Materials Science Forum
ISSN: 1662-9752, Vols. 539-543, pp 362-367
doi:10.4028/www.scientific.net/MSF.539-543.362
© 2007 Trans Tech Publications, Switzerland
Online: 2007-03-15
In-Situ SEM/EBSP Analysis during Annealing
in a Pure Aluminum foil for Capacitor
M. Kobayashi1,2,a Y. Takayama1,b，H. Kato1,c and H. TODA2,d
1
Department of Mechanical Systems Engineering, Utsunomiya University,
Utsunomiya, Tochigi 321-8585, Japan
2
Department of Production Systems Engineering, Toyohashi University of Technology,
Toyohashi, Aichi 441-8580, Japan
a
[email protected], [email protected],
c
[email protected] [email protected]
Keywords: Cube orientation, recrystallization, grain growth, textural evolution, scanning electron
microscopy/ electron backscattered diffraction pattern (SEM/EBSP) technique.
Abstract. In-situ SEM/EBSP analysis has been performed during the evolution of the cube texture
in a pure aluminum foil. In general, foils for capacitor are manufactured in an industrial process of
casting, homogenizing, hot rolling, cold rolling (CR), partial annealing (PA), additional rolling (AR)
and final annealing (FA). The foil samples after CR or AR in the process were analyzed by the
SEM/EBSP technique at a constant temperature which was step-heated repeatedly by 10-20K from a
room temperature to 623K or 598K. In a CRed sample, cube ({001}<100>) grains begin to grow
preferentially at 503K to cover the sample. On the other hand, in a sample subjected to PA at 503K
and AR, cube grains coarsened rapidly and preferentially at more than 533K in contrast to other
oriented small grains remaining their sizes. Further, intragranular misorientation analysis revealed
that the misorientation, which corresponds to dislocation density or strain, was much smaller in cube
grains than in S ({123}<634>) and Cu ({112}<111>) ones.
Introduction
It is well known that the sharp cube texture is evolved in pure aluminum foils for capacitors in the
interest of capacitance augmentation [1]. The foils are often produced in an industrial process
patented by Company Pechiney [2], that is, hot rolling, heavily cold-rolling, partial annealing (PA),
additional rolling (AR) and final annealing (FA). Numerous studies on the cube texture formation
were performed extensively in the production process from the hot rolled sheets to the final annealed
foils up to date [1, 3-8]. It was understood in the previous studies that cube-oriented ({001}<100>)
nuclei formed during the hot rolling or cold rolling, they became stable grains in partial annealing
process, and in final annealing process the cube grains were able to grow preferentially with
strain-induced grain boundary migration (SIBM) driven by difference in stored energy introduced by
additional rolling process. It was mentioned that cube grains were less affected than other oriented
grains by deformation of additional rolling [8-11] or were able to recover earlier than other oriented
ones [9, 12].
The purpose of the preset study is to reveal how the cube grain has an advantage in comparison
with the other oriented grains during microstructural evolution by using in-situ scanning electron
microscopy/ electron backscattered diffraction pattern (SEM/EBSP) analysis [13]. Application of the
analysis enables us to find the process where the cube grain gets priority in growth and a critical
temperature where it does. Further, effect of PA and AR on preferential growth of the cube grain is
investigated from comparison of conditions with and without them.
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Materials Science Forum Vols. 539-543
363
Experimental Procedure
Samples were 99.99% pure aluminum foils for capacitors, which contain with 8ppm Fe, 8ppm Si and
50ppm Cu. The foils were produced by the process consisting of casting, homogenizing, hot rolling
and cold rolling with a reduction of 97%. Furthermore, they were partially annealed (PAed) at 503K
and 17% additionally rolled (ARed). The CRed and PA/ARed samples were of 132μm and 110μm
thickness, respectively.
The crystallographic orientation analyses were performed repeatedly in the same position of
samples during heating by the SEM/EBSP technique using HITACHI S-3500H SEM / TSL OIM
systems. For such in-situ SEM/EBSP analyses, a specially designed sample holder with a built-in
ceramic heater was used. The temperature of the sample was controlled by the heater current. First,
the temperature was raised from room temperature to a given one, which was 493K for a cold rolled
(CRed) and 523K for a PAed and ARed (PA/ARed) samples, at a constant heating rate of 0.16/K.
Next, after a temperature which was 10 to 20K lower than the given one was held to stabilize grain
structure (or suppress grain boundary migration), the SEM/EBSP analysis was carried out for about
80min. Immediately after the analysis the temperature was raised to a 10 or 20K higher given one.
Then, the SEM/EBSP analysis was performed at a 10 to 20K lower temperature, and raising the
temperature. In the same way the foil sample was analyzed at a constant temperature which was
step-heated repeatedly by 10-20K from a room temperature to 623K or 598K. The standard analyzed
area was 300×600μm2 and the standard step size of 3.0μm. Grain interior misorientation was
measured with a step size of 0.5μm.
Results and Discussion
Orientation maps obtained at each temperature by the in-situ SEM/EBSP analyses in the cold rolled
sample are displayed in Fig. 1. Although S ({123}<634>) oriented area covers more than a half of the
sample at (a) 493K, cube oriented grains multiply gradually with increasing temperature. This means
the gradual conversion from rolling texture to cube texture. Cube grains are evidently observed in (a)
493K map though their number and size are not so large. As temperature rises, not only cube grains
grow on the surface but also some of them appear from inside of the foil. An arrowed cube grain in
Fig. 1(d), which appears from inside and beside is very close to ideal orientation, coarsens extremely
later. S grains appearing from inside grow and remain to an equivalent size to the cube grains. These
facts are probably related to the size effect, that is, the grains appearing from inside are larger in three
dimensions than those on surface.
Figure 2 shows orientation maps representing change of microstructure in the PA/ARed sample.
Fairly large equiaxed grains of 20 to 30μm are observed in (a) as received sample. The grains involve
numbers of cube ones. The cube grains probably grew during partial annealing (PA) [14]. The
microstructure does not change markedly until temperature reaches the PA temperature of 503K. At
(c) 533K the cube grains become larger rapidly while the other grains remain small. The cube grains
grew likely with a driving force of consuming the stored energy of unrecrystallized fine grains. [15]
At (e) 543K and later, the cube grains cover extended area of the sample instantaneously.
Microstructure where the same oriented grains occupy nearly whole area includes lots of low angle
and irregular boundaries. It is noted that the microstructure is smaller in grain size compared with that
of CRed sample.
Changes in texture components against the temperature during heating in both of the samples are
compared in Fig. 3. In CR sample, fraction of the cube grains increases gradually to about 80%. The
competitive orientation S slightly increases in fraction after about 20% decrease at 570K. Another
orientation Cu ({112}<111>) lowers in fraction slowly toward near zero at 613K. On the other hand,
the cube fraction of PA/ARed sample shows a drastic increase about 533K while both competitive
orientations S and Cu decrease in fraction to zero. Evolution of cube orientation in the PA/ARed
sample is regarded as sensitive to temperature owing to stored energy of the additional rolling
364
THERMEC 2006
compared with that in the CRed one, though difference in heating conditions between both samples
should be considered for comparison.
(a)
493K (477K-478K)
(e)
568K (545K-546K)
(b)
503K (486K-488K)
(f)
593K (570K-572K)
(c)
523K (505K-506K)
(g)
613K (578K-581K)
(d)
543K (524K-528K)
(h)
623K (606K-609K)
：Cube
：Brass
：Cu
：Goss
：S
RD
TD
Fig. 1
Orientation maps obtained at each temperature by the in-situ SEM/EBSP
analyses in a cold-rolled (CRed) foil.
Materials Science Forum Vols. 539-543
365
(a)
as-received
(e)
543K (535K-536K)
(b)
523K (500K-503K)
(f)
558K (547K-549K)
(c)
533K (517K-520K)
(g)
573K (573K-568K)
(d)
533K → Cooling
(h)
598K (588K-599K)
：Cube
：Brass
：Cu
：Goss
：S
RD
TD
Fig. 2
Orientation maps obtained at each temperature by the in-situ SEM/EBSP
analyses in a 17% additionally rolled foil after partial annealing at 503K (PA/ARed).
Changes in numbers of grains of each texture component as a function of temperature is displayed
in Fig. 4. In both samples, the total number of grains decreases gradually during microstructural
evolution. The number of cube grains in contrast increases steadily to keep a fixed level above 543K
for CRed sample. For PA/ARed sample, the number also increases until 543K, and then decreases
slightly. The increase in the number of cube grains for both samples in lower temperature range
means that numbers of cube grains appear from inside to surface as described above. Further, the
decrease in the number reveals that even growing cube grains can be consumed in the process of cube
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THERMEC 2006
100
80
(a)CR
60
(b)PA503K+AR17%
Cube
S
Cu
Brass
Goss
Fraction (%)
Fraction (%)
100
40
80
60
Cube
S
Cu
Brass
Goss
40
20
20
0
as-received
0
as-received 500
500
550
600
Temperature, T / K
550
600
Temperature, T / K
Fig. 3 Changes in fraction of main texture components as a function of
temperature in (a) CRed and (b) PA/ARed foils.
105
104
(a)CR
103
All
Cube
S
Cu
Brass
Goss
102
101
100
as-received
500
550
600
Temperature, T / K
Number of grain, N
Number of grain, N
105
(b)PA503K+AR17%
104
103
102
All
Cube
S
Cu
Brass
Goss
101
100
as-received
500
550
600
Temperature, T / K
Fig. 4 Changes in number of grains of main texture components as a function of
temperature in (a) CRed and (b) PA/ARed foils.
texture formation. The numbers of S- and Cu-oriented grains decrease with evolving cube texture in
both samples.
In order to investigate the reason why cube grains grow more preferentially in a PA/ARed sample,
a detailed in-situ EBSP analysis with a step size of 0.5μm was conducted to measure grain interior
misorientation or kernel average misorientation (KAM) [16]. KAM is defined for a given point as the
average misorientation of that point with all of its neighbors, which is calculated with the proviso that
misorientations exceeding a tolerance value of 5° are excluded from averaging calculation. From the
KAM value the density of geometrically necessary dislocation can be derived [17]. Thus, stored
energy can be evaluated by KAM.
Figure 5 illustrates distributions of KAM in cube, S and Cu grains of the PA/ARed sample at each
heating step. Comparing three orientations, the distribution of the cube grain shows a sharp peak at
the lowest misorientation angle among them. The misorientation of the KAM peak becomes higher in
order of cube, S and Cu grains. It is, therefore, understood that the stored energy of the cube grains is
lower than that of the other oriented grains, which results in retarding growth of the latter and
stimulating more preferential growth of the former. In addition, the KAM distributions shifted to the
right side as the temperature rose. This may be due to not only microstructural change but also
thermal expansion of the sample.
Materials Science Forum Vols. 539-543
367
Summary
Fraction(%)
25
20
15
10
(a)Cube
as-received
483K
503K
523K
533K
5
Fraction(%)
0
0.5
1.0
1.5
2.0
25
Kernel average misorientation (degree)
(b)S
20
as-received
483K
15
503K
523K
10
533K
5
0
Fraction(%)
In order to reveal how the cube grain has an
advantage in comparison with the other oriented
grains during microstructural evolution, the in-situ
SEM/EBSP analysis was performed in a pure
aluminum foil. The results obtained are
summarized as follows.
1. In the CRed sample, when the temperature
reached 503K, cube grains started to grow
preferentially to cover the sample. The grains
appearing from inside tended to grow and remain
finally.
2. In the PA/ARed sample, the cube grains
coarsened rapidly and preferentially at more than
533K in contrast to the other oriented small grains
remaining their sizes.
3. The cube grains in the CRed sample were larger
than those in PAed one.
4. The intragranular misorientation analysis
revealed that the stored energy of the cube grains
was lower than that of the other oriented grains,
which resulted in retarding growth of the latter and
stimulating more preferential growth of the
former.
0.5
1.0
1.5
2.0
Kernel average misorientation (degree)
25
as-received
(c)Cu
20
483K
503K
523K
15
533K
10
5
0
0.5
1.0
1.5
2.0
Kernel average misorientation (degree)
Fig. 5 Distributions of KAM for (a) Cube,
(b) S and (c) Cu orientated grains of the
PA/ARed sample at each heating step.
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THERMEC 2006
10.4028/www.scientific.net/MSF.539-543
In-Situ SEM/EBSP Analysis during Annealing in a Pure Aluminum Foil for Capacitor
10.4028/www.scientific.net/MSF.539-543.362
DOI References
[17] Y. Takayama and J.A. Szpunar: Mater. Trans., 45(2004) 2316-2325. 25 Fraction (%) 20 15 10 5 0.5 1.0
1.5 2.0 25ern el averag e misorien tation (d eg ree) K (b )S 20 as-received 483K 15 503K 523K 10 533K 0
(a)Cu b e as-received 483K 503K 523K 533K Fraction (%) Fraction (%) 5 0.5 1.0 1.5 2.0 K 25ern el averag e
misorien tation (d eg ree) as-received (c)Cu 20 483K 503K 523K 15 533K 10 5 0.5 1.0 1.5 2.0 Kern el averag
e misorien tation (d eg ree) 0 0 Fig. 5 Distributions of KAM for (a) Cube, (b) S and (c) Cu orientated grains
of the PA/ARed sample at each heating step.
10.2320/matertrans.45.2316