Hot working of AA1050—relating the microstructural and textural developments

Acta mater. 49 (2001) 1759–1769www.elsevier.com/locate/actamat

HOT WORKING OF AA1050—RELATING THEMICROSTRUCTURAL AND TEXTURAL DEVELOPMENTS

I. SAMAJDAR†‡, P. RATCHEV, B. VERLINDEN and E. AERNOUDTDepartment MTM, Katholieke Universiteit Leuven, de Croylaan 2, 3001 Heverlee, Belgium

( Received 9 November 2000; received in revised form 5 February 2001; accepted 11 February 2001 )

Abstract—An aluminum alloy AA1050 was deformed in plain strain at different hot working conditions.An increase in temperature or a decrease in strain rate reduced the relative drop in cube {001}�100� and therelative increase in rolling texture components of Cu {112}�111� and S {231}�346�, especially apparent atthe higher strain. Along with such textural changes, significant differences in hot worked microstructureswere observed. The two distinct microstructural features, as observed by polarized light optical microscopy,were grain boundary serrations (GBS) and in-grain inclined lines (IIL), typically observed at an approximateangle of 35° with rolling direction (RD). At higher temperatures and lower strain rates, and correspondinglylower Zener–Holloman factors (Z�109�1010 s�1), coarse but nearly equiaxed grain interior substructuresand GBS were observed. Interestingly, orientation imaging microscopy (OIM) clearly showedinsignificant/non-noticeable differences between the substructures of different orientation components. Anincrease in Z aligned the grain-interior low angle boundaries at an angle of approximately 35° with RD andat higher Z (Z�1012�1013 s�1) the main microstructural feature was the IILs. Development of in-grain longrange misorientation (LRM) was estimated to be the mechanism behind the optical visibility of the IILs. Theappearance of IILs had two apparent effects—first the substructures of different orientation components weredifferent, and secondly the stability of cube grains dropped noticeably. Generalizing the IILs or 35° inclinedcell walls as plastic instabilities or strain localizations, the observed differences in their relative appearanceat different deformation conditions and/or texture components could be explained. When formation of suchstrain localizations are considered as “necessary” for the reorientation of grain segment(s), the cube stabilityat low Z deformation could also be understood. 2001 Acta Materialia Inc. Published by Elsevier ScienceLtd. All rights reserved.

Keywords: Hot deformation; Texture; Aluminum; Microstructure

1. BACKGROUND

One of the first, but often most critical, steps in theindustrial production of aluminum alloys, is the hotworking. For prediction and control of final mechan-ical properties and for process optimization, under-standing the developments in microstructure and tex-ture as a function of hot working parameters (e.g.strain, strain rate and temperature) is of critical impor-tance [1–3]. Along with such obvious technologicalimplications, hot working of aluminum is also anissue of intense academic interest, as the hot workedmicrostructure often contains the “seed” of the sub-sequent cube recrystallization texture [3–11]. Pre-vious studies have shown that the cube recrystalliz-ation texture depends on two factors: the presence of

† On leave from the Department of Metallurgy, Engineer-ing and Material Science, IIT Bombay, Powai, Mumbai-400076, India.

‡ To whom all correspondence should be addressed. Fax:+91-22-578-3480.

E-mail address: [email protected] (I. Samajdar)

1359-6454/01/$20.00 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.PII: S13 59-6454( 01 )0 0083-0

deformed cube bands and their ability to form recrys-tallized cube grains [4–14]. The factors responsiblefor the preferred nucleation and/or selective/microgrowth advantage of cube grains, has been a subjectof intense scientific debate [4, 7–9, 12–14]. The rela-tive stability, or metastability, of a cube grain underhot working conditions is perhaps an even more baf-fling question [4, 15, 16]. Recent studies have indi-cated that part of each original cube grain may sur-vive a large deformation and even “rotate” (as thenumber fraction of deformed grains within 10° ofexact cube orientation was observed to increase withstrain) closer to the exact cube orientation [4, 6–8,11, 12, 15, 16]. An octahedral fcc slip system andusual Taylor simulations cannot explain such “metast-ability” of a cube [18–20], or in general, the develop-ment of deformation texture at elevated temperatures[21]. An alternative has been proposed through poss-ible activation of non-octahedral slip systems [16, 21,22]. On the other hand, studies on aluminum singlecrystals [16, 22] have shown that the relative stabilityof cube and possible activation of non-octahedral slip

1760 SAMAJDAR et al.: HOT WORKING OF AA1050

systems at higher temperatures (approximately above0.6 times the melting temperature) are alsoaccompanied by the absence of so-called “defor-mation banding”. This raises an interesting ques-tion—if and how the changes in bulk texture arerelated to changes in microstructure and microtextureat different hot working conditions (e.g. strain, strainrate and temperature). Seeking an answer to this ques-tion is the objective of the present study.

2. EXPERIMENTAL METHODS

In the present study, an aluminum alloy AA1050(a dc cast ingot, with 0.1 wt% Si and 0.31 wt% Fe,hot rolled using standard industrial practice) wasrecrystallized (450°C, 1 h) and then deformed in plainstrain compression (PSC) in a thermo-mechanicaltreatment simulator (from Servotest Ltd, with col-loidal graphite used as a lubricant). The hot workingconditions were selected to cover the usual industrialpractices and samples were quenched within 1 s afterthe deformation. The hot working conditions used inthe present study are given in Table 1.

For each of the deformation conditions, sampleswere obtained for measurement of X-ray texture, pol-arized light optical microscopy (PLOM) and orien-tation imaging microscopy (OIM). X-ray texture wasobtained by inversion of four incomplete pole figures,using the standard series expansion method [23] andthe MTM–FHM program [24]. For volume fractionmeasurements, X-ray orientation distribution func-tions (ODFs) were convoluted with suitable modelfunctions, with an integrated ODF value of 1 and16.5° Gaussian spread. For deformation texturesimulation, the initial non-deformed X-ray texturewas discretized to 3000 individual orientations andthen different Taylor simulations were obtained byMTM–Taylor and Lamel programs [24, 25].

Samples for PLOM and OIM were made on thelong transverse sections (i.e. planes containing therolling and normal directions, RD & ND) using stan-

Table 1. Hot working conditions used in the plain strain compression testsa

Sample code Strain Strain rate (s�1) Temperature (°C) Z values (s�1)

0.4-10-400 0.4 10 400 1.28×1013

0.4-1-350 0.4 1 350 1.2×1013

0.4-10-450 0.4 10 450 1.86×1012

0.4-1-400 0.4 1 400 1.28×1012

0.4-1-450 0.4 1 450 1.86×1011

0.4-1-500 0.4 1 500 3.48×1010

0.4-0.1-450 0.4 0.1 450 1.86×1010

0.4-0.1-500 0.4 0.1 500 3.48×109

1-10-400 1.0 10 400 1.28×1013

1-1-350 1.0 1 350 1.2×1013

1-1-400 1.0 1 400 1.28×1012

1-1-450 1.0 1 450 1.86×1011

1-1-500 1.0 1 500 3.48×1010

1-1-550 1.0 1 550 7.96×109

1-0.1-500 1.0 0.1 500 3.48×109

a Z values or Zener–Holloman factors were calculated using the expression Z = e exp(Q/RT), where e, Q, R and T are respectively the strainrate, activation energy, molar gas constant and temperature in absolute scale [26, 27]. A Q value of 156 kJ/mol was used [26].

dard techniques [4]. The entire deformed sampleswere studied by PLOM to confirm homogeneity ofthe PSC tests and at least two different regions ineach sample were photographically recorded. At leasttwo different regions of each sample were scannedby OIM—measuring a total minimum area of500 µm×500 µm. Typically a minimum of 3deformed grains from each class of texture compo-nents (i.e. within 20° of ideal texture components)was analyzed to obtain information on stored energyand long range misorientation (LRM). In case a grainwas within 20° of more than one ideal component, itwas considered as the component of least misorien-tation. OIM measurements and analysis wereobtained using the standard TSL (Tex-Scan Ltd.)package.

3. RESULTS

As mentioned in Table 1, true strains of 0.4 and1.0 were used, while PSC (plain strain compression)temperatures ranged between 350 and 550°C andstrain rates between 0.1 and 10 s�1. Hot working con-ditions (e.g. temperature and strain rate) are oftencombined, albeit empirically, in the Zener–Hollomanfactor, Z, where Z = e exp(Q/RT). e, Q, R and T arerespectively the strain rate, activation energy, molargas constant and temperature in absolute scale [26–28]. Using a Q value of 156 kJ/mol [27], a range ofZener–Holloman factors, between 109 and 1013 s�1 isobtained from the deformation conditions mentionedin Table 1. Results on developments in bulk textureand microstructure/microtexture for this range of Zvalues are presented separately.

3.1. Bulk texture

In the present study, mainly the following texturecomponents have been studied: Brass {011}�211�,S {231}�346�, Cu {112}�111�, cube {001}�100�,CG (RD rotated cube) {012}�100� and CH (NDrotated cube) {001}�210�. The other two usual fcc

1761SAMAJDAR et al.: HOT WORKING OF AA1050

components, Goss {011}�100� and H {001}�110�,were observed to be relatively weak (less than 6 and4 vol% at 16.5° Gaussian spread) and hence werenot considered separately. As shown in Fig. 1(c) and(d), i.e. between nearly two extreme Zener–Hollo-man factors at a strain e = 1, the hot deformationtextures were observed to be significantly different.Figure 2 plots the volume fractions of the major tex-ture components at strains of 0.4 [see Fig. 2(a)] and1.0 [see Fig. 2(b)], in the order of descending Z. Asseen in Fig. 2(b), an increase in strain, withoutchanging temperature and strain rate, reduced thecube fraction but increased the fraction of Cu andS. At the lower strain, i.e. 0.4, no clear trend wasapparent as a function of temperature or strain rateor combination of the two in the form of Z values.At the higher strain of 1, however, a distinct trend,albeit with some scatter, is noticed: with an increasein temperature or a decrease in strain rate or adecrease in Z values, cube increased but Cu and Sdropped. This trend, shown in Fig. 2(b), alsoinvolved small increase in CG, a small decrease ofbrass and almost no change in CH. Since thechanges in the CG, CH and brass components are

Fig. 1. j2 = 45°, 65° and 90° sections of the X-ray ODFs for(b) non deformed (i.e. hot rolled but recrystallized material),(c) sample 1-1-350 (Z = 1.2×1013 s�1) and (d) sample 1-0.1-500 (Z = 3.48×109 s�1). Contour levels were drawn at 2, 2.8,4, 5.6, 8, 11, 16, 22, 32 and 44 times random. In (a) the maintexture components of the respective ODF sections are marked.

relatively small, only the evolution of cube andCu/S components will be considered in this study.Such textural changes, i.e. increase in cube butdecrease in Cu and S with increase in temperatureor decrease in strain rate, are not unique and similartrends have been reported [8, 16, 29] in several earl-ier studies. However, the emphasis of the presentstudy is not to confirm possibilities of such a trendin the present material and deformation conditions,but to explore links, if any, between such texturalchanges and changes in microstructure andmicrotexture.

3.2. Microstructure and microtexture

Observations on the grain size and shape by PLOMbefore and after deformation and by less statisticalbut more accurate OIM measurements, failed to showany evidence of recrystallization—dynamic, static ormetadynamic. However, several distinct features ofthe deformed microstructure/microtexture wereobserved as a function of temperature and strain rateor Z values.

3.2.1. Polarized light optical microscopy. Polar-ized light optical microscopy (PLOM) showed twodistinct microstructural features. As shown in Fig.3(b), at lower Z values (and correspondingly highertemperature and/or lower strain rate) the main micro-structural feature was a somewhat serrated appear-ance of the grain boundaries. In the present study thisfeature will be termed as GBS or grain boundary ser-rations. Appearance of GBS at low Z deformation isa well established phenomenon [33]. However, atintermediate Z values along with GBS, in-graininclined lines at approximately 35° with RD wereobserved. These will be termed as IIL in the presentstudy. At the highest Z values, only IILs were seen—see Fig. 3(a). Table 2 lists the relative appearance ofGBS and IIL after different deformation conditions,used in the present study. As shown in Table 2, atboth strains, the appearance of GBS and IIL followsthe same trend:

� at low Z (109–1010 s�1) only GBS were observed.� at high Z (1012–1013 s�1) only IIL were observed.� there were a few samples of intermediate Z values

where both IIL and GBS were seen.

Another interesting feature, at least for the higherstrain of 1.0, is the general trend that the decrease incube or the increase in Cu + S, during plain straincompression was less with GBS but more with IIL,see Table 2. In order to understand the reason behindthe optical visibility of the IILs and orientation depen-dence, if any, of substructural features—detailed OIMstudies were carried out.

3.2.2. Substructural differences studied by OIM.OIM pictures at the low Z values typically

showed nearly equiaxed, but rather largesubgrains/cells, with serrated grain boundaries—see


Fig. 2. Volume fractions of major orientation components, as obtained by deconvoluting X-ray ODFs withrespective model functions of 16.5° Gaussian spread, after true strains of (a) 0.4 and (b) 1.0 and for differentdeformation conditions (i.e. strain and strain rates). It has to be noted that in both (a) and (b) deformationconditions are arranged in a descending order of Z, and in (b) generalized trends of volume fraction changes

are also indicated by dotted arrows.

Fig. 4. Figure 4(a) illustrates how cell/subgrainwalls often marked or accommodated such grainboundary serrations (GBS). Perhaps the most inter-esting microtextural feature at such low Z defor-mation was the observation that cell/subgrain sizeor misorientation among neighboring subgrains didnot depend, in general, on the crystallographicorientation or Taylor factor. For example: thedeformed cube and S grains in Fig. 4(a), respect-ively with relative high and low Taylor factors—seeFig. 4(b), had similar substructures.

With increasing Z values, the cell/subgrain wallswere increasingly aligned at approximately 35° withRD. Such effects were more vivid at the higher Zvalues, see Fig. 5, i.e. Z = 1012�1013 s�1. Appearanceof such aligned cell walls also accompanied:

� the disappearance of GBS or serrated grain bound-aries

� appearance of IILs or in-grain inclined linesunder PLOM

� refinement of the substructure� general orientation dependence of the substructure

In general, more frequent inclined cell walls wereobserved at grains/orientations of higher Taylor fac-tors—see the Cu/S grain of Fig. 5. This, on the otherhand, reflected in a generallized orientation depen-dence of the substructure at both strains of higherZ values. The ratio of q/d, cell misorientation/cellsize, is expected to scale with the stored energy,provided loose cell interior dislocations or statisti-cally stored dislocations are neglected [4, 7]. Sucha ratio can be, and is often [4, 7, 13, 30], used toillustrate the so-called orientation dependence ofstored energies. Measurements from the presentstudy, as shown in Fig. 6, present an interestingcomparison between two extreme orientations of


Fig. 3. PLOM (polarized light optical micrograph) of twoextreme Zener–Holloman factors (a) sample 0.4-1-350 (Z = 1.2×1013 s�1) and (b) sample 0.4-0.1-500 (Z = 3.48×109

s�1). In (b) GBS (grain boundary serrations) are the predomi-nant feature, while in (a) IILs (in-grain inclined lines) are also

observed.

cube and Cu/S†. At lower Z values, nonoticeable/significant differences in stored energiesexist between Cu/S and cube. At intermediate Zvalues such differences were noticeable, thoughsmall. At higher Z values, differences in stored ener-gies were significant. An apparent reason for suchdifferences in stored energies is the increasedappearance of 35° RD inclined cell walls in grainsof certain (in general of high Taylor factor) orien-tations.

3.2.3. Developments in long range misorientation(LRM). During deformation, a grain may reorientand/or get subdivided [15, 31, 32]. This may lead tothe development of long range misorientation ororientation changes inside a single deformed grain.In the present study, the development of long rangemisorientation (LRM) was measured by drawing linesalong RD inside a deformed grain and then consider-

† Although there is some expected [4, 13] differencebetween Cu and S, the magnitude of such difference is insig-nificant compared to differences between cube and Cu/S.Another reason for considering Cu and S together was theobservation that many deformed grains [e.g. Fig. 5 (a)] hadorientations that alternate between the two and bulk texturaldevelopments of Cu and S also follow the same trend.

ing gradual changes in misorientation along theselines with respect to the starting point or origin, seeFigs 4(c) and 5(c). Such point to origin “misorien-tation vs distance” plots, were obtained using the TSLpackage and averaging of LRM angles/distances wasobtained using recognizable/distinct peaks/cusps, asin Figs 4(c) and 5(c).

Figure 7 illustrates average values ofsignificant/noticeable misorientation changes, see Fig.7(a), and the average distance for such changes, seeFig. 7(b). Typically two types of LRMs wereobserved—repetititive, e.g. LRM in Cu/S grain inFig. 5(c) or LRMs in both cube and S grains in Fig.4(c), and cumulative, as in the cube grain of Fig. 5(c).

At lower Z (Z = 109�1010 s�1) LRMs wereobserved to be repetititive in almost all cases (onlyone grain, within 20° of exact brass, showed cumulat-ive LRM) and no significant difference betweenLRMs of cube and Cu/S grains were observed. Athigher Z (Z = 1012�1013 s�1) on the other hand, about75% of the cube grains showed cumulative LRM,while 65% of the Cu/S grains had repetititive LRM.In addition, LRM values in Cu/S were more signifi-cant, both in terms of increased misorientation angles(with respect to point of origin orientation) and higherfrequency (i.e. shorter spacing) of such high misorien-tations, than those seen in cube oriented regions, seeFig. 7. As will be discussed subsequently, develop-ment of greater than 5° LRMs may explain the opticalvisibility of the IILs.

4. DISCUSSION

In the present study deformation conditions weregeneralized by Zener–Holloman factors or Z values[26–28], although corresponding temperatures andstrain rates are also listed in Table 1. Such use of Zvalues is convenient and is a usual practice [16], asit combines two variables into one. Nevertheless theremight be problems in considering such values as anabsolute numerical index, especially between differ-ent studies. It not only has an empirical origin [26],but more importantly, the activation energy or Qvalues (considered as 156 kJ/mol in the present study)may change between different material and defor-mation conditions, which is not taken into account.The important issue of the present study is, however,to describe possible links between textural and micro-structural changes and changes in generalized defor-mation conditions, for which Z values are adequateas a relative index of comparison.

4.1. Main microstructural/microtextural features

The main microstructural features, at the levels ofPLOM, were the GBS and IIL, see Table 2 and Fig.3. As shown in Table 2, at e = 1, optical visibility ofGBS and greater cube stability appeared to be related.Deformation at low Z typically formed large equiaxedsubgrains, while grain boundary serrations weremarked/accommodated by the subgrain/cell bound-


Fig. 4. OIM results on sample 0.4-0.1-500 (Z = 3.48×109 s�1). (a) OIM image with boundaries drawn at 1–20° (light boundaries) and >20° (dark boundaries). One cube and one S deformed grain are also marked. Ithas to be noted that the magnification of (a) is higher than in (b). (b) Taylor factor map for the same regionas in (a). Dark and light regions correspond respectively to low and high Taylor factors—range of Taylorfactors 2.36–4.53. (c) Long range misorientation (LRM) development along lines drawn in the cube and S

grains of (a). Such misorientation is estimated with respect to the initial orientation, i.e. point to origin.

Table 2. Compilation of textural changes (in terms of changes in volume fractions of cube and Cu+S) and of microstructural changes (i.e. PLOM)during plain strain compression

Textural changes

Sample code Z values (s�1) Microstructural features Change in cube Change in Cu+S

0.4-10-400 1.28×1013 IIL �0.05 +0.040.4-1-350 1.2×1013 IIL �0.04 +0.0070.4-10-450 1.86×1012 IIL �0.05 +0.030.4-1-400 1.28×1012 IIL �0.08 +0.060.4-1-450 1.86×1011 IIL �0.05 +0.040.4-1-500 3.48×1010 GBS+IIL �0.05 +0.040.4-0.1-450 1.86×1010 GBS �0.08 +0.10.4-0.1-500 3.48×109 GBS �0.05 +0.0041-10-400 1.28×1013 IIL �0.12 +0.121-1-350 1.2×1013 IIL �0.113 +0.121-1-400 1.28×1012 IIL �0.11 +0.1451-1-450 1.86×1011 GBS+IIL �0.10 +0.131-1-500 3.48×1010 GBS �0.066 +0.0661-1-550 7.96×109 GBS �0.08 +0.121-0.1-500 3.48×109 GBS �0.05 +0.05


Fig. 5. OIM results on sample 0.4-1-350 (Z = 1.2×1013 s�1). (a) OIM image with grain boundaries drawn andcube and Cu/S deformed grains marked. (b) Taylor factor map—range of Taylor factors being 2.35–4.46.Graphics convention in (a) and (b) is the same as in Fig. 4. (c) LRM development along the lines marked in

(a).

aries, see Fig. 4. Development of GBS, possiblythrough dynamic recovery, with the wavelength of theserrations being similar to subgrain size, has beenreported before at low Z [33]. What is surprising inthe present study is the observation that in microstruc-tures with GBS, the dislocation substructure did notdepend noticeably on orientation or Taylor factor, seeFigs 4 and 6. Dependence of stored energy on orien-tation (or Taylor factor) in deformed fcc metals is awidely acknowledged fact based on different levelsof experimental techniques [4, 7, 8, 30, 34–36]. Butit seems that at low Z values such differences are notsignificant/noticeable, at least by OIM scans of sub-micron step sizes.

The difference in stored energies between cube andCu/S, insignificant at low Z values, wasnoticeable/significant at higher Z, see Fig. 6. Thesewere also the conditions where optically IILs were

Fig. 6. Average values of q/d for the two major texture compo-nents, cube and Cu/S, as function of Z values. q and d arerespectively the cell size and cell misorientation (consideredamong adjoining cells), as obtained from the OIM scans.Microstructural features (e.g. GBS or IIL), typical of Z value

ranges, are marked on the plot.


Fig. 7. Long range misorientation or LRM as function of Zvalues. LRM values are based on misorientation changes in asingle deformed grain, obtained with respect to the point oforigin along an imaginary line drawn in the RD. (a) Averagevalues of significant/noticeable misorientation changes, asshown in Figs. 4(c) and 5(c). (b) Average distance for suchchanges in LRM. Microstructural features (e.g. GBS or IIL),

typical of Z value ranges, are marked on the plot.

visible† and in the OIM scans cell walls appearedaligned at �35° with RD inside individual grains, seeFig. 5. Appearance of these microstructural featurescoincided with an increased drop in cube and with alarger increase in Cu+S during plain strain com-pression, clearly identifiable at e = 1 (see Table 2).The question is, are these 35° aligned cell walls andthe IILs and the cube instability, related?

4.2. Visibility of IILs

ILLs and inclined cell walls, see Figs 3(a) and 5(a),appeared at similar deformation conditions, i.e. highZ values, and both were inclined at �35° with RDinside individual grains. At first glance, this bringsthe possibility of a direct correspondence between the

† PLOM visibility of IILs in cube oriented grains werenot observed/expected, as under polarized light cuberemains black at all possible stage rotations [48].

two. However, two issues make such a correspon-dence “improbable” as: (I) typical thickness of IILsunder PLOM were 7–30 µm, while inclined cell wallseparations were in the sub-micron to (at the most)a few micron range. (II) Misorientation across suchindividual cell walls rarely exceeded 5° and hencemay not be enough for optical visibility under PLOM[35]‡. This raises another interesting possibility—PLOM visibility through long range misorientation(LRM) [35]. A previous study [35] on hot torsion ofAA5182 had also shown that appearance of smallgrains under PLOM were actually due to build-up ofLRM in cell clusters. At high Z, LRM build-up,across several of the 35° cell walls, can explain thePLOM visibility, as in Fig. 7, and also explain thesize discrepancy. In a word, “individual” inclined cellwalls may not define exact boundaries of opticallyvisible IIL; but regions containing several of suchwalls may acquire, as in Fig. 7, sufficient misorien-tation (i.e. LRM values) necessary for PLOM visi-bility.

At low Z values, the average LRM did not exceed5°, and optically IILs were not visible, and LRM inboth cube and Cu/S grains were similar. On the otherhand, at high Z deformation, average LRMs weremore than 5° and LRM in cube was less than Cu/S,see Fig. 7. Larger than 5° LRM may explain theoptical visibility of IILs, but the exact nature of 35°inclined cell walls can perhaps be debated. 35° (withRD) shear bands are a well known phenomenon incold rolled fcc metals [37]. The IILs reported in thepresent study have two major differences with such35° shear bands: (I) misorientations (typically within10°) are much smaller than those of fully developedshear bands in cold rolled fcc metals (often greaterthan 40°), and (II) such shear bands may cut/growacross several grains, while IILs were largely restric-ted inside single grain(s).

4.3. Formation of IILs

In-grain inclined lines may be considered as defor-mation bands (DBs) [16, 32] or first generation/non-crystallographic micro bands [31] or grain interiorshear bands [37]. Detailed TEM studies are currentlybeing conducted to resolve such issues. However, forthe present study of relating microstructural changes,i.e. appearance of 35° cell walls at high Z defor-mation, with textural changes, i.e. decreased cubestability, 35° inclined cell walls can be generallizedas plastic instabilities or strain localizations. Macro-scopically, formation of plastic instabilities can beconsidered from the so-called instability criteria [37]:

‡ Two other important differences between PLOM andOIM are: (a) the much lower spatial resolution of PLOMthan in OIM and (b) OIMs show misoriented boundarieswhile PLOM show misoriented areas.


1s�ds

de� =ne

+me�de

de� +1 + n + m

M �dMde � (1)

�mr�dr

de��0

where s and e are the macroscopic stress/strain, nand m are strain hardening exponent and strain ratesensitivity, e is the strain rate, M is the Taylor factorand r is the mobile dislocation density. It is perhapsinteresting to note that Dillamore et al. [37] had pre-dicted, based on textural softening or (dM/de), anglesof about 35° or 54° with RD for macroscopic plasticinstabilities/shear bands in fcc metals.

Formation of macroscopic plasticinstabilities/strain localizations is expected to be gov-erned by the macroscopic instability criteria [35, 37-39]—i.e. positive terms in equation (1) should be lowand the negative terms should be high to form plasticinstabilities. This may explain the Z dependence ofthe “appearance” of inclined cell walls. At low strainrate the second positive term becomes larger, whileat high temperatures an increased dislocation mobilitymay reduce the last negative terms. Thus low andhigh Z deformations are expected respectively to hin-der and to favor formation of plasticinstabilities/strain localizations. Formation of strainlocalizations will also depend on the textural soften-ing or dM/de, i.e. if Z values are not low enough inmaking equation (1) “strongly” positive. dM/de isnegative for Cu/S but positive for cube—about �0.2and +1.0, as estimated for ideal orientations by fullconstraint Taylor model. With deviations from exactcube dM/de reduces and at about 10° from ideal cube(depending on being RD, TD or ND rotated cube, thismay differ significantly) may even turn negative.Such differences in dM/de may explain the observeddifferences in numbers/frequencies of 35° inclinedcell walls between grains of different orientations orTaylor factors, see Fig. 5. Relative frequency of plas-tic instabilities, e.g. 35° inclined cell walls, may alsopossibly explain differences in LRM and q/d valuesbetween cube and Cu/S (see Figs 6 and 7).

4.4. Relation of IILs with cube (meta)stability

The most significant microstructural change athigh Z deformation is the appearance of �35°inclined cell walls. Clusters of such cell walls mayachieve sufficient LRM necessary for appearance ofIILs. Appearance of inclined cell walls or IILsaccompanied: (I) reduced cube stability, especiallyapparent at e = 1.0 (see Table 2), and (II)noticeable/significant stored energy differencebetween cube and Cu/S, see Fig. 6. These two effectsand their possible links with IILs or strain localiza-tions are discussed in the subsequent sections.

At least at the strain of 1, hot worked AA1050showed definite trends of textural changes withchanges in Z values. In general, the drop in cube vol-ume fraction and increase in Cu+S increased with Z,

see Table 2 and Figs 1 and 2. This is, however, nota unique observation. Several researchers [8, 16, 22,29] had made similar observations in hot workedaluminum alloys. The only difference with the presentstudy is the observation that lower Z did not increasethe brass component, although such increases are gen-erally reported [29].

An interesting issue is that none of the existingTaylor type deformation texture models may predictthe low cube drop and Cu+S increase, typical atlower Z, see Table 3 and also Table 2, using theclassical octahedral slip system—i.e. {111}�1–10�†.Instability of near cube, except ND rotated cube,using octahedral slip systems and both full constraint[18, 19] and relaxed constraint [20] Taylor models isa well established fact. Metastability of deformedcube grains, on the other hand, has been shown manytimes experimentally [3, 4, 6–8, 12, 15–17, 29, 30,43–48]. To explain such differences between Taylortype predictions and actual texture development,possible activation of non-octahedral slip systems, athigh temperatures, have been proposed [16, 21, 22,29] and used with some success [21, 29]. Plain straincompressed aluminum samples [16] have shown pres-ence of {110} slip traces at higher deformation tem-peratures, which also supports the argument of non-octahedral slip.

In the present study, textural developments weresimulated using different Taylor type models [24, 25,40, 41]—i.e. full constraint, relaxed constraint lathand pancake and lamel. Simulations were conductedfor octahedral, i.e. {111}�1–10�, and non-octahedral[16], i.e. {111}+{110}�1–10�, slip systems. The otherfactors (e.g. critical resolved shear stress switch,strain rate sensitivity, etc.) were kept the same, onlyslip systems were changed.

The results of such simulations are summarized inTable 3 and show a predicted drop in cube and anincrease in Cu+S when {110} slip is included. At firstglance, the lower cube drop and increase of Cu+S,typical of relaxed constraint lath with non-octahedralslip systems, as in Table 3, appears similar to the tex-tural developments at lower Z, as in Table 2. How-ever, both the full constraint Taylor and the relaxedconstraint lath models, with non-octahedral slip, over-estimate‡ the strength/volume fractions of Goss andBrass, especially the former, see Table 4. This per-haps raises some doubts about explaining deformationtexture developments at elevated temperatures onlythrough activation of non-octahedral slip systems, atleast in the present case. Another interesting issue isthat non-octahedral slip may not explain the observedcube metastability in cold rolled aluminum [30] or

† It is perhaps important to remember that Taylor typedeformation texture models are typically used to predictcold worked textures.

‡ This is less of a problem in octahedral slip, which pre-dicts lower (and closer to the actual experimental values)volume fractions for Goss and Brass.


Table 3. Textural changes, in terms of changes in volume fractions of cube and Cu+S, predicted by different deformation texture models (RCstands for relaxed constraint). Simulations were obtained using both octahedral and non-octahedral slip systems

Strain (e) = 0.4 Strain (e) = 0.4

Imposed slip system Deformation texture model cube Cu+S cube Cu+S

{111}�1–10� octahedral slip Full Constraint Taylor �0.083 +0.08 �0.135 +0.199RC Lath �0.0976 +0.049 �0.1274 +0.0696

RC Pancake �0.184 +0.1 �0.24 +0.189Lamel �0.116 +0.086 �0.188 +0.184

{111}+{110} �1–10� Non- Full Constraint Taylor �0.091 +0.06 �0.125 +0.1octahedral slip [16] RC Lath �0.058 +0.05 �0.08 +0.071

RC Pancake �0.1 +0.081 �0.135 +0.13Lamel �0.07 +0.075 �0.11 0.121

Table 4. Volume fractions of Goss and Brass components as obtained by full constraint (FC) and relaxed constraint lath (RCL) models using non-octahedral slip, {111}+{110}�1–10�. Average volume fractions of Goss and Brass obtained experimentally, at both low and high Z values, are also

included

e = 0.4 e = 1.0

Source of data Goss Brass Goss Brass

FC 0.125 0.106 0.12 0.119RCL 0.135 0.091 0.136 0.098Low Z 0.050 0.072 0.058 0.078High Z 0.050 0.076 0.047 0.084

copper [12, 17]. In the latter case, the deformationtemperature being only 0.02 Tm (Tm = meltingtemperature), elevated temperature activation of non-octahedral slip is perhaps questionable. Suchexamples do not rule out the possibility that non-octa-hedral slip can play a role at elevated temperatures,but may indicate that some other mechanism(s) canalso be responsible for cube metastability.

In a classical Taylor approach of strain homogeniz-ation [40–42], cube instability is expected, see [18–20] and also Table 3; while the present study, as wellas observations of Maurice and Driver [16], hasshown that absence of strain inhomogenities are“related” to cube stability. Even in cold rolling ofcopper with different levels of purity, Ridha and Hut-chinson [34] reported “reduced retained cube texturewith more shear banding”. The correlation that lessstrain localization is related with increased cube stab-ility seems quite convincing. What remains unknownis: (I) the exact rationale behind such a correlationand (II) the way to incorporate this in deformationtexture modeling.

5. SUMMARY

� During plain strain compression of AA1050, thevolume fraction of cube grains decreases and thatof Cu and S grains increases.

� At e = 1, a larger decrease of cube and a largerincrease of Cu+S is observed with increasing Zvalues (Zener–Holloman factor, whereZ = e exp(Q/RT).

� At low Z deformation grain boundary serrations

(GBS) were observed by PLOM, while at high Z,in-grain inclined lines (IIL), at �35° with RD,were seen.

� Low Z deformed grains contained large, but equi-axed, cells/subgrains. Increased Z aligned the cellwalls at �35° with RD.

� Substructures were studied in terms of q/d (cellmisorientation/cell size) and LRM (long rangemisorientation). At low Z both q/d and the vari-ation of the LRM values (typically at less than 5°misorientation) were similar between grains of dif-ferent orientations. Appearance of 35° inclinedcell walls, increased the variation in the LRMvalues to greater than 5° misorientation and per-haps as a result, the 35° inclined cell clustersobtained optical visibility as IILs. More frequentappearance of 35° inclined cell walls in grains ofhigher Taylor factors, was reflected in differencesin q/d and LRM values between grains of differentorientations. In general, Cu/S grains had largerq/d and greater variation in the LRM values thancube grains.

� Generalizing the IILs or 35° inclined cell walls asplastic instabilities or strain localizations, andhence considering the instability criteria, mayexplain their presence/absence at high/low Zdeformation. Textural softening (dM/de, whereM is the Taylor factor) may expain the observeddifferences in relative appearance of such insta-bilities in grains of different orientations.

� Textural developments at low Z deformation, e.g.cube stability, cannot be predicted using any of theTaylor type models and octahedral slip systems.


Use of non octahedral slip systems, e.g.{111}+{110}�1–10�, predicts (especially forrelaxed constraints lath) higher cube stability butoverestimates Goss development considerably. Onthe other hand, experimental evidence seems con-vincing in relating increased cube stability withabsence of strain localizations.

Acknowledgements—This research was funded by the BelgianNational Science Foundation (FWO) under contract numberG.0220.98. Corus–Aluminium is acknowledged for the plainstrain compression tests.

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Documents

Hot working of AA1050—relating the microstructural and textural developments