ϕ is the atomic volume] and Path is the electron wind forceiven by Path = (�D/ND)enej, where �D/ND is the specific resistivityer unit dislocation length, ND is the dislocation density, ne is thelectron density and j is the current density.

In EPT, the total driving pressure is thus:

EP = P +�P = PV + PR + Pth + Path (5)

According to Eqs. (1) and (5), the velocity of the moving bound-ry during EPT is given by:

EP =MPEP (6)

Accordingly, the effects of the rolling reduction correspondingo the driving pressure P and the current density corresponding tohe�P on the velocity of the moving boundary are quite apparent.

In the single phase magnesium alloy AZ31, the stored energynduced by a large rolling strain (31%) exists in the regions ofhear band, twin boundaries, and near-boundary regions [15]. Theicrostructure in Fig. 2(b) exhibits good correspondence. As Jm and

e become larger, substantial athermal and thermal effects are pro-uced as Path and Pth increase sharply. Consequently, a higher storednergy results and faster boundary migration ensues thus stimulat-ng the twin recrystallization process at relative low temperature.

Complete recrystallization due to the twin recrystallization pro-ess results in the refined grain and weakened and tilted basal-typeexture shown by Fig. 4(b) and (d). In the short time during EPTrocess, acceleration of the boundary migration is responsible forhe increased nucleation rate in the early stage of recrystalliza-ion and retardation of the subsequent grain growth as shown inig. 4(b). The selective effect of twin recrystallization can offer theeason of weakened and tilted basal texture shown in Fig. 4(d). Byhe combined thermal and athermal effects, nucleation occurs byither intersection of various systems of twins or rearrangementf lattice dislocations within the twin lamellae. As a result, theseuclei are transformed into recrystallized grains. Since the treat-ent times too short, some of these tiny recrystallized grains give

ise to the random misorientation and another tilt around the RD.urther clarification of the exact effect of EPT on the recrystalliza-ion mechanisms of SIBM and twin recrystallization is being carriedut and it includes determination of misorientation distributionscross nucleus-deformation matrix interfaces.

. Conclusions

When electropulsing is performed to AZ31 magnesium alloyample with a low rolling reduction of 10%, recrystallized grains

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mpounds 487 (2009) 309–313 313

are formed along the grain boundaries via the SIBM mechanism.With regard to samples with a high rolling reduction of 31%, twinrecrystallization results in the fine grain structure. As recrysatal-lization progresses, the local randomized texture corresponding torecrystallized grains gives rise to the tilted basal texture. During EPT,the combined thermal and athermal effects offer additional drivingpressure to recrystallization with selective effects. The increaseddriving pressure results in recrystallizaton occurring at a rela-tively low temperature and acceleration of boundary migration isresponsible for the increased nucleation rate in the early stage ofrecrystallization and retards subsequent grain growth.

Acknowledgements

The authors acknowledge support from the Tsinghua – CityUCollaboration Scheme and express their thanks to Mr. J. Ma forhis discussions. The work was supported by the National Natu-ral Science Foundation of China (No. 50571048) and Hong KongResearch Grants Council (RGC) General Research Funds (GRF) No.CityU 112307.

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