Full text of DOI:10.1557/JMR.2002.0173

Effect of silver on superplastic deformation in
YBa2Cu3O7−x/Ag composites
Jondo Yun
Division of Advanced Materials Engineering, Kyungnam University, Masan, 631-701, Korea
Ye T. Chou and Martin P. Harmer
Department of Materials Science and Engineering and Materials Research Center, Lehigh University,
Bethlehem, Pennsylvania 18015
(Received 15 August 2001; accepted 25 February 2002)
Superplastic deformation was studied in fine-grained (0.7–1.1 ␮m) YBa2Cu3O7−x/Ag
composites containing 2.5–25 vol% Ag. The compression tests were conducted in
the temperature range of 750–875 °C and at strain rates of 10−5 to 10−3/s. For the
YBa2Cu3O7−x/25%Ag composites with grain size of 0.7–1.1 ␮m, deformed at
800–850 °C and 10−5 to 10−3/s, the stress exponent, grain size exponent, and the
activation energy of deformation were 2.0 ± 0.1, 2.5 ± 0.7, and 760 ± 100 kJ/mol,
respectively. These values were the same as those of the pure YBa2Cu3O7−x, indicating
that the deformation of the composite was controlled by that of the rigid YBa2Cu3O7−x
phase. However, the strain rate was increased by the addition of silver as explained by
the soft inclusion model of Chen. The dependence of the flow stress on the silver
content was in close agreement with the prediction of the model.
I. INTRODUCTION
Recently, a large deformation over 100% was ob-
served in YBa2Cu3O7−x/Ag composites both in compres-
sion and tensile mode at high temperatures by Yun,
Albuquerque, Chou, and Harmer.7–11 These investigators
showed that the composites with submicron grain size
have a great potential to exhibit superplasticity. How-
ever, the superplastic behavior of the composites was not
fully examined and understood in those works.
In this study, the superplastic nature of the composites
was investigated by high-temperature compressive test-
ing at constant strain rates, by the determination of de-
formation parameters, and by microstructural analysis.
The deformation mechanism will be described in refer-
ence to that of the pure YBa2Cu3O7−x, and the role of
silver in the deformation behavior will be discussed.
The high critical temperature YBa2Cu3O7−x supercon-
ductors and metallic silver are nonreactive and form a
composite with a duplex microstructure composed of
a matrix phase of YBa2Cu3O7−x and large pockets
of silver.1 The superconductivity of this composite is as
good as or better than that of pure YBa2Cu3O7−x. Al-
though its chemical and physical properties are well
understood,2–4 there is limited information on creep
and high-temperature plastic deformation, probably
due to its brittleness despite the presence of the ductile
silver phase.
Routbort et al. investigated the high-temperature de-
formation behavior of the YBa2Cu3O7−x/Ag composites
containing 15–30 vol% Ag at temperatures between 830–
900 °C.5 They found that the stress exponent and activa-
tion energy were about one and 800 kJ/mol respectively,
II. EXPERIMENTAL PROCEDURES
and were the same as those of the pure YBa2Cu3O7−x
.
Hendrix et al. studied the high-temperature deformation
of the composite with 20 vol% Ag and observed a higher
ductility as compared to the pure YBa2Cu3O7−x. They
observed a yield drop and a low strain rate sensitivity and
concluded that there was no evidence of superplasticity
in this system. However, it should be noted that the grain
sizes of the samples used by both groups were about
15–25 ␮m. With such large grain sizes, the occurrence of
superplastic deformation would be highly restricted.
Powders of YBa2Cu3O7−x (Rhone Poulence, Inc.) and
2.5, 10, and 25 vol% of metallic silver (Johnson & Mat-
tey, 1–3 ␮m) were mixed and ground together. It was
processed in the same way as that for the pure
YBa2Cu3O7−x,
12 except for the sintering conditions. The
sintering temperature was 875 °C, and the sintering times
of 3, 6, 12, and 24 h were selected to prepare samples of
different grain sizes of YBa2Cu3O7−x phase. The details
of the sintering condition, grain size, and final density are
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J. Yun et al.: Effect of silver on superplastic deformation in YBa2Cu3O7−x /Ag composites
listed in Table I. For comparison, the pure YBa2Cu3O7−x
samples were also prepared by sintering at 875 °C
for 6 h.
The XRD analysis was performed to examine the
phase stability during the deformation. Figure 3 shows
the XRD patterns for samples before and after a 50%
deformation at 850 °C. Distinct peaks for YBa2Cu3O7−x
and Ag phases are seen, indicating the phase stabil-
ity during the deformation. The YBa2Cu3O7−x peaks
in the deformed samples showed those of tetragonal
structure due to the loss of oxygen during the tests per-
formed in air.
The sintered samples were machined and tested in
compression in the same manner as that for the pure
12
YBa2Cu3O7−x
.
They were tested at temperatures of
750, 775, 800, 825, 850, and 875 °C at a constant strain
rate ranging from 1.0 × 10−5/s to 1.0 × 10−3/s. A cali-
bration sample was placed near the test sample during the
test for comparison. The calibration sample was not
loaded but was kept under the same heating and cooling
cycle. Before the compression test was started, each
sample was kept at the assigned test temperature for 1.5 h
for the stabilization. Deformation parameters such as
stress exponent, grain size exponent, and activation en-
ergy of deformation, were determined from the plot of
the applied stress versus strain rate, grain size, or the
temperature. When determined, the applied stress was
always measured at 4% true strain for each test. The
x-ray diffractometry (XRD) and density measurement
were performed for the selected samples. The as-
sintered, calibration, and deformed samples were then
characterized by the light optical microscopy (LOM),
scanning electron microscopy (SEM), and transmission
electron microscopy (TEM).
FIG. 1. Stress–strain curves for YBa2Cu3O7−x /Ag composites (25F).
In curve 775 °C, the load drop near 0.2 strain was due to the readjust-
ment in the stroke range of the test unit.
III. RESULTS AND DISCUSSION
A. Deformation of 25 vol% Ag composites
Similar to pure YBa2Cu3O7−x, all composite samples
with 25% Ag exhibited superplasticity except for those
tested at 750 °C. Figure 1 shows the stress–strain curves
at various testing conditions, and Fig. 2 shows the shape
change of the samples after deformation. When tested at
750 °C, the samples failed after a 5% strain with shear
cracks in the diagonal directions. At 775 °C, the sample
deformed up to 46% without surface cracks, but a yield
drop was observed at the beginning of the test. The de-
formation curve has a load drop near a 0.2 strain due to
the readjustment in the stroke range of the unit. At
800 °C or higher, the samples were sufficiently ductile to
deform up to 110% at a slightly increasing plateau stress.
TABLE I. Grain size of YBa2Cu3O7−x phase and density of the
YBa2Cu3O7−x /Ag composite samples sintered at 875 °C.
Sintering
time (h)
Grain
size (␮m)
Relative bulk
density (%)
%Ag
Sample
0
2.5
10
25
25
25
25
00FF
2.5F
10F
25FF
25F
6
6
6
3
6
0.54
0.65
0.72
0.69
0.75
0.99
1.09
70
82
85
87
90
91
92
25L
25LL
12
24
FIG. 2. Shape change of a YBa2Cu3O7−x /25%Ag composite sample
during deformation at 850 °C and a strain rate of 1.0 × 10−4/s.
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J. Yun et al.: Effect of silver on superplastic deformation in YBa2Cu3O7−x /Ag composites
The deformation parameters for the following Eq. (1)
were determined from the slopes of the curves shown in
the graphs (Figs. 4–7):
The stress exponent (n) was measured to be 2.0 ± 0.1
in the temperature range of 800–850 °C and in the grain
size range of 0.7–1.1 ␮m. The grain size exponent (p)
was determined to be 2.5 ± 0.7 at 850 °C in the same
grain size range. The activation energy determined
from the data of 0.8-␮m-grained samples was 760 ±
100 kJ/mol in the temperature range of 800–850 °C. The
invariance of the parameters in the range studied indi-
cated the existence of a unique deformation mechanism.
In determining the grain size exponent, the porosity
effect had been taken into account because of the change
in porosity with the grain size of the tested samples. The
porosity would increase the effective stress. The effective
n
d⑀
dt
␴
ր
RT
סA eמQ
,
(1)
dp
where d⑀/dt is strain rate, A is a constant, ␴ is stress, d is
grain size, n is the stress exponent, p is the grain size
exponent, Q is the activation energy for the deformation,
T is temperature, and R is the gas constant.
FIG. 5. Log–log plot of applied stress versus strain rate for the sample
of different grain sizes at 850 °C. Note that all plots show the same
slope (LL ס 1.09 ␮m, L ס 0.99 ␮m, F ס 0.75 ␮m, FF ס 0.69 ␮m).
FIG. 3. XRD patterns of the YBa2Cu3O7−x /25%Ag composite
samples before and after 50% deformation at 850 °C.
FIG. 4. Log–log plot of applied stress versus strain rate at 800 and
850 °C. The slope determines the stress exponent in the rate equation.
FIG. 6. Log–log plot of effective stress versus grain size. The slope
determines the grain size exponent in the rate equation.
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J. Yun et al.: Effect of silver on superplastic deformation in YBa2Cu3O7−x /Ag composites
stress ␴ was determined by the equation, ␴ ס e3.8P
eff
eff
␴a, where P is the porosity of the sample and ␴ is the
a
applied stress.9,13,14 On the other hand, the stress expo-
nent and the activation energy were independent of the
porosity, as they were measured from the samples with
the same porosity.
For the details of the grain structures of the deformed
samples, a SEM study was performed. Figure 8 shows
the SEM micrographs from the cross-sections of the
samples before and after the 50% deformation at 850 °C.
In this case, there was no significant change in grain
morphology and no distinct texture was detected. How-
ever, after 110% deformation at 875 °C, a considerable
amount of a grain elongation and alignment was ob-
served, as shown in Fig. 9.
As shown in Fig. 10, there was no intergranular liquid
phase at the grain boundary, indicating no contribution
from the liquid phase. However, a thin layer of unknown
phase was observed at the interface between
YBa2Cu3O7−x and Ag. Since the average size of the sil-
ver pockets was about 10 times larger than the grain size
of YBa2Cu3O7−x, the fraction of the interface area to the
total boundary area was very small. Therefore, the phase
at the interfaces may not be attributed to the deformation
of large scale. Dislocations were not observed in the
silver phase. Since the test temperature was high in com-
parison with the melting point of silver (approximately
0.9 Tm), the deformation of silver may be dominated by
diffusion rather than the movement of dislocations. The
deformation map of silver constructed by Ashby also
indicated the predominant role of diffusional creep when
the grain size is 5–10 ␮m, flow stress is 1–10 MPa, and
FIG. 8. SEM micrographs of the YBa2Cu3O7−x /25%Ag composite
samples (a) before and (b) after 50% deformation at 850 °C.
FIG. 7. Arrhenius plot of applied stress versus inverse temperature.
The slope determines the activation energy for the deformation.
FIG. 9. SEM micrograph of an YBa2Cu3O7−x /25%Ag composite
sample after 110% deformation at 875 °C.
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J. Yun et al.: Effect of silver on superplastic deformation in YBa2Cu3O7−x /Ag composites
the temperature is about 0.9 Tm.15 If diffusion predomi-
nates the creep, one would not observe dislocations in-
side the samples.
A constitutive equation was constructed by use of de-
termined parameters as
silver inclusion in the composite is shown in Fig. 11. The
solid line is the result of theoretical calculations based on
the soft inclusion model by Chen,16 who provided a so-
lution to the non-Newtonian fluid mixture containing a
volume fraction Vi of incompressible spherical soft in-
clusions dispersed in a hard matrix. According to his
analysis, the stress exponent n should not change, but the
flow stress ␴ would decrease with increasing Vi, given by
the equation,
2
d⑀
dt
␴
ր
RT
= 63.4
e−760,000
,
(2)
d2.5
where d⑀/dt is in s−1, ␴ in Pa, d in m, Q in J/mol, T in K,
and R is 8.3144 J/mol/K.
n
d⑀
dt
␴
͑
͒
− 53n+7 36
ր
= 1 − V ͒
␣
,
(3)
͑
The determined parameters of the 25% Ag composite
are almost identical to those of the pure YBa2Cu3O7−x, as
listed in Table II. This suggests that the deformation
mechanism for the composite is the same as that for the
pure oxide; i.e., grain boundary sliding (GBS) with ac-
commodation by diffusion.12 Because the rigid
YBa2Cu3O7−x matrix forms a skeletonlike structure, its
deformation may control the overall deformation of the
composite.
ͩ ͪ
i
␴
o
where ␣ is a reference strain rate, and ␴ a reference
o
stress. Based on this theory, the exponent of (1 − Vi) is
calculated to be −3.1. Experimentally, the exponent de-
termined by a least-square method from the data points
was −3.0, in close agreement with the theoretical value.
The observation of the identical stress exponents in both
pure YBa2Cu3O7−x and the composite is also consistent
with the theory.
Figure 12 shows the stress–strain curves for tested
samples containing different amounts of silver. It is noted
that the strain hardening decreases with an increase in
silver content. In general, the stress increases with strain
when there is a grain growth, densification, or an increase
in the dislocation density. Since no dislocations were
observed, they would not have played a role in strain
hardening. On the other hand, the grain size was found to
increase during the deformation. Table III lists the values
of grain size of as-sintered (ds), calibration (dc), de-
formed samples (dd), and the estimated value (de) which
B. Effect of silver content on
deformation behavior
The flow stress of the composite containing silver was
found to be lower than that of pure YBa2Cu3O7−x of the
same grain size and porosity. This softening effect of
FIG. 10. TEM micrograph of a grain boundary in the YBa2Cu3O7−x
/
Ag sample showing lattice fringes from the neighboring YBa2Cu3O7−x
grains. Note the absence of intergranular liquid phase.
TABLE II. Deformation parameters of pure YBa2Cu3O7−x and its
composites (n: stress exponent, p: grain size exponent, and Q: acti-
vation energy for deformation). Note that the parameters of two ma-
terials are almost the same.
n
p
Q (kJ/mol)
FIG. 11. Change in flow stress with silver content in the YBa2Cu3O7−x
/
YBa2Cu3O7−x
YBa2Cu3O7−x /25%Ag
2.0 ± 0.3
2.0 ± 0.1
2.7 ± 0.7
2.5 ± 0.7
745 ± 100
760 ± 100
Ag composites deformed at 850 °C and 10− 4/s. The stress was nor-
malized with respect to the grain size.
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J. Yun et al.: Effect of silver on superplastic deformation in YBa2Cu3O7−x /Ag composites
(2) The deformation parameters were measured for
the 25% Ag composite with grain size, 0.7–1.1 ␮m, com-
pressed at 800 to 850 °C and 10−5 to 10−3/s. The stress
exponent, grain size exponent, and the activation energy
for deformation were 2.0 ± 0.1, 2.5 ± 0.7, and 760 ±
100 kJ/mol, respectively. These values were the same as
those of pure YBa2Cu3O7−x
.
(3) The deformation of the composite was controlled
by the rigid matrix of YBa2Cu3O7−x phase in the pres-
ence of the ductile silver phase.
(4) The composites were softened by silver inclu-
sion, as explained by the soft inclusion model of Chen.16
The dependence of flow stress on the silver content was
found to be in close agreement with the predictions of
the model.
ACKNOWLEDGMENTS
The authors wish to thank Dr. Wiederhorn for valu-
able discussions. They are also grateful to Dr. Wang,
Dr. J.M. Albuquerque, and Mr. D. Ackland for technical
assistance.
FIG. 12. Stress–strain curves for YBa2Cu3O7−x /Ag composite
samples with various silver contents. Note that strain hardening rate
decreases with an increase in Ag%.
REFERENCES
TABLE III. Grain size of as-sintered (ds), calibration (dc), and de-
formed samples (dd). The estimated grain size at the start of compres-
sion is de. The sample was deformed by 50% at 850 °C and 10−4/s. The
ratio of dd/de indicates the amount of grain growth during deformation.
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IV. CONCLUSIONS
(1) Superplastic deformation was observed in fine-
grained (0.7–1.1 ␮m) YBa2Cu3O7−x/Ag composites con-
taining 2.5 to 25 vol% silver in the temperature range of
775–875 °C and strain rates of 10−5 to 10−3/s. The de-
formation was measured from 25% to 110% without any
indication of failure.
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