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Materials Characterization 52 (2004) 165–177

Polarized light microscopy: utilization in the investigation of the

recrystallization of aluminum alloys

M. Slamovaa,*, V. Ocenaseka, G. Vander Voortb

aVUK Panenske Brezany, s.r.o., Panenske Brezany 50, 250 70 Odolena Voda, Czech RepublicbBuhler Ltd., 41 Waukegan Road, Lake Bluff, IL 60044, USA

Received 1 September 2003; accepted 31 October 2003

Abstract

To obtain specific properties and performance from a metallic material, it is necessary to control the microstructure. This

imposes a need to monitor closely and/or predict microstructural transformations during downstream processing. One of the

main features that determine the properties of wrought metals and alloys is the status of their matrix, which may be changed by

thermomechanical treatment procedures. Different softening processes, such as recovery, recrystallization (RX) or grain growth,

may occur in cold- and hot-worked materials. The present paper focuses on different aspects of RX and the microscopic

methods used for its study. Examples of the usefulness of light microscopy examinations in the study of RX of several

aluminum alloys are given.

D 2004 Elsevier Inc. All rights reserved.

Keywords: Recrystallization; Aluminum alloys; Light microscopy; Polarized light

1. Introduction these phenomena can be found in Refs. [1–3]. In

The effort to control the microstructure of metallic

materials to obtain specific properties and perfor-

mance imposes the necessity to monitor closely and/

or predict microstructural transformations during

downstream processing. One of the main features

determining the properties of wrought pure metals

and their alloys is the status of the matrix, which may

be changed by thermomechanical treatment proce-

dures. Annealing phenomena have been the subject

of extensive research, and a comprehensive guide to

1044-5803/$ - see front matter D 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.matchar.2003.10.010

* Corresponding author. Tel.: +420-283-971-894; fax: +420-

283-971-816.

E-mail address: [email protected] (M. Slamova).

aluminum alloys, especially in alloys not hardened by

precipitates, the most important transformation pro-

cess, resulting in specific microstructures determining

material strength and formability, is recrystallization

(RX). Recent basic approaches to the recovery and

RX of aluminum alloys are discussed in Refs. [4–7].

The specific problems concerning particular alloys

and the physical and metallurgical aspects of the

process have been largely studied and published in

materials science journals. Modern and sophisticated

observation and analysis methods are used to under-

stand the underlying relationships between process

kinetics and the resulting grain structures. However,

the most powerful and easily accessible method of

microstructure observations, and one that is largely

used in industry and applied research, is light micros-

M. Slamova et al. / Materials Characterization 52 (2004) 165–177166

copy. The present paper gives a brief overview of the

different aspects of recovery and RX and the micro-

scopic methods used to study them. An emphasis is

put on polarized light microscopy. Several examples

of its usefulness in the study of RX of aluminum

alloys are given.

2. Recovery, RX and grain growth

During the deformation processing of metals (e.g.,

extrusion, rolling, forging and drawing), thermody-

namically unstable microstructures containing lattice

defects (vacancies and dislocations) and interfaces

(subgrain and grain boundaries) are produced. These

changes increase the strength and decrease the form-

ability of the materials. To make possible further

forming, the initial nondeformed microstructure of

Fig. 1. Schematic representation of the main softening processes: (a)

deformed state, (b) recovered, (c) partially recrystallized, (d) fully

recrystallized, (e) grain growth, and (f) abnormal grain growth (from

Ref. [2]).

Fig. 2. Preferred sites and mechanisms of RX nucleation in

AlCu4Mg1 (AA2024-T42) extrusion: (a) nucleation of a new grain

by strain-induced grain boundary migration at grain boundary; (b)

nucleation by PSN at coarse intermetallic particles.

the material has to be partly or completely restored

by annealing. The microstructural changes that occur

upon annealing a deformed metal are schematically

shown in Fig. 1. These processes are commonly

described in terms of recovery, RX and grain growth,

generally referred to as softening processes.

When annealing is carried out only at elevated

temperature, the microstructure and alloy properties

are only partially restored to their original values by

recovery, in which annihilation and rearrangement of

the dislocations occurs. The microstructural changes

during recovery are relatively homogeneous and do

not usually affect the boundaries between the de-

formed grains (Fig. 1b).

M. Slamova et al. / Materials Characterization 52 (2004) 165–177 167

Recovery is easier in metals with high stacking

fault energy (SFE), such as aluminum and lead, and is

relatively difficult in copper and silver, where the SFE

is low. The SFE is a metal characteristic, determining

the ease of dislocation dissociation into partial dis-

locations. Low SFE is indicative of easy dissociation

and, therefore, restricted cross slip and climb of

dislocations. When climb and cross slip do not occur,

dislocations cannot be annihilated; that is, recovery is

difficult.

Recovery generally results in only a partial resto-

ration of properties because the dislocation structure is

not completely removed but reaches a metastable

state. A further restoration process called RX may

occur, in which new dislocation-free grains are formed

within the deformed or recovered structure (Fig. 1c).

These then grow and consume the old grains, resulting

Fig. 3. Illustration of the effect of as-cast grain structure on 0.18 mm gau

alloy (AA8006): (a) and (b) coarse and unevenly sized as-cast grains resu

(c) and (d) optimal final gauge microstructure obtained in a material with

the final condition (d).

in a new grain structure with a low dislocation density

(Fig. 1d).

Although RX removes the dislocations, the mate-

rial still contains a large fraction of grain boundaries,

which are thermodynamically unstable. Further

annealing may result in grain growth, in which the

smaller grains are eliminated, the larger grains grow

and the grain boundaries form a lower energy config-

uration (Fig. 1e). In certain circumstances, the normal

grain growth may give way to the selective growth of

a few large grains (Fig. 1f), a process known as

abnormal grain growth or secondary RX.

Recovery and RX are driven by the stored energy

of the deformed state, ED, which is related to the

dislocation density according to the relationship [2]

ED ¼ c2qGb2; ð1Þ

ge soft condition strips produced from continuously cast Al-Fe-Mn

lt in coarse and unevenly sized grains at final gauge, respectively;

good as-cast microstructure (c) and using appropriate processing to


where c2 is a constant of the order of 0.5, q is the

density of dislocations, G is the shear modulus and b

is the Burgers vector. If the deformation microstruc-

ture consists of subgrains, then the stored energy may

be estimated from the subgrain diameter (D) and the

specific energy (cs) of the low-angle boundaries

forming the walls of the subgrains [2]

EDcð3=DÞcs ¼ acs=R; ð2Þwhere a is approximately 1.5. The boundary energy

cs is directly related to the misorientation of adja-

cent subgrains (h), and thus, the stored energy

depends on the subgrain misorientation, according

to Ref. [2]

EDcð3=DÞcs ¼ Kh=D: ð3Þ

Recovery and RX are competing processes, as both

are driven by the stored energy of the deformed state.

Fig. 4. Effect of dynamic recovery during cold rolling—the lower RX rate

indicative for a more intensive dynamic recovery during rolling: (a) and (b

and 4.0, respectively, and annealed at 280 jC/240 s; (c) and (d) alloy AlF

respectively, and annealed at 300 jC/300 s.

Once RX has occurred and the deformation substruc-

ture has been consumed, then no further recovery can

occur. The extent of recovery will therefore depend on

the ease with which RX occurs. Conversely, because

recovery lowers the driving force for RX, a significant

amount of prior recovery may, in turn, influence the

nature and the kinetics of RX. The division between

recovery and RX is sometimes difficult to define

because recovery plays an important role in nucleating

RX.

Particles in two-phase alloys may strongly affect

RX. They have three important effects [2]:

(a) The stored (deformation) energy and, hence, the

driving force for RX, may be increased.

(b) Large particles may act as nucleation sites for RX.

(c) Particles, particularly if closely spaced, may exert

a significant pinning effect on both low- and high-

in the samples with larger plastic deformation prior to annealing is

) alloy AlFe0.7Si0.7 (AA8011)–TRC strips rolled to strains e = 5.3e1.4Mn0.4 (AA8006)–TRC strips rolled to strains e = 4.6 and 3.9,


angle boundaries and thus can hinder the progress

both of recovery and of RX.

Although the softening processes are studied

mainly in connection with annealing, they may

occur also during the cold or hot working. Especial-

ly in metals with high SFE, such as high-purity

aluminum or low-alloyed Al alloys, recovery can

easily occur, even during cold rolling, when high

deformation is imposed. When recovery and RX

occur during annealing, they are referred to as static

processes, whereas in the case of working, dynamic

processes are studied. The occurrence of dynamic

recovery and RX affects the course and resulting

grain structure of subsequent static softening pro-

cesses during annealing.

Fig. 5. Effect of Sc and Zr additions on the as-cast grain size of an

Al-Mg-Mn-Cr alloy (AA5754): (a) standard alloy AA5754; (b)

alloy AA5754 with 0.25 wt.% Sc and 0.08 wt.% Zr.

3. Methods of RX study

The progress of RX can be followed by several

methods, such as calorimetry, measurements of den-

sity, electrical resistivity, hardness and proof stress.

However, the structural changes expressed in the

property changes are often small and difficult to

measure. For the early stages of RX, when only a

few recrystallized nuclei of very small size are pres-

ent, examinations in the scanning electron microscope

(SEM), by using grain contrast regime, and in the

transmission electron microscope (TEM) are more

suitable. The progression of RX and also the complete

evaluation of grain orientations, i.e., of crystallograph-

ic texture, are conventionally done using X-ray dif-

fraction and, more recently, SEM-based electron

backscatter diffraction (EBSD) [8]. However, the most

direct and easily accessible method for the investiga-

tion of RX is still light microscopy. In aluminum

alloys, the best resolution of individual grains is

achieved by the examination of electrolytically anod-

ized samples in a polarized light microscope (PLM).

This technique is used mainly for the study of the later

stages of RX because the first germs of new grains are

often too small to be detected by light microscopy.

Anodizing is suitable for alloys of both low and high

alloy content. Moreover, the grain structure in highly

alloyed materials can be revealed either by etch pit

and alloying elements film deposition (in Al–Cu

alloys with Cu content larger than 1 wt.%) or due to

precipitation on grain boundaries that are thus delin-

eated upon chemical etching by the darkened precip-

itates [9].

The samples for PLM observations of grain struc-

ture are prepared by the usual procedure of mounting,

mechanical grinding and polishing. After polishing,

the samples are anodized by electrolytic etching at 20

V DC in Barker’s reagent, consisting of 4–5 ml HBF4(48%) in 200 ml H2O. The etching is aimed at

depositing a film of Al2O3 on the surface. In poly-

crystalline materials, the thickness of the film formed

on each particular grain depends on the grain’s crys-

tallographic orientation. When viewed with plane-

polarized illumination passed through an analyzer,

the film can rotate the plane of polarization regarding

the orientation of the underlying grain, thus producing


various shades of black, gray or white. The contrast

effects can be converted to striking color contrast by

inserting before the polarizer a sensitive tint or quar-

ter-wave plate.

Several aspects of RX and the factors affecting its

kinetics can be studied by PLM:

(i) the temperature of RX start (only to some extent,

more precisely by TEM);

(ii) the preferred sites for RX nucleation, such as

grain boundaries, localized deformation bands,

deformation zones at coarse particles;

(iii) the extent of RX described by the volume

fraction of the recrystallized material, XV;

(iv) the temperature of RX (usually taken as the

temperature at which XV= 50%);

(v) the grain size, shape and their uniformity

through sheet thickness or other directions in

extrusions and forgings;

Fig. 6. Effect of thermally stable precipitates on RX in an AlMg3 alloy: (a)

wt.% Zr. (a) and (b) Samples processed by homogenization, hot and cold ro

by homogenization, hot and cold rolling and annealed at 595 jC for 1.5

(vi) the effect of work hardening and its recovery on

the material’s ability to recrystallize at given

conditions;

(vii) the hindering effect of small particles (dis-

persoids) both on RX nucleation and grain

growth;

(viii) some particular crystallographic orientations of

individual grains can be determined by exam-

ination with polarized light.

4. Examples of application of the PLM in RX

studies of aluminum alloys

Grain boundaries, localized deformation bands

and deformation zones at coarse particles are pre-

ferred sites for RX nucleation. Fig. 2 shows two of

these preferred sites in an AlCu4Mg1 alloy. The

small new grain in the central part of the micrograph

and (c) standard alloy; (b) and (d) alloy with 0.25 wt.% Sc and 0.08

lling and annealed at 360 jC for 0.2 h; (c) and (d) samples processed

h.


in Fig. 2a has apparently formed by the strain-induced

grain boundary migration of the deformed grain above

it. The small new grains in proximity of the coarse

(black) particles are a typical example of particle-

stimulated nucleation (PSN) of RX in the deformed

zone formed at second-phase particles during the

plastic deformation by cold rolling.

The PLM can be successfully used also in the

investigation of the as-cast grain structure of alumi-

num alloys. Two of the authors have been working

intensively on the development of technologies for the

production of aluminum rolled materials by twin-roll

continuous (TRC) casting, and for this reason, many

of the examples are of TRC materials. Casting and

processing conditions have a significant effect on the

grain structure and formability of Al-Fe-Mn strips

prepared by TRC casting, cold rolling and annealing

[10]. Materials with a nonuniform as-cast grain struc-

Fig. 7. Evolution of RX in twin-roll cast (TRC) samples of alloy AA5182:

370 jC; (b) rolled material heated to 400 jC; (c) as-cast material heated to 4

lower temperature in the cold rolled material (Panels (a) and (b)) as compar

hot rolling component, therefore, as-cast materials are plastically deforme

ture (Fig. 3a) cannot be processed to yield good

quality thin strips. Such materials, even if they are

processed by an optimum procedure, exhibit in the

final condition a coarse-grained microstructure (Fig.

3b). Coarse grains deteriorate the deep drawing capa-

bility of the material and make it unusable for its main

application, i.e., for fins in heat exchangers. When the

as-cast grain structure is uniform (Fig. 3c) and an

appropriate downstream processing is used, the final

gauge microstructure is fairly uniform and fine

grained (Fig. 3d). Therefore, a regular inspection of

the as-cast grain structures in the PLM is indispens-

able in industry.

It is fair to conclude that the research aimed at

optimizing casting conditions and downstream pro-

cessing of TRC materials is highly effective when

PLM examinations are regularly used in the course of

the experiments. An example of the utility of PLM

effect of cold rolling prior to annealing. (a) Rolled material heated to

00 jC; (d) as-cast material heated to 500 jC. The RX starts at much

ed with the as-cast samples (Panels (c) and (d)). TRC involves also a

d and possess a nonzero stored deformation energy.


examinations in this application area can be found in

Ref. [11]. The effects of alloy pretreatment and cold

working rate on RX response were studied in the case

of Al-Fe-Mn (AA8006) and Al-Fe-Si (AA8011) TRC

strips. An inspection of the annealed samples showed

that those with heavier cold rolling prior to annealing

(Fig. 4a and c) are recrystallized to a smaller extent

than the less deformed samples were (Fig. 4b and d).

Therefore, it was revealed that heavy cold rolling

results in significant in situ deformation recovery in

both alloys. The occurrence of deformation recovery

due to heavy cold working was afterwards proved by

TEM observations that showed the presence of dislo-

cation walls and subgrain boundaries. Such disloca-

tion structures are less energetic, and thus, it was

established that the as-rolled materials are partly

recovered; that is, the driving force for RX is low.

Therefore, metallographic analyses of the grain struc-

tures helped substantially the development work on

Fig. 8. Evolution of RX in TRC samples of alloy AA5049: effect of cold

rolled material heated to 400 jC; (c) as-cast material heated to 370 jC; (temperature in the cold-rolled material (Panels (a) and (b)) as compared w

TRC AA8006 and AA8011 strips for application in

heat exchangers. These analyses allowed the selection

of annealing conditions most suited for achieving the

necessary level of alloy RX and thus the required

values of proof stress, strength and tensile elongation.

It is well known (see, e.g., Ref. [1,2]) that RX can be

slowed down also by the presence in the materials of a

fine dispersion of second-phase particles, usually

formed by precipitation of the alloying elements from

supersaturated solid solution. Such precipitation is

usually deliberately induced by a specific heat treat-

ment consisting of solution annealing, quenching and

precipitation annealing. However, in Al alloys contain-

ing Sc and Zr, it can occur spontaneously during the

cooling from the solidus temperature. The effect of

stable Al-Sc-Zr precipitates on the resistance to RX due

to the addition of Sc and Zr to Al-Mg alloys has been

studied using PLM [12]. In that work, it was observed

that the addition of Sc and Zr to the standard aluminum

rolling prior to annealing. (a) Rolled material heated to 340 jC; (b)d) as-cast material heated to 400 jC. The RX starts at much lower

ith the as-cast samples (Panels (c) and (d)).


alloy AA5754 (AlMg3MnCr) significantly refined the

as-cast grain structure (Fig. 5). Furthermore, PLM

examinations of the standard and modified (with Sc

and Zr) alloys processed by homogenization, hot and

cold rolling, indicated that the standard alloy was

completely recrystallized when annealed for short

times at 360 jC (Fig. 6a), whereas the modified alloy

resists RX at this temperature. The alloy with Sc and Zr

was not completely recrystallized (Fig. 6d), even after

annealing for hours at very high temperatures, such as

595 jC, which is close to the alloy’s melting point. In

the same time, the standard alloy presents a relatively

coarse grain structure due to grain growth (Fig. 6c). The

presence was detected (by TEM) of nanosized coherent

Al3(Sc,Zr) precipitates that formed during cooling,

either after solidification or homogenization and/or

during hot rolling. These precipitates exhibit a very

high thermal stability and are supposed to be the main

reason for restricted RX in alloys to which Sc and Zr

have been added.

Fig. 9. Effect of composition on the grain size in Al-Mg TRC alloys annea

AA5049 heated to 560 jC; (b) alloy AA5052 heated to 530 jC; (c) alloy A

The evolution of grain structure during the ho-

mogenization of TRC Al-Mg alloys has also been

studied using PLM [13,14]. This investigation dem-

onstrated once more that the PLM observations of

samples at the early stages of RX are very important.

Fig. 7 shows that it is not appropriate to follow the

progress of the process only by hardness measure-

ments on strip surfaces, especially in cases when RX

starts at the surface of the sheet (Fig. 7a) and

gradually propagates deeper into the thickness (Fig.

7b). It is evident that surface hardness measurements

in these cases necessarily give wrong information on

the starting temperature and the progress of the

process because RX occurs at much lower temper-

atures in the surface layers than in the interior. The

study showed that the response of TRC Al-Mg alloy

to homogenization (high temperature) annealing

depends on alloy composition and is affected by

the application of cold rolling prior to annealing. The

effect of cold rolling manifests itself by different RX

led at 560 jC after a slow heating (industrial-like) regime. (a) Alloy

A5754 heated to 560 jC; and (d) alloy AA5182 heated to 560 jC.


extent and grain size (Fig. 8). The RX starts and is

completed at lower temperatures and also results in

finer grain size in the materials homogenized after a

cold rolling pass (Fig. 9). It must be pointed out that

TRC involves a hot rolling component, and there-

fore, as-cast TRC materials are plastically deformed

and possess a nonzero stored deformation energy.

This is the reason why as-cast TRC materials can

recrystallize even if they have not been previously

work hardened. The grain size and shape in the

different alloys annealed under the same conditions

depend on alloy composition (Fig. 9). It was found

that not only the content of the main alloying

element, i.e., of Mg, is important, but also, the

content of other elements, especially of Mn and Cr,

influences RX rate and the resulting grain size and

shape. These findings are of great importance and

were used in the optimization of the downstream

processing of the TRC Al-Mg alloys intended for

automotive applications.

Fig. 10. Examples of joints between Al tubes and

The grain structure in brazed all-aluminum heat

exchangers used in automobiles has been also exam-

ined by PLM to evaluate the effectiveness of the

brazing process and changes in microstructure result-

ing from brazing (NOCOLOK) performed at 600 jC.Fig. 10 shows examples of brazed joints between

tubes and fins (typically of alloys AA3003–AlMn1)

in automotive heat exchangers. It can be seen that the

grain structure of tube core alloys (typically AA3003)

and clad layers (typically AA4xxx Al-Si alloys) pres-

ent some differences. These may result in materials

with different corrosion resistance and, subsequently,

differences in the length of service life of the exchang-

ers. PLM examinations of the grain structure of tubes

used in exchangers prior to mounting can reveal

differences in the deformed grains of the core tube

material (Fig. 11a and b) and defects in tube seam

(Fig. 11c and d).

The PLM has been also very useful in the devel-

opment of TRC foils (typically of thickness of 75 Am)

fins in brazed automotive heat exchangers.

Fig. 11. Examples of the microstructure of Al tubes used in automotive heat exchangers before the brazing process. (a) and (b) Core tube

material; (c) partial RX near tube seam; and (d) defective tube seam.


for applications as fins in heat exchangers. The

producers of heat exchangers impose very strict

requirements on the mechanical properties of the

materials in the postbrazed condition. To satisfy these

requirements, the foil grain structure after the brazing

procedure should be fine grained. However, PLM

examination indicated that, in many cases, the grain

size in the short-transverse direction was as large as

the foil thickness (Fig. 12), making the fin soft and

very susceptible to intergranular corrosion. The effort

of development research is thus oriented to materials

with finer postbrazed grain structure.

The PLM in conjunction with an interactive

image analysis system may be used for grain size

and shape measurements when quantitative informa-

tion on grains is required. In rolled materials, even

in fully recrystallized samples, the grains are often

flattened in the direction normal to the rolling

plane. In such cases, grain sizes in the rolling,

short and long transverse directions are of interest.

One of the quickest and comfortable ways of grain

size measurement in Al alloys is the semiautomatic

counting method based on the lineal intercept

concept. A square grid of suitable size or a set of

parallel lines (oriented in direction of interest) is

projected as an overlay image on the grain struc-

ture. The number of intercepts of grain boundaries

with the set of measuring lines is counted and

recorded, and the mean intercept length is then

calculated. The method is suitable especially in

materials with nonequiaxed grains, where it is

meaningless to determine an equivalent grain diam-

eter based on the measurement of grain area. The

latter method, when the reconstruction of grain

boundaries is possible using sophisticated software,

is not suitable in many aluminum alloys due to the

fact that grain boundaries cannot be etched to a

sufficient extent. For this reason, the PLM repre-

sents the only observation tool that can yield sa-

tisfactory results.

Fig. 12. Examples of grain structures in AlMn1 (AA3003) foils used as fins in brazed heat exchangers in after-brazing condition. Foil thickness

is � 75 Am.


The fraction of recrystallized grains can be also

measured by semiautomatic counting using a regular

grid of points projected on the PLM image of grain

structure. Although the method is time consuming, it

is easily accessible and faster when compared with

more sophisticated methods using, e.g., the EBSD

(orientation imaging) technique [15].

5. Conclusion

The examples presented of the use of polarized

light microscopy prove that it is an essential method

for investigating the RX of aluminum alloys. Obser-

vations in polarized light represent a quick and

precise method of obtaining both qualitative and

quantitative descriptions of the softening level of

aluminum alloys. The use of polarized light micros-

copy greatly facilitates research aimed at the optimi-

zation of alloy downstream processing and final

product properties.

Acknowledgements

This work was supported by the Ministry of

Education, Youth and Sports of the Czech Republic

under Project No. 2003-039-1 and by the Grant

agency of the Czech Republic under Project No. 106/

00/1486. The financial support of both institutions is

gratefully acknowledged.

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