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Materials Science and Engineering A 528 (2011) 3249–3252

Contents lists available at ScienceDirect

Materials Science and Engineering A

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

ccumulation and annihilation effects of electropulsing on dynamicecrystallization in magnesium alloy

ing Xua,1, Guoyi Tanga,∗, Yanbin Jiangb,2, Guoliang Hua,1, Yaohua Zhuc,3

Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, ChinaInstitute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, ChinaDepartment of Industrial and Systems Engineering, Hong Kong Polytechnic University, Kowloon, Hong Kong, China

r t i c l e i n f o

rticle history:eceived 11 November 2010eceived in revised form

a b s t r a c t

The effect of electropulsing treatment (ET) on the microstructure of Mg–3Al–1Zn (AZ31) alloy was inves-tigated. Combing with the calculated temperature of the sample after ET, it was found that electropulsingaccelerated dynamic recrystallization (DRX) at a relatively low temperature and a high strain rate. Based

0 December 2010ccepted 30 December 2010vailable online 7 January 2011

eywords:agnesium alloys

on the interplay between the accumulation and annihilation effects of electropulsing, a tentative mech-anism of DRX induced by electropulsing was discussed.

© 2011 Elsevier B.V. All rights reserved.

ecrystallizationrain boundaries

. Introduction

Electropusling, as an instantaneous high energy input method,as been applied in materials science and engineering extensively.

n the early years, studies of electroplastic effect focused on plas-ic deformation, such as the drop in flow stress and the increasen stress relaxation [1]. This mechanism was mainly based on theffect of the electric current on dislocation motion. Conrad et al.2] proposed that the electron wind force induced by electropuls-ng assisted dislocation movement and improved the plasticity of

etals.In recent years, researchers have investigated the effect of elec-

ropulsing on controlling microstructure [3]. Zhu et al. [4,5] showed

hat electropulsing induced phase transformation. Jiang et al. [6–9]ndicated that electropulsing treatment decreased the apparentolid solution temperature and accelerated the dissolution of �hase in Mg–9Al–1Zn alloy. Xu et al. [10,11] indicated that DRX

∗ Corresponding author at: Room 204, Building J, Tsinghua Campus, The Univer-ity Town, Shenzhen 518055, China. Tel.: +86 755 2603 6752;ax: +86 755 2603 6752.

E-mail addresses: xuq08@mails.tsinghua.edu.cn (Q. Xu),anggy@mail.tsinghua.edu.cn (G. Tang), jyb05@mails.tsinghua.edu.cn (Y. Jiang),gl07@mails.tsinghua.edu.cn (G. Hu), yaohuazhu@hotmail.com (Y. Zhu).1 Address: Room 307, Building J, Tsinghua Campus, The University Town, Shen-

hen 518055, China. Tel: +86 755 2603 6382.2 Address: Room 318, Fushi Building, 30 Xueyuan Road, Haidian District, Beijing

00083, China. Tel.: +86 010 6233 2253.3 Tel: +852 2766 4983.

921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2010.12.101

occurred at a relatively low temperature under electropulsing ten-sion. This mechanism was mainly based on the coupling of thethermal and electromigration effects of electropulsing [9,12]. How-ever, little has been done on the interplay between these twomechanisms. Undoubtedly, this lack of work impedes the develop-ment of electropulsing in both theoretical research and engineeringapplication.

Based on experimental results of DRX induced by electropulsing,the present work will establish a tentative interplay between thetwo mechanisms.

2. Experimental

The hot-rolled sheets of AZ31 Mg alloy (3.1 wt.% Al, 0.9 wt.% Zn,0.4 wt.% Mn, balance Mg) with a thickness of 0.8 mm were used inthis investigation. Uniaxial tensile test was performed on a SANSCMT universal testing machine, using the sample of 10 mm widthand 65 mm gauge length. The test was carried out at a rate of100 mm/min and the tensile direction was parallel to the trans-verse direction (TD) of the sheets. The dynamic electropulsing wasperformed on the sample being tensile deformed.

A self-made electropulsing generator was applied to dis-charge positive direction multiple pulses. Multiple electropulses

were applied on two electrical contactors with a distance of45 mm. Current parameters, including frequency (f = 200 Hz), root-mean-square current (I = 80 A), amplitude (Jm = 123.8 A/mm2) andduration (�p = 72 �s) of current pulses were monitored by a Halleffect sensor connected to an oscilloscope. The surface tempera-

3250 Q. Xu et al. / Materials Science and Engineering A 528 (2011) 3249–3252

ET. (a

ttwpmK

3

3

mnεt

lolsmtsatppoWsfstbdadbis

Fig. 1. Microstructure evolution of DRX under

ure of the sample, measured by a thermocouple, was 351 K. Dueo the short time of ET at the high strain rate, the temperatureas measured on the evenly deformed part rather than the neckart. The sample was quenched in water in order to retain theicrostructure. Metallurgical structure was observed by HiROX

H-7700 microscope.

. Results and discussion

.1. Microstructure evolution

Compared with conventional warm methods [13–16], the defor-ation under ET is inhomogeneous due to the obviously extended

ecking stage [10]. By ignoring the decrease in thickness, the straincan be determined by ε = (w0 − w)/w0 (where w0 is the width of

he sample before ET, and w is the width of the sample after ET.).Fig. 1 shows microstructure evolution of the sample [11]. With

ow strain, grains are prolonged slightly and deformed twins arebserved, as shown in Fig. 1a. Since the temperature is relativelyow and the tensile direction is parallel to the TD, the basal slip isuppressed. Moreover, twins prone to nucleate and grow to accom-odate further deformation due to the high strain rate [17]. With

he increasing strain, serrated grain boundaries are observed, ashown in Fig. 1b. The electron wind force induced by electropulsingssists dislocation movement so that some non-basal slip sys-ems, such as {112̄2}〈112̄3〉 and {112̄0}〈112̄0〉 [14], are activated torovide five independent slip systems required for homogeneouslasticity [18]. As a result, dislocations are piled up in the vicinityf grain boundaries and coarser grain boundaries are formed [14].hen the strain is increased to 0.44, new grains initiates along the

errated grain boundaries and a “necklace” structure, as a typicaleature of the initiation of DRX in Mg alloys [15], are observed, ashown in Fig. 1c. By the coupling of the thermal and electromigra-ion effects of electropulsing, piled-up dislocations are rearrangedy climb and cross-slip controlled by self-diffusion [18,19]. Thus,islocation density near grain boundaries is decreased and low-

ngle boundaries are formed [14,15]. Furthermore, by absorbingislocations, low-angle boundaries are converted to high-angleoundaries and new grains are formed [18,19]. When the strain

s increased to 0.63, DRX completes and fine grains are obtained, ashown in Fig. 1d.

) ε = 0.17; (b) ε = 0.33; (c) ε = 0.44; (d) ε = 0.63.

3.2. Final temperatures

The measured value (351 K) represents the temperature of theevenly deformed part, while DRX occurs in the neck part. Therefore,the real temperatures representing DRX initiates and completesneed to be calculated. The final temperature of the sample T, as afunction of the strain, can be determined by [11]:

T = T0 + �T1 + K1t − [1 − exp(−K2t)]K3

K1 = I2�eε(2 − ε)

2hw20dt(1 − ε)2

K2 = 2h

CP�md

K3 = I2�e�mCPε(2 − ε)

4h2w20t(1 − ε)2

− I2�e

2hw20d

+ �T1

(1)

where T0 is the room temperature, �T1 is the increased tempera-ture in the evenly deformed stage, t is the time of the necking stage,I is the RMS current value of ET, �e is the electrical resistivity, h isthe heat transfer coefficient between the sample and the surround-ing air, d is the thickness of the sample, Cp is the specific heat, and�m is the density of the sample.

For numerical evaluation of Eq. (1), the param-eters used are T0 = 298 K, �T1 = 20.2 K, t = 6.74 s,�e = 9.2 × 10−8 � m, h = 22.30 W/(m2 K), w0 = 10−2 m,d = 8 × 10−4 m, CP = 1.13 × 103 J/(kg K), �m = 1.78 × 103 kg/m3.The calculated result is shown in Fig. 2. Combining with themicrostructure observation (Fig. 1c, d), the temperatures at whichDRX initiates and completes under ET are 376.04 K (ε = 0.44)and 434.51 K (ε = 0.63), respectively. It means that electropulsingaccelerates DRX at a relatively low temperature and a high strainrate.

3.3. Accumulation and annihilation effects of electropulsing

From the above analysis, DRX in Mg alloy is the result of thechange in dislocation density in the vicinity of grain boundaries.The electron wind force accumulates dislocations and leads to theformation of coarse grain boundaries. The coupling of the ther-mal and electromigration effects annihilate dislocations and leads

Q. Xu et al. / Materials Science and Engineering A 528 (2011) 3249–3252 3251

teteet

embivt

(

wNcc

j

wmcd

b

wegpm

Fig. 2. The final temperature of the sample after ET.

o the rearrangement of dislocation structures. It means that theffect of electropulsing is composed of two parts: the accumula-ion effect induced by the electron wind force and the annihilationffect induced by the coupling of the thermal and electromigrationffects. Therefore, electropulsing accelerates DRX dramatically dueo the joint action of the accumulation and annihilation effects.

Previous studies [12] showed that vacancy concentration is anffective parameter to evaluate dislocation density and rearrange-ent of dislocation structures. The flux of vacancies contributed

y the accumulation effect (jacc) represents the rate of the increasen piled-up dislocations near grain boundaries, while the flux ofacancies contributed by the annihilation effect (jann) representshe rate of decrease in piled-up dislocations.

The vacancy concentration caused by the accumulation effectCacc) can be determined by:

Cacc = Cacc0Nl exp(

−QFE

RT

)

Cacc0 =√

3

6�(d/b)2

(2)

here Cacc0 is the pre-exponential factor, d/b is the splitting width,l is the atom density, QFE is the activation energy of Friedel–Escaigross-slip, and R is the gas constant. Thus, the flux of vacanciesontributed by the accumulation effect can be determined by:

acc = Caccvacc (3)

here vacc = J/ene [1], J = 2�pfJm/� [6], vacc is the velocity of theovement vacancy dragged by the electron wind force, J is the

urrent density, e is the electron charge, and ne is the electronensity.

The flux of vacancies contributed by the annihilation effect cane determined by [11]:

jann = jann(th) + jann(em)

jann(th) = D0C0�pQvI2Ret0Nl

2(R0 − r0)CPmRT2exp

(− Qd

RT

)

jann(em) = 2D0Z∗e�e�pfJmNl

�kTexp

(− Qd

RT

)(4)

here jann(th) is the flux of vacancies contributed by the thermalffect, jann(em) is the flux of vacancies contributed by the electromi-ration effect, D0 is the diffusion pre-exponential factor, C0 is there-exponential factor, Qv is the activation energy for vacancy for-ation, Qd is the activation energy for vacancy diffusion, Re is the

Fig. 3. The ration q, as a function of ε after ET.

electrical resistance, t0 is the unit time, R0 and r0 are the distancesof the boundaries conditions, m is the mass of the sample, and Z* isthe effective valence.

In order to study the interplay between the two effects, theration q is introduced by:

q = jann

jacc(5)

Combing with Eq. (1), Eq. (3) and Eq. (4), the parametersused for Eq. (5) are: d/b = 5; Nl = 4.48 × 1028/m3; QFE = 96 KJ/mol[15]; R = 8.314 J/(mol K); e = 1.6 × 10−19 C; ne = 8.96 × 1028/m3;D0 = 1.2 × 10−3 m2/s; C0 = 10; Qv = 69.35 KJ/mol; Qv = 135 KJ/mol;Re = 7.48 × 10−4 �; t0 = 1 s; R0 = 7.07 × 10−6 m; r0 = 3.21 × 10−10 m;m = 9.26 × 10−4 kg; Z* = 10. The ration increases with the strain, asshown in Fig. 3. With low strain, the accumulation effect is moredominant than the annihilation effect. It means that dislocationsprone to be piled up in the grain boundaries and form serratedboundaries, as show in Fig. 1b. When the strain is increased to0.50, the contributions of the two effects are equal. It means thatpiled-up dislocations can be rearranged and new grains are formed.Moreover, the calculated value (εC = 0.50) is in good agreementwith the measure value (0.44) that DRX initiates, as shown inFig. 1c. With the increasing strain, the annihilation effect is moredominant. It means that dislocations prone to be rearranged inthe grain boundaries and DRX finally completes, as shown inFig. 1d. The above analysis shows that the process of DRX inducedby electropulsing can be evaluated by the interplay between theaccumulation and annihilation effects.

4. Conclusions

DRX can be accelerated dramatically under ET due to the twoeffects of electropulsing: the accumulation effect induced by theelectron wind force and the annihilation effect induced by the cou-pling of the thermal and electromigration effects. The process ofDRX induced by electropulsing can be evaluated by the interplaybetween the accumulation and annihilation effects.

Acknowledgements

The work was supported by the National Natural Science Foun-dation of China (No. 50571048) and State Key Lab of AdvancedMetals and Materials, University of Science and Technology Beijing,China.

3 d Engi

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252 Q. Xu et al. / Materials Science an

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