Materials Transactions, Vol. 52, No. 11 (2011) pp. 2022 to 2026
#2011 The Japan Institute of Metals
Formation of Nanotwins in Cold-Rolled Cu-Zn Alloy by Electric Current Pulses
Wenbin Dai1 , Xinli Wang1; * , Lin Zhao2 and Jingkun Yu1
1
2
School of Materials and Metallurgy, Northeastern University, Shenyang 110004, P. R. China
China First Heavy Industries, Qiqihaer 161042, P. R. China
A significant number of nanotwins was fabricated in a cold-rolled Cu-Zn alloy after an electric current pulse (ECP) treatment in this study.
Results showed that the formation of nanotwins was related not only to the to phase transformation, but also to the effect of the enhanced
directional nucleation along the current direction during the heating caused by the ECP treatment. A theoretical analysis revealed that the
nucleation rate of the higher conductivity phase ( phase) was significantly accelerated by the application of an electric current, which provided
more nucleation places for the phase during the subsequent rapid cooling process. Especially, during the cooling process, nanotwins with
{111} twin planes in the phase would be formed due to the stronger internal stress induced by the rapid cooling rate after the ECP
treatment. [doi:10.2320/matertrans.M2010139]
(Received April 19, 2010; Accepted August 18, 2011; Published September 28, 2011)
Keywords: electric current pulses, nanotwins, nucleation rate, rapid cooling
Because of the blocking of the motion of dislocations and
the retaining of the strain hardening capability, twins with
nanoscale lamellar always lead to the unusual combination of
high tensile strength and ductility.1–4) With the formation of
high density nanoscaled growth-in twins, Lu et al.1) obtained
a pure copper with a tensile strength above 1 GPa and Zhang
et al.2) achieved a 330 stainless steel thin film with a hardness
of about 6.5 GPa.
Up to now, high density nanoscaled twins could be formed
via chemical or physical depositions, such as pulsed electrodeposition,1) magnetron-sputter deposition,2) surface mechanical attrition treatment3) and dynamic plastic deformation.4) Recently, Zhang et al. and Zhou et al. reported that
nanoscaled phases could be obtained in a coarse-grained
alloy by electric current pulse (ECP) treatment.5–7) Our
previous study also showed that inclusions in a Cu-Zn alloy
were refined by the ECP treatment.8) In the present study, the
effects of ECP on the formation of nanotwins in a cold-rolled
Cu-Zn alloy are described and the possible formation
mechanism is discussed.
2.
Experimental
A 30% reduction in the thickness direction of a cold-rolled
commercial Cu-Zn two-phase alloy sheet with a composition
of Cu 59.4 mass% and Zn 40.6 mass% was selected as the
research object in this study. The crystal structure of the
phase is f.c.c. (A1) with a crystal lattice constant of
a ¼ 0:3696 nm, the phase is b.c.c. (A2) and the 0 phase is a
simple cubic (CsCl-type structure, B2) type structure with a
lattice constant of a ¼ 0:2948 nm. As known, due to the low
stacking fault energy in the f.c.c. crystal, twins can form in
the f.c.c. crystal but hard to form in the b.c.c. crystal.
By using the electrospark discharge technique, samples of
3 mm length, 2 mm width, and 1 mm thick were prepared and
treated under different conditions, i.e., (a) as-worked, (b) by
traditional heat treatment of water quenching after heating
*Corresponding
author, E-mail: wangxl520@hotmail.com
(a)
Control
circuit
Trigger
circuit
Sample
Charge
220V circuit
Switch
Capacitor
banks
Copper
electrodes
Current
probe
To
oscilloscope
(b)
16
-2
Introduction
Current density (KA·mm )
1.
8
0
-8
-16
0.0
0.2
0.4
0.6
0.8
Time, t/ms
Fig. 1 (a) Schematic of experimental arrangement for ECP treatment,
(b) typical waveform of ECP.
at 1023 K for 2 min, and (c) by ECP treatment at a current
density of 18.6 kAmm2 . As schematically shown in
Fig. 1(a), the ECP treatment was performed under ambient
conditions through the discharge of capacitor banks, and the
two ends of each sample were put into copper electrodes
during this process. The waveform of the ECP was detected
to be a damped oscillation wave using a Rogowski coil and
a TDS3012 digital storage oscilloscope (Tektronix Inc.,
Beaverton, OR). As revealed in Fig. 1(b), the period of the
ECP (tp ) was about 110 ms and the pulse duration (td ) was
about 800 ms.
The phase composition of the samples was examined by Xray diffraction (XRD) with Cu K radiation. The morphology
of the samples was characterized by an optical microscope,
Formation of Nanotwins in Cold-Rolled Cu-Zn Alloy by Electric Current Pulses
2023
(a)
α(220)
β'(110)
α(111)
β’ phase
β'(211)
α phase
β'(200)
β'(111)
α(200)
Intensity (a.u.)
ECP treated
Quenched
As-worked
40°
50°
60°
70°
Diffraction angle, 2θ/deg.
Fig. 2
50μm
80°
(b)
β’ phase
XRD results of samples treated under different conditions.
α phase
scanning electron microscope (SEM, JSM-6301F; JEOL,
Tokyo, Japan) equipped with energy dispersive spectroscopy
(EDS), and high resolution transmission electron microscopy
(TEM, JEM-2010, JEOL, Tokyo, Japan). Samples for the
TEM observations were prepared by mechanical grinding
and electrochemical polishing at about 263 K.
3.
Results
50μm
Figure 2 shows the XRD patterns of the samples. The
intensities of the phase and 0 phase in the as-worked
sample and water-quenched sample were identical, and the
phase composition of these two samples had no obvious
differences. For the ECP-treated samples, it was noted that
the intensity of the phase had increased, and a wider
half-width of the 0 phase was obtained. Hence, a phase
transformation must have occurred during the ECP process.
Figure 3 shows images of the samples observed using an
optical microscope in which the white and black areas are the
and 0 phase, respectively. Obviously, the as-worked
sample shown in Fig. 3(a) consisted of an elongated phase
and 0 phase. In comparison, the elongated grains remained
and the microstructure had not obviously changed for the
water-quenched sample shown in Fig. 3(b). It was very
interesting to find in Fig. 3(c) that the elongated grains
became irregular and fragmentation occurred in the elongated phase even when the ECP treatment was as short as
800 ms. A further EDS determination revealed that the
statistical average chemical composition of the as-worked
sample was Cu 62.8 mass%, Zn 37.2 mass% (white areas),
and Cu 55.9 mass%, Zn 44.1 mass% (black areas), and that in
the ECPed sample was Cu 61.1 mass%, Zn 38.9 mass%
(white areas), and Cu 57.4 mass%, Zn 42.6 mass% (black
areas). Hence, a significant diffusion occurred during the
ECP treatment.
Figure 4 shows the microstructures of the samples obtained by TEM. The grains of the as-worked sample shown in
Fig. 4(a) were coarse, only a few twins could be found,
and an identical microstructure could be seen in the waterquenched sample (Fig. 4(b)). Interestingly, a significant
difference was observed in the ECP-treated sample and
(c)
α phase
β’ phase
10μm
Fig. 3 Optical microstructures of samples treated under different conditions (a) as-worked sample, (b) water-quenched sample, and (c) ECPed
sample.
many nanoscaled twin lamellas were formed in the phase
grains (Fig. 4(c)). Through the corresponding selected area
electron diffraction pattern (SAED), it is known that these
nanotwins had a {111}/[112] twin relationship for a facecentered cubic structure. The statistical analysis of many
images showed that the nanotwins had a thickness of 5–
40 nm and a length of 50–500 nm. Figure 4(d) shows a
HRTEM image of the ECP-treated sample observed along
the ½11 0 directions. The straight twin boundary revealed
that nanotwins were formed through the typical growth
process, and the phase transformation occurred during the
ECP treatment.
2024
W. Dai, X. Wang, L. Zhao and J. Yu
(a)
300nm
(b)
(d)
(111) M
(111) T
(111)
2nm
300nm
Fig. 4 Bright-field TEM images of (a) as-worked sample, (b) water-quenched sample, (c) ECPed sample and the corresponding selected
area electron diffraction (SAED) pattern, and (d) HRTEM image of the twin boundary (marked by the black arrow).
Discussion
Figure 5 shows a Cu-Zn binary equilibrium phase diagram.9) The lines A and C represent the chemical composition of the phase and 0 phase in an as-worked sample, and
the lines B and D represent that of the phase and 0 phase of
the ECP-treated sample. In addition, the chemical composition of the alloy is marked by the straight line H. Apparently,
the original phase can completely transform into the phase when the temperature increases to 1003 K. According
to the phase diagram, it is known that the transition
temperature from ! decreases in the phase, while
the transformation temperature from ! increases in the
phase with the increasing of Zn. In the present study, the
temperature rise in the ECP-treated sample is induced by
Joule heating and it is dominated by the energy input of the
ECP. According to the wave form and the current density
(18.6 kAmm2 ), the temperature of the ECP-treated sample
reaches 1023 K, which is higher than the phase transforma-
A B H
D C
1250
L
36.8
1175K
1150
Temperature (K)
4.
1050
1003K
950
850
β
α
750
727K
650
550
25
30
β'
38.9
42.6
37.2 40.6 44.1
35
40
45
Content of Zinc, wt /mass%
Fig. 5
Cu-Zn binary phase diagram.
50
55
Formation of Nanotwins in Cold-Rolled Cu-Zn Alloy by Electric Current Pulses
tion temperature (1003 K). According to the change in the
composition detected by EDS, a drastic diffusion between the
phase and 0 phase occurred during the ECP treatment,
while the 0 $ phase transformation belongs to the orderdisorder transformation at 727 K, which is ineffective for
grain refinement. In addition, due to the cold-rolled Cu-Zn
alloy ahead of the ECP treatment, many defects are inevitably
formed in the matrix. Since the resistivity in the area with
defects is higher than that in the area without defects, a nonhomogeneous temperature rise is generated. As known, the
activation energy of nucleation of a second phase on a
dislocation decreases even more rapidly with the increasing
thermodynamic driving force than does the nucleation energy
for homogeneous nucleation, and becomes zero at a finite
value of the supersaturation.10) Therefore, the evolution
mechanism of the structure is ascribed to the $ solidstate phase transformation during the ECP treatment.
As for the change in nucleation caused by the ECP electric
current, Dolinsky and Elperin found that the passing of an
electric current in a conductor should promote the formation
of a phase with a higher conductivity and prevent the
formation of a phase with a lower conductivity.11,12) Through
the investigation of the effect of the ECP on the grain size
during casting, Qin et al. proposed that the passing of an
electric current with a density of 10 Amm2 could produce
a one-order-of-magnitude decrease in the casting grain
size.13–15) The theoretical model developed by Qin et al.
can not only be used to deal with the liquid-solid phase
transformation during solidification, but it can also be
employed to deal with the solid state phase transformation.7)
When an electric current passes through a conductor able to
promote a phase transform, it is assumed that a spherical phase nucleus would be formed in the phase. The energy
change in the system U can be written as11–15)
U ¼ U0 þ Ue
ð1Þ
where U0 is the free-energy change in a current-free
system, and Ue is the energy change caused by deformation
of the current lines which can be given as follows11,13)
Ue ¼ gða; RÞð1 ; 0 Þj2 ðtÞV
0 1
ð0 ; 1 Þ ¼
1 þ 20
ð2Þ
ð3Þ
where is the magnetic susceptibility ( ¼ 0 , 0 is
magnetic susceptibility
in a vacuum),
g is a geometric factor,
2
5
gða; RÞ ¼ 32 ln Ra 65
R
,
R
is the radius of the
48
48
spherical -matrix approximately equal to the average
thickness of sample C, a 80 nm is the critical radius of
the phase nucleus, V is the volume of a nucleus, and is a
factor determined by the conductivity () of the nucleus and
the medium. In the temperature region of the to phase
transformation, ¼ 0:25 108 1 m1 > ¼ 0:15 108 1 m1 ,16) which gives ð ; Þ ¼ 0:182 < 0. For
the above ideal model, Ue 23400 Jmol1 < 0 from
eq. (2); this means that the passing of an electric current is
more beneficial to the phase transformation from the to phase. Furthermore, it implies that the higher the conductivity of the phase, the more negative Ue is, and the
thermodynamic barrier for the to phase transformation
significantly depends on . This result further tells us that
2025
the thermodynamic barrier (Ue ) for this phase transformation significantly depends on . Therefore, ECP can
enhance the nucleation rate of the higher conductivity phase
( phase in present) through decreasing the thermodynamic
barrier during the phase transformation. In other words, such
phase nuclei would be preferably formed by directional
nucleation on the highly conductive crystalline planes along
the current direction. It is expected that these phase nuclei
are hardly able to fully grow in so short an ECP application
(heating-up time). In fact, Zhang et al. observed that in the
coarse-grained Cu-Zn alloy subjected to the ECP treatment,
a nanophase of about 11 nm was formed.17) In our previous
study,8) the diffusivity of the Pb inclusions in a Cu-Zn alloy
was significantly enhanced due to the ECP application.
Based on the above theoretical analysis and the corresponding experimental results, it is reasonably deduced that
diffusive phase transformation is induced in the ECP heating
treatment.
To clarify the effect of the cooling behavior of the ECP on
the phase transformation ! , a one-dimensional transient
heat-transfer analysis is applied based on the supposition that
the temperature distribution is the same in the effective part,
and the initial temperature is 293 K. Therein, the equation of
the heat transfer is as follows:
Q ¼ dcp @T=@t r ðkrTÞ
3
ð4Þ
3
where d ¼ 8:4 10 kgm
is the density, cp ¼ 378
Jkg1 K1 is the specific heat, k ¼ 125 Wm1 K1 is the
thermal conductivity, T is the temperature, and Q is the heat
source, which can be given by
Q ¼ j2 ðtÞ
8
ð5Þ
where ¼ 6:2 10 m is the electrical resistivity, and
jðtÞ is the amplitude of the electric current density. The
cooling rate variation with time is plotted in Fig. 6 and the
maximum cooling rate is about 8:0 103 Ks1 which is
higher than that of the water quenching (102 –103 Ks1 ). This
rapid cooling process after ECP treatment leads to a higher
internal stress that promotes the to phase transformation
in the Cu-Zn alloy.18) As known, the relationship between the
habit planes during the transformation from b.c.c. to f.c.c.
ensures a ð111Þ½1 1 2 shear in the f.c.c. corresponding to
a f110gh110i shear in the b.c.c.18) Therefore, it can be
Fig. 6 Simulated cooling rate of ECP-treated sample.
2026
W. Dai, X. Wang, L. Zhao and J. Yu
concluded that the ð111Þ½1 1 2 nanotwins in the phase
should originate from the phase nuclei directionally
nucleated on the {110} planes. In this process, the not-fully
grown and sub-micrometer-sized phase grains would be
transformed into phase nanotwins. In addition, since the
formation of twins is accompanied by a decreasing free
energy, and very low energies, < 0:1 Jmm2 are achieved
for the twins on {111},19) the potential reduction in free
energy for the twins is far in excess of that for any other type
of boundary. Consequently, many nanotwins on {111} in
the phase are generated from the refined phase nuclei.
5.
Conclusion
The main mechanism of the formation of nanotwins can be
ascribed to a two effects caused by the ECP; one effect is the
nucleation rate of a new phase with a higher conductivity
along the electric direction, which is enhanced by the
decreasing thermodynamic barrier in a current-carrying
system, and the other is the stronger internal stress induced
by the rapid cooling rate, which is high enough to accelerate
the phase developed from the phase nuclei directionally
nucleated on the {110} planes. Combined with the low
energy for the formation of twin boundaries, many nanotwins
are formed. Therefore, the ECP treatment is a new method
to prepare nanotwin materials.
Acknowledgement
This study was supported by the National Nature Science
Foundation of China, Grant No. 50901018. The authors thank
Prof. J. D. Guo and Mr. B. Q. Wang of the Institute of Metal
Research, Chinese Academy of Sciences, for their help with
the ECP treatment experiments.
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