Scripta Materialia 55 (2006) 995–998

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Magnetically enhanced recrystallization in an aluminum alloy

S. Bhaumik, X. Molodova, D.A. Molodov* and G. Gottstein

Institute of Physical Metallurgy and Metal Physics, RWTH Aachen University, D-52056 Aachen, Germany

Received 21 July 2006; revised 8 August 2006; accepted 9 August 2006Available online 8 September 2006

The recrystallization behaviour of cold rolled (71%) aluminum alloy 3103 was investigated by measuring the crystallographictexture and grain microstructure during heat treatment at 288, 310 and 330 �C in a magnetic field of 17 T. The results demonstratethat the recrystallization kinetics is substantially accelerated by the application of a magnetic field.� 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Recrystallization; Magnetic annealing; Texture

In a previous paper [1] we reported preliminary re-sults of experiments with 71% cold rolled sheet of alumi-num alloy AA3103 annealed in a high magnetic field.These results revealed a higher intensity of the typicaldeformation texture components after annealing in themagnetic field than after annealing without field. Sincean increase of the orientation density of deformationtexture components during annealing is typical forrecovery of the deformed material, the results of thatstudy demonstrated that annealing in a magnetic fieldpromotes recovery. Moreover, an analysis of the texturedata indicated that annealing in the magnetic fieldcaused an earlier start of subsequent primary recrystalli-zation. The current paper reports further investigationsinto magnetically affected recrystallization behaviourof cold rolled commercial aluminum alloy.

Specimens of an AA 3103 aluminum alloy sheet,similar to those used in our previous experiment [1], wereannealed in a salt bath at 288, 310 and 330 �C for differ-ent annealing times at zero field and in a high magneticfield of 17 T. The specimens were placed in a salt bathat constant magnetic field. After exposure for the desiredannealing time the samples were removed from the saltbath and water quenched. Reference specimens weretreated at the same annealing temperatures and timesat zero field. The experiments were carried out using adirect current resistive, 20 T Bitter magnet with a195 mm bore diameter at the National High MagneticField Laboratory in Tallahassee, Florida, USA.

1359-6462/$ - see front matter � 2006 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2006.08.018

* Corresponding author. Tel.: +49 2418026873; fax: +492418022301; e-mail: molodov@imm.rwth-aachen.de

The evolution of crystallographic texture and micro-structure after the heat treatment was characterized byX-ray diffraction and orientation microscopy (electronback scatter diffraction (EBSD) in a scanning electronmicroscope). The crystallographic texture was deter-mined in terms of the orientation distribution function(ODF) from a set of four incomplete pole figuresmeasured with Co K radiation by means of an auto-mated X-ray texture goniometer in back reflectionmode. Details concerning the sample preparation andmacrotexture measurements were given elsewhere [1].The ODFs were calculated using the series expansionmethod with positivity correction [2–4]. The samplesymmetry was assumed to be orthorhombic.

Individual orientation measurements by EBSD wereperformed on the plane perpendicular to the transversaldirection (TD) of the sheet. After wet grinding with SiCpapers of 1200, 2400 and 4000 grit the samples for thesemeasurements were also mechanically polished succes-sively with 6, 3 and 1 lm diamond paste, colloidal silicasuspension and then electrolytically polished in a solu-tion of 20% HClO4 + 80% C2H5OH at a temperatureof about �10 �C.

Figure 1 depicts the texture of the investigated alumi-num alloy sheet after annealing at 288 �C for 100 minwithout field and in a magnetic field of 17 T. The textureafter conventional (zero field) annealing (Fig. 1a) is verysimilar to the texture of the as-received (71% cold rolled)material, given elsewhere [1], and characterized by ahigh intensity of orientations that compose the b-fibre:{112} h111i (Cu–), {12 3} h634i (S–) and {011}h211i (Brass-component). Magnetic annealing at 17 Tdramatically changed the texture (Fig. 1b). The

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ϕ1

Levels (max 12.4):1.2 2.0 4.0 7.0 12.0

ϕ2 = const

ΦBrass

S

Cu

Levels (max 12.4):1.2 2.0 4.0 7.0 12.0

Brass

S

Cu

ϕ1 ϕ2 = const

Φ

Cube

Cube

Levels (max 9.1):1.2 2.0 4.0 7.0

Cube

Cube

Levels (max 9.1):1.2 2.0 4.0 7.0

a b

Figure 1. Texture of 71% cold rolled aluminum alloy (AA3103) sheetannealed 100 min at 288 �C (a) without field and (b) in a magnetic fieldof 17 T.

ϕ1

Levels (max 8.2):1.2 2.0 4.0 7.0

ϕ2 = const

Φ

ϕ1

Levels (max 8.2):1.2 2.0 4.0 7.0

ϕ2 = const ϕ1 ϕ2 = const

Φ

Levels (max 12.4):1.2 2.0 4.0 7.0 12.0

ϕ1 ϕ2 = const

Levels (max 12.4):1.2 2.0 4.0 7.0 12.0

ba

Figure 3. Texture of 71% cold rolled aluminum alloy (AA3103) sheetannealed for 1 min at 330 �C (a) without field and (b) in a magneticfield of 17 T.

Figure 4. Volume fraction of deformation components vs. annealingtime at 288 �C, 310 �C and 330 �C. Solid symbols indicate annealingsin a magnetic field of 17 T, open symbols at zero field.

996 S. Bhaumik et al. / Scripta Materialia 55 (2006) 995–998

deformation components were much weaker than afterannealing without field. Concurrently, a high intensityCube-component ({100} h0 01i) developed (Fig. 1b).A similar behaviour, i.e. a degradation of the deforma-tion texture components and an enhancement of theCube-component in a magnetic field can be also seenin the texture after 5 min annealing at 310 �C (Fig. 2)and after 1 min at 330 �C (Fig. 3).

From the ODFs the volume fractions of the majororientations can be calculated [2–4]. Figure 4 showsthe development of the b-fibre with annealing time atdifferent temperatures. Subsequent to an initial slightincrease, the volume fraction of b-fibre orientationsdecreased distinctly earlier and faster during annealingin a field than without field for all three investigatedtemperatures. The unexpected increase of the b-fibre ori-entations after 60 min of magnetic annealing at 288 �Cand after 30 min of conventional annealing at 310 �Cis presumably due to the formation of the so calledR-component [5,6]. Since the R-orientation {124}h211i is close to the S-component component of thedeformation texture, the total b-fibre volume fraction,calculated from the ODF intensity of the Cu–, S– and

ϕ1

Levels (max 10.4):1.2 2.0 4.0 7.0

ϕ2 = const

Φ

Levels (max 10.4):1.2 2.0 4.0 7.0

ϕ1 ϕ2 = const

Φ

Levels (max 8.9):1.2 2.0 4.0 7.0

Levels (max 8.9):1.2 2.0 4.0 7.0

ba

Figure 2. Texture of 71% cold rolled aluminum alloy (AA3103) sheetannealed for 5 min at 310 �C (a) without field and (b) in a magneticfield of 17 T.

Brass-orientations, is increased. It is worth noting thatthe R-component is characteristic for annealing texturesof some aluminum alloys and generally attributed, aswell as the Cube-component, to recrystallization of thedeformed material.

A degradation of the deformation texture compo-nents is a typical sign of the progress of recrystallizationin deformed Al-alloy. Correspondingly, Figure 4 sug-gests that recrystallization is promoted by annealing ina magnetic field. The same conclusion can be drawnfrom the dependence of the calculated volume fractionof the Cube-component on the annealing time. Thisfraction rises much faster during annealing in a mag-netic field of 17 T, than at zero field. The results of theX-ray texture analysis are fully confirmed by investiga-tions of the grain microstructures after conventionaland magnetic annealing. For example, Figure 5 showsthe grain structure after 100 min annealing at 288 �Creconstructed from individual orientation data obtainedby EBSD. The microstructure after annealing out offield is composed of a few recrystallized grains sur-rounded by a deformed/recovered structure, whereasheat treatment in the magnetic field results in an almostcompletely recrystallized structure with a few partlyrecovered areas.

1.6 1.65 1.7 1.75 1.8 1.85

1/T [103/K]

101

102

103

104

recr

ysta

lliza

tion

time

t R [s

]

330 310 290

T [ºC]

Figure 7. Recrystallization time tR (for recrystallized volume X(tR) =0.63) during the annealing at different temperatures at zero field (opensymbols) and in a magnetic field of 17 T (solid symbols).

Figure 5. Microstructure after 100 min annealing at 288 �C (a) without field and (b) in a magnetic field of 17 T.

S. Bhaumik et al. / Scripta Materialia 55 (2006) 995–998 997

The change of the b-fibre volume fraction during theannealing shown in Figure 4 can be utilized for an esti-mation of the recrystallization kinetics. Assuming thehighest value of the b-fibre volume fraction in Figure 4to reflect the non-recrystallized state of the investigatedmaterial and the lowest measured value of b-fibrevolume fraction to correspond to completely finishedrecrystallization, the recrystallized volume fraction canbe calculated for all annealing times. The temporalchange of the recrystallized volume fraction obtainedin such a manner for all three annealing temperaturesis presented in Figure 6. Apparently, the incubation timeof recrystallization during magnetic annealing is sub-stantially decreased compared to conventional anneal-ing. This can be attributed to magnetically enhancedrecovery in the deformed aluminum alloy as observedand discussed in previous work [1]. It is further noticedthat the recrystallization kinetics are significantly accel-erated by a magnetic field. This is illustrated in Figure 7,where the characteristic time for recrystallization,tR, corresponding to a recrystallized volume fractionX(tR) = 0.63, is plotted versus the annealing tempera-ture. As can be seen, recrystallization in a magnetic fieldof 17 T takes distinctly less time than without field in theentire investigated temperature range.

According to the classical Johnson–Mehl–Avrami–Kolmogorov model of recrystallization [7–9] for homo-geneous nucleation at a constant rate and isotropic

Figure 6. Recrystallized volume fraction of investigated 71% coldrolled AA3103 vs. time of annealing at zero field and in a magneticfield of 17 T. Solid symbols indicate annealings in a magnetic field of17 T.

nucleus growth, the recrystallization time can be writtenas

tR ¼p3

_Nv3� ��1=4

¼ 3

p _N0v30

!1=4

� expQ _N þ 3Qv

4kT

� �ð1Þ

where _N and v are rates of nucleation and nucleusgrowth, Q _N and Qv are the activation enthalpies ofnucleation and nucleus growth, and _N0 and v0 arethe respective preexponential factors. According to Eq.(1) the slope of curves in Figure 7 determines theapparent activation enthalpy of recrystallization QR ¼ðQ _N þ 3QvÞ=4. The value of about 3.5 eV for the activa-tion enthalpy of recrystallization is abnormally high andobviously has little physical meaning. However, as istypical for aluminum alloys, besides nucleation andgrowth of new grains recovery also takes place concur-rently during annealing and thus, may disguise theactual activation enthalpy.

Nevertheless the observed decrease of the recrystalli-zation time in a magnetic field gives an opportunity todiscuss the magnetic effect on recrystallization kinetics.It is unlikely that the nucleation rate can be affectedby an external magnetic field, since in particle containingAl-alloys nucleation is site saturated, i.e. it occurs at

998 S. Bhaumik et al. / Scripta Materialia 55 (2006) 995–998

particles at the very beginning of recrystallization.Furthermore if a magnetic field were to affect the acti-vation enthalpy of nucleation, this would result in a dif-ferent recrystallized grain size after magnetic andconventional annealing, but the mean grain size aftermagnetic annealing in such a case would be smaller.From the time dependence of the b-fibre volume fractionin the annealed specimens (Fig. 4) the conclusion can bedrawn that recrystallization is complete at 330 �C after1 min annealing in the field and 20 min without field.Individual grain orientation measurements revealedthat the mean grain size in both cases was equal andamounted to 7 lm.

Therefore, we suggest that the reduction of therecrystallization time in a field can be attributed to mag-netically accelerated growth kinetics. This means thatthe mobility of the boundaries in a deformed matrix isincreased in the presence of a magnetic field. Up tonow there has been no comparative study of grainboundary mobility in the presence of a magnetic fieldand without field. It is known, however, that a magneticfield can induce the motion of dislocations without anyexternal mechanical stress [10,11]. Such a magneticcause of dislocation motion was attributed to spin-dependent interactions between dislocations and para-magnetic defects in the crystal structure [12,13] that leadto the release of dislocations from their pinning centers.The enhanced dislocation motion can accelerate recov-ery of a deformed structure as observed in our previousstudy [1]. The grain boundary structure can be repre-sented by dislocation arrangements. Therefore, it isreasonable to assume that also the mobility of grainboundaries can be affected by a magnetic field. The bestway to prove this hypothesis would be to study the mag-netic effect on the kinetics of the curvature driven graingrowth. This is, however, not possible in the investigatedmaterial in the temperature range used in the currentexperiment. It is known that in aluminum alloys con-taining about 1% Mn capillary driven grain coarseningsubsequent to primary recrystallization is suppresseddue to the pinning of grain boundaries by precipitates[14]. This is also confirmed in the current study – themean grain size attained at the end of primary recrystal-lization does not change during further annealing.

The measurement of grain growth kinetics in theinvestigated material, however, can be carried out at ele-vated temperatures, over 640 �C, when the alloy has asingle phase structure. Such experiments to study themagnetic effect on grain growth in aluminum alloyAA3103 are in progress.

In summary, for the first time it has been experimen-tally demonstrated by means of texture and microstruc-

ture analysis that the application of a magnetic fieldsubstantially enhances recrystallization in cold rolledcommercial aluminum alloy. The incubation time forrecrystallization was decreased and recrystallizationkinetics were significantly accelerated. The observedphenomena are obviously due to a magnetically en-hanced mobility of dislocations and grain boundariesthat may promote recovery and nucleus growth kineticsduring recrystallization, respectively.

The authors express their gratitude to the DeutscheForschungsgemeinschaft for financial support (GrantMO 848/6-1) and Hydro Aluminum Deutschland AGfor supplying the material. Part of this work wasperformed at the National High Magnetic Field Labo-ratory, which is supported by NSF Cooperative Agree-ment No. DMR-0084173, by the State of Florida andby the DOE. The help of Dr. Bruce Brandt and his team(DC Field Facility of NHMFL), Peter Konijnenbergand Bobby Joe Pullum in performing magnetic annea-lings is gratefully acknowledged.

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