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Materials Science and Engineering A 527 (2010) 5826–5830

Contents lists available at ScienceDirect

Materials Science and Engineering A

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

ffect of stress on the annealing behavior of severely plastically deformedluminum alloy 3103

. Bhaumik ∗, X. Molodova, G. Gottsteinnstitute of Physical Metallurgy and Metal Physics, RWTH Aachen University, Kopernikusstrasse 14, D-52056 Aachen, Germany

r t i c l e i n f o

rticle history:eceived 29 January 2010eceived in revised form 18 May 2010

a b s t r a c t

The influence of a mechanical stress during annealing on the softening behavior of an ECAP deformedand a cold rolled aluminum alloy 3103 was investigated. An analysis of the work hardening behavior andthe microstructural evolution during annealing proved that recrystallization is substantially retarded by

ccepted 19 May 2010

eywords:ecoveryecrystallization

the applied stress. This can be attributed to significantly accelerated recovery processes.© 2010 Elsevier B.V. All rights reserved.

nnealinghermo-mechanical treatment

. Introduction

Microstructural restoration processes, like recovery and in par-icular recrystallization, have significant impact on the mechanicalroperties of a material. Therefore, these softening processes are ofreat scientific and technological interest in particular with regardo thermo-mechanical processing. Recovery comprises the rear-angement and annihilation of deformation induced dislocations. Iteduces the driving force for recrystallization but concurrently it isprecursor of recrystallization since it aids the nucleation of recrys-

allized grains [1]. Recrystallization proceeds by the nucleation androwth of strain free grains. The progress of these restoration pro-esses depends on time and temperature. But in addition, externalmpacts, e.g. mechanical stresses [2–6] or magnetic fields [7], haveeen shown to affect their kinetics and the microstructural evolu-ion. Fifty years ago, it was already shown that mechanical stressesan influence the recrystallization behavior of cold deformed pureluminum [2,3]. The results indicated that recovery processes wereromoted by an external stress and could retard the onset of recrys-allization. Recently, Winning and Schäfer found that even smalllastic shear stresses could decelerate or even inhibit recrystalliza-ion in a cold rolled aluminum alloy [8]. The experimental results

ere interpreted as stress induced decelerated subgrain growth

hat would cause a retardation of recrystallization. Furthermore,he observed complete suppression of recrystallization at hightresses was attributed to the occurrence of dynamic recovery,

∗ Corresponding author. Tel.: +49 241 8020257; fax: +49 241 8022301.E-mail address: bhaumik@imm.rwth-aachen.de (S. Bhaumik).

921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2010.05.053

which was considered to strongly reduce the dislocation densityand hence to eliminate the driving force for recrystallization.

The aim of the current study was an investigation into theinfluence of a constant mechanical stress during annealing on thesoftening processes recovery and recrystallization. For this, spec-imens of a commercial aluminum alloy AA3103 were severelydeformed by equal channel angular pressing (ECAP) or cold rollingand subsequently annealed under the influence of an appliedmechanical stress in both the elastic and the elastic–plastic regime.

2. Experimental

A commercial aluminum alloy AA3103 with the chemical com-position listed in Table 1 was used in this study. The material wasdeformed by cold rolling to a thickness reduction of 71% or byECAP deformation. The ECAP experiments were carried out at roomtemperature (RT) on 10 mm × 10 mm × 60 mm samples up to twopasses with a 90◦ die angle using route Bc [9,10]. From these heavilydeformed materials tensile samples were fabricated with a thick-ness of d = 1.2 mm for cold rolled and d = 1.0 mm for ECAP deformedmaterial by electric discharge machining (Fig. 1). The width of thesamples was 4 mm. The tensile direction of the sample was parallelto either the rolling direction (RD) or the extrusion direction (Fig. 1).In the following we will mainly focus on the cold rolled samples. Alltests were carried out on an electro-mechanical testing machine,

which was equipped with a 2 kN load cell, a strain gage and heatedrods which conveyed the heat to the specimens.

In order to determine the extent of the elastic regime the tensilespecimens of the initial cold rolled material were firstly annealedat 285 ◦C at a clamping load of 5 N for 8 min and 5 h, the later-

S. Bhaumik et al. / Materials Science and Engineering A 527 (2010) 5826–5830 5827

Fig. 1. Geometry of the used tensile samples.

Table 1Chemical composition (in wt.%) of aluminum alloy AA3103.

oSsltstapTtdotl

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Element Si Mn Fe Ti

Content 0.065 1.03 0.54 0.006

n applied shortest and the longest annealing times, respectively.ubsequently, tensile tests were conducted at 285 ◦C and a con-tant strain rate of 5 × 10−4 s−1 (Fig. 2). As obvious from Fig. 2,oads of 20 N ↔ 4.2 MPa and 60 N ↔ 12.5 MPa were deep in the elas-ic regime. Then, the specimens were annealed under a constanttress of 4.2 MPa and 12.5 MPa for different annealing times in aemperature range between 255 ◦C and 290 ◦C for cold rolled andt a temperature of 320 ◦C for ECAP deformed material. A timeeriod of maximum 8 min was required to heat up the samples.he final temperature was held constant within ±1 K. Both heat-up

ime and annealing temperature were determined to be indepen-ent of the applied stress by means of a thermocouple in the centerf the sample. Reference specimens were similarly treated usinghe same equipment and parameters but with the lowest possibleoad of 5 N ↔ 1 MPa. This minimum stress was essential to assure

ig. 2. Flow curves of the initially cold rolled material after annealing at 285 ◦C formin (black) and 5 h (grey).

Fig. 3. Flow curves at RT of the initially cold rolled material (thick grey line) andafter annealing at 255 ◦C for 8 min (dashed line), 30 min (dotted line) and 5 h (solidline) depending on the applied stress of 1 MPa (black) or 12.5 MPa (grey).

the contact and therefore heat transmission from the heating rodsto the sample. After annealing for the desired time all samples wereremoved from the mechanical testing machine and subsequentlywater quenched.

To investigate the influence of an additional mechanical stressduring annealing, tensile tests at RT were carried out at a con-stant strain rate of 5 × 10−4 s−1 subsequent to annealing (Fig. 2). Inaddition, the microstructure after the heat treatment was charac-terized by orientation microscopy (electron back scatter diffraction(EBSD)) in a field emission gun-scanning electron microscope (FEG-SEM). These measurements were performed on the sheet plane, i.e.perpendicular to the normal direction (ND) of the cold rolled sheetand on the plane perpendicular to TD for ECAP deformed mate-rial, as shown in Fig. 1. After wet grinding with SiC papers of 1200,2400 and 4000 grit the samples were additionally mechanicallypolished with 6 �m, 3 �m and 1 �m diamond paste, colloidal silicasuspension and finally electrolytically polished in a solution of 20%HClO4 + 80% C2H5OH at a temperature of about −10 ◦C.

3. Results

To shed light on the governing mechanisms during annealingwith and without load, the evolution of microstructure as well asthe work hardening behavior was recorded by using orientationmicroscopy and tensile tests at room temperature. Fig. 3 showsthe stress (1 MPa and 12.5 MPa) and time dependent flow curves ofthe initially cold rolled material at room temperature. Any anneal-ing treatment lowered the flow stress in comparison to the as-coldrolled material (thick grey line). However, the conspicuous resultwas that the flow curves after annealing at 12.5 MPa (grey) weresignificantly lower than the flow curves after annealing with theminimal stress of 1 MPa (black). Annealing treatments at a temper-ature of 270 ◦C yielded similar results.

The obtained flow curves reveal the dependency of the yieldstress �y on annealing time and applied stress. The yield stress�y was determined by forward extrapolation of the elastic regimeand back extrapolation of the incipient plastic regime. The yieldstress was plotted over time after annealing at 255 ◦C (filled sym-bols) and 270 ◦C (open symbols) under mechanical stresses of 1 MPa(black) and 12.5 MPa (grey) (Fig. 4). Fig. 4 demonstrates that theyield stress decreased with increasing annealing time, irrespectiveof temperature and applied stress. However, not only an increased

temperature but also a higher applied stress caused a stronger yieldstress decrease, i.e. a more pronounced softening.

The respective microstructure evolution was investigated byorientation microscopy. It is stressed that these investigations weresolely used in order to determine the onset of recrystallization and

5828 S. Bhaumik et al. / Materials Science and Engineering A 527 (2010) 5826–5830

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ig. 4. Experimental stress/time evolution of the initially cold rolled material afternnealing at 255 ◦C (filled symbols) and 270 ◦C (open symbols) under the influencef an additional stress of 1 MPa (black) and 12.5 MPa (grey).

ot to elucidate the mechanisms underlying recovery. The obtainedicrostructures are shown in Figs. 5 and 6 for the cold rolled mate-

ial and in Fig. 7 for the ECAP deformed material. In Fig. 5, theicrostructure after cold rolling and annealing at 270 ◦C and 2 h

nder the influence of an applied stress of 12.5 MPa is shown. Nondication of recrystallization is seen in the fine grained structuren Fig. 5. Hence, the respective decrease of the yield stress must bettributed to recovery only.

In Fig. 6, the microstructures after annealing at 290 ◦C for 1 hnder different stresses (1 MPa, 4.2 MPa and 12.5 MPa) are shown.vidently, the fraction of coarse (recrystallized) grains embeddedn a fine grained structure decreased with increasing mechan-cal stress. This is even more obvious for the ECAP deformedample, where the fraction of the coarse grained (recrystallized)icrostructure decreased with increasing stress during annealing

Fig. 7).

. Discussion

.1. Origin of stress effect on annealing

As evident from Fig. 4, the yield stress decreases with growingnnealing time, regardless of whether the annealing is carried outith or without applied stress. This behavior has to be attributed

o the occurrence of softening processes, recovery and/or recrys-allization, during annealing, which is common to aluminum and

ts alloys. With respect to the effect of a concurrent stress dur-ng annealing on the subsequent room temperature flow curve,ig. 3 substantiates that the flow stress decreases significantly moretrongly the higher the applied stress. For instance, the specimen

ig. 6. Microstructure of the initially cold rolled material after annealing at 290 ◦C for 1 hc).

Fig. 5. Microstructure of the initially cold rolled material after annealing at 270 ◦Cfor 2 h under the influence of an additional stress of 12.5 MPa.

annealed at a high stress of 12.5 MPa revealed a substantially loweryield stress than the specimen subjected to the minimal stress of1 MPa during annealing at both temperatures 255 ◦C and 270 ◦C(Fig. 4). This softening of the material has to be attributed to recov-ery processes, since no indication of coarse recrystallized grains isapparent in the fine grained microstructure after annealing for 2 hat a temperature up to 270 ◦C (Fig. 5). By contrast, the microstruc-ture after annealing at 290 ◦C with an applied stress of 1 MPa, asshown in Fig. 6(a), comprised coarse grains embedded in a finegrained structure. These coarse grains give evidence of the onset ofprimary recrystallization. Additionally, it was determined by subse-quent microstructure investigations whether and how an increaseof stress influences primary recrystallization during such annealingtreatment at 290 ◦C (Fig. 6(b) and (c)). After annealing at a stressof 12.5 MPa the microstructure revealed a substantially smallernumber of recrystallized grains than after annealing at 1 MPa. Evi-dently, primary recrystallization was retarded during annealingunder stress. Since even in the absence of recrystallization the yieldstress decreased faster with rising stress during annealing (Fig. 4),we have to conclude that an applied mechanical stress – in theelastic regime – promotes recovery.

These observations confirm results of Auld et al. and Thorntonand Cahn [2,3]. They investigated the influence of a small addi-tional stress on the softening behavior of materials with high andlow stacking fault energy. In pure aluminum a marked retardationof recrystallization was found with increasing stress and explained

ing force for primary recrystallization, which eventually delays theonset of recrystallization. The authors proposed that the appliedstress promoted forward recovery by accelerating climb and glideof dislocations. Corresponding investigations on copper – which is

under the influence of an additional stress of 1 MPa (a), 4.2 MPa (b) and 12.5 MPa

S. Bhaumik et al. / Materials Science and Engineering A 527 (2010) 5826–5830 5829

F te Bc) after annealing at 320 ◦C under the influence of an additional stress of 1.25 MPa (a),5

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ig. 7. Microstructure of initially ECAP deformed material (two ECAP passes in RouMPa (b) and 15 MPa (c).

nown to undergo only marginal recovery – demonstrated a negli-ible influence of an applied stress on recrystallization [3]. This, inurn, substantiated the occurrence of stress–recovery in aluminum.

An analogous behavior of the investigated aluminum alloy waslso observed after ECAP deformation [9]. Fig. 7 demonstrates thatlready a small stress in the elastic regime strongly affects theecrystallization kinetics. Recrystallization became significantlyetarded with increasing stress and was almost completely sup-ressed during annealing at 320 ◦C under a stress of 15 MPa.nnealing treatments of the same material with growing number ofCAP passes were also conducted and yielded similar results [9]. Its emphasized that this observation is important for the thermaltability of severely plastically deformed materials with submi-ron grain size. The highly deformed structure and the fine grainize dramatically increase the propensity of the material for recrys-allization during annealing which would seriously deteriorate itseneficial mechanical properties. The control of the thermal sta-ility is, therefore, a sensitive issue for highly deformed materials.pplication of a mechanical stress during annealing is a promis-

ng measure to improve the thermal stability of ultra fine grainedaterial with high stacking fault energy like aluminum and its

lloys.

.2. Recovery mechanisms

To investigate the recovery behavior, the change of yield stressith annealing time was measured. The recovery of the flow stressith time after isothermal annealing can generally be expressed by

he relation [11–13]:

= �0 − ˛ · ln(1 + ˇt) (1)

here � and �0 are the current and initial (t = 0) stress values, t ishe time, and ˛ and ˇ are recovery constants. Fig. 8 exemplarilyhows the experimentally measured stress evolution for an annealt 255 ◦C and a fit according to Eq. (1). Apparently, the experimen-al data can be well fitted by the logarithmic dependency given byq. (1). The ˇ value in Eq. (1) represents the recovery rate. From thetted stress/time evolution with a loading state of 1 MPa in Fig. 8, aecovery rate ˇ of 0.0178 s−1 was deduced, whereas ˇ = 0.1057 s−1

esulted from the fitted curve with a loading state of 12.5 MPa.ence, the recovery rate ˇ increased substantially with increasing

tress. Furthermore, the value of ˇ enables to evaluate the activa-ion energy for the underlying process by [11]:

= � · exp(

− G

kT

)(2)

here � is a frequency, G is the activation free energy, k is the Boltz-ann constant, and T is the absolute temperature. To determine the

ctivation energy, investigations covering more temperatures haveo be conducted, which is work in progress.

Fig. 8. Experimental (symbols) and fitted (line) stress/time evolution of initially coldrolled material after annealing at 255 ◦C under the influence of an additional stressof 1 MPa (black) and 12.5 MPa (grey).

Although the obtained results do not lend themselves to a quan-titative assessment of the underlying physical mechanisms thataffect the recovery kinetics, the qualitative behavior allows to drawsome general conclusions. The fact that recovery proceeds fasterunder load than without applied stress, as evident from Fig. 2,proves that the applied external stress promotes the recoverymechanisms. From the high temperature creep behavior of alu-minum Eisenlohr and Blum [14] concluded that annihilation ofdislocation dipoles by climb controls the recovery kinetics. In fact,if one assumes that the climb kinetics are affected by both the inter-nal and the applied stress, a dislocation based recovery model cansuccessfully account for the effect of an external stress on recovery[15].

Furthermore, the current investigation proves that such effectsoccur independent of the particular strain path during deformation,i.e. irrespective of the preceding forming process as substantiated inthis study for rolled and ECAP processed material. But the effect ofeither a compressive or a tensile stress during annealing was not yetinvestigated. This aspect should be subject of future investigations.

5. Conclusions

The current study proved that the softening of deformed AA3103 by recovery and recrystallization can be significantly influ-enced by application of an additional mechanical stress during

annealing. Therefore, the deformed microstructure can be mademore stable against recrystallization during annealing by adjust-ment of an external mechanical stress as demonstrated for ECAPdeformed and cold rolled Al-alloy. A superior thermal stability is, inturn, an essential prerequisite for potential applications at elevated

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[11] D. Kuhlmann, G. Masing, J. Raffelsieper, Z. Metallkunde 40 (1949) 241–246.[12] A.H. Cottrell, V. Aytekin, J. Inst. Met. 77 (1950) 389–422.

830 S. Bhaumik et al. / Materials Science

emperatures of ultra fine grained materials produced by severelastic deformation.

cknowledgements

The authors would like to thank the Deutsche Forschungs-emeinschaft for financial support through the research unitMechanical properties and interfaces in ultra-fine grained mate-ials” (TP 4-Go 335/31-1(2)). The authors express their gratitude toydro Aluminum Deutschland AG for supplying the material.

eferences

[1] G. Gottstein, in: J. Hirsch (Ed.), Virtual Fabrication of Aluminum Products,Wiley-VCH, Weinheim, 2006, pp. 157–175.

[[[

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13] E. Nes, Acta Metall. Mater. 43 (1995) 2189.14] P. Eisenlohr, W. Blum, Mater. Sci. Eng. A 400–401 (2005) 175–181.15] S. Bhaumik, V. Mohles, G. Gottstein Proceedings of ICAA 12 (submitted for

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