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
M. Slamova et al. / Materials Characterization 52 (2004) 165–177168
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,
M. Slamova et al. / Materials Characterization 52 (2004) 165–177 169
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
M. Slamova et al. / Materials Characterization 52 (2004) 165–177170
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.
M. Slamova et al. / Materials Characterization 52 (2004) 165–177 171
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.
M. Slamova et al. / Materials Characterization 52 (2004) 165–177172
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)).
M. Slamova et al. / Materials Characterization 52 (2004) 165–177 173
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.
M. Slamova et al. / Materials Characterization 52 (2004) 165–177174
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.
M. Slamova et al. / Materials Characterization 52 (2004) 165–177 175
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.
M. Slamova et al. / Materials Characterization 52 (2004) 165–177176
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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