APPLIED PHYSICS LETTERS 91, 211901 共2007兲
High strength and high electrical conductivity in bulk nanograined Cu
embedded with nanoscale twins
Y. Zhang, Y. S. Li, N. R. Tao,a兲 and K. Lub兲
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, Shenyang 110016, China
共Received 8 October 2007; accepted 1 November 2007; published online 20 November 2007兲
A bulk nanograined Cu sample embedded with nanoscale twins is produced by means of dynamic
plastic deformation at cryogenic temperatures. It exhibits a tensile yield strength of 610 MPa and an
electrical conductivity of 95% IACS at room temperature. The unique combination of a high
strength and a high conductivity is primarily attributed to the presence of a considerable amount of
nanoscale twins which strengthen the material significantly while having a negligible influence on
electrical conductivity. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2816126兴
Mechanical strength and electrical conductivity are the
most important properties of conducting materials. However,
high strength and high electrical conductivity are mutually
exclusive in materials strengthened via conventional approaches such as strain hardening, alloying, or grain refinement. Trade-off between strength and conductivity is always
encountered in developing conducting materials.1 A recent
investigation2 indicated that introducing a high density of
nanoscale twins 共NTs兲 in an ultrafine-grained 共UFG兲 Cu may
significantly strengthen Cu while keeping its high electrical
conductivity. A high tensile strength 共⬃1 GPa兲 combined
with a high electrical conductivity 关97% International Annealed Coppper Standard 共IACS兲兴 was achieved when the
average twin/matrix 共T / M兲 lamellar thickness is as small as
15 nm in Cu thin foils produced by means of pulsed
electrodeposition.2
However, such a unique strength-conductivity combination could only be achieved in thin foil Cu samples, of which
technological applications are limited. Synthesis of bulk Cu
samples with NTs becomes a challenge. In this letter, we
report our success in processing bulk nanograined Cu embedded with a considerable amount of NTs by means of dynamic plastic deformation 共DPD兲 at cryogenic temperatures,
which exhibits an excellent combination of a high strength
and a high electrical conductivity.
A copper cylinder 共18 mm in diameter and 25 mm in
height兲 with a purity of 99.995% was processed by means of
DPD at liquid nitrogen temperature 共LNT兲. The setup and
parameters of the DPD processing were described in detail in
Ref. 3. The basic principle of the DPD process is that the
sample is compressed at very high strain rates 共⬃103 s−1兲
and low temperatures, which ensure that the dislocation recovery is effectively suppressed. Under such circumstances,
the critical stress for twinning is ready to be exceeded.
Hence, deformation twinning 共instead of dislocation activities兲 becomes the dominant plastic deformation mechanism.
The Cu cylinder was deformed into a disk with a total true
strain of 2.1, where the strain is defined as ␧ = ln共L0 / L f 兲 共L0
and L f are the initial and the final thicknesses of the treated
sample, respectively兲. The final sample dimension is about
50 mm in diameter and 3 mm in thickness. The deformation
a兲
Electronic mail: nrtao@imr.ac.cn.
Electronic mail: lu@imr.ac.cn.
b兲
is fairly uniform and the as-processed sample is free of
cracks. Measurements showed that the density and purity of
the DPD Cu samples are identical to those of the original
ones.
Figure 1共a兲 is a typical cross-sectional transmission electron microscopy 共TEM兲 image of the bulk DPD Cu, which
shows a mixed microstructure of nanograins and a considerable amount of twins. The T / M lamellar thickness is in the
nanometer regime. Nanoscale T / M lamellae form bundles of
several hundred nanometer to several micrometer thick and
micrometer long, most of which are roughly aligned perpendicular to the loading direction. Detailed observations revealed that most twin boundaries 共TBs兲 are stepped or
curved, typical characteristics of boundaries for deformation
twins at which a high density of dislocations exists, as shown
in Fig. 1共b兲. The statistical measurements show that the volume fraction of twin bundles is about 35% and the average
T / M lamellar thickness is about 44± 2 nm 关see Fig. 1共d兲兴.
Most nanograins are elongated with an aspect ratio of ⬃2.5
and some grains are with diffuse grain boundaries 共GBs兲 关as
shown in Fig. 1共c兲兴. The histogram of the overall grain size
distribution shows an average grain size 共in short axis兲 of
66 nm 关Fig. 1共e兲兴.
Formation of a considerable amount of NTs in the DPD
Cu sample is a direct consequence of the suppression of dislocation activities during the plastic deformation at high
strain rates and low temperatures. The deformation twinning
becomes a dominant mechanism for plastic deformation. In
addition, formation of NTs is crucial for the formation of
nanograins. As described in Ref. 3, most of the nanograins
are derived from the fragmentation and shear banding of the
nanoscale T / M lamellae, while a fraction of nanograins is
evolved from dislocation cells which are associated with a
high density of dislocations.
The tensile tests show that the bulk DPD Cu sample
exhibits an impressive yield strength of 610± 10 MPa, which
is about one order of magnitude higher than that of coarse
grained 共CG兲 Cu. The elongation to failure is about 8%. It
should be noted that the yield strength of the DPD Cu is
significantly higher than those ever achieved for bulk Cu
processed by means of severe plastic deformation 共SPD兲 at
room temperature 共usually below 400 MPa兲.4 It is also
higher than that of Cu processed by cold drawing at LNT, in
which a small volume fraction of twin bundles were
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91, 211901-1
© 2007 American Institute of Physics
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211901-2
Zhang et al.
Appl. Phys. Lett. 91, 211901 共2007兲
FIG. 2. 共Color online兲 A plot of room-temperature electrical conductivity vs
yield strength for SPD Cu and Cu alloys 共in shaded regime at the lower-left
side兲, with the data points for the DPD and DPD+ CR Cu samples of the
present work. For comparison, literature data for nanocrystalline Cu,
ultrafine-grained Cu are also included.2,5,7,15
FIG. 1. 共Color online兲 A typical cross-sectional microstructure of the DPD
Cu sample with ␧ = 2.1: 共a兲 the overview microstructure; detailed observations of 共b兲 T / M lamellae and 共c兲 nanograins; statistical distributions of 共d兲
T / M lamellar thickness and 共e兲 grain size determined from a number of
TEM images.
identified.5 Additional cold rolling 共CR兲 of the SPD Cu at
LNT can barely elevate the strength to about 460 MPa.6
Electrical conductivity of the DPD Cu was measured on
strip specimens with a cross-sectional area of 0.5⫻ 1 mm2
and a length of 12 mm using the standard four-probe technique at ambient temperature. At least five specimens were
measured to obtain an average value. The average conductivity value is 共0.55± 0.01兲 ⫻ 108 ⍀−1 m−1, which is about
95共±2 % 兲 IACS. It is noteworthy that electrical conductivity
of the DPD Cu is slightly lower than that of the CG Cu
共0.595⫻ 108 ⍀−1 m−1兲 and is comparable to that of the UFG
Cu processed via SPD 共e.g., equal channel angular pressing
or cold rolling兲, as plotted in Fig. 2. The conductivity of the
DPD Cu is slightly lower than that of the UFG Cu thin foils
with NTs 共0.571⫻ 108 ⍀−1 m−1兲.2 However, the conductivity
of DPD Cu is significantly higher than the reported values of
pure Cu samples with nanoscale grain sizes. The nanograined Cu produced by ball milling exhibited a similar
strength with our present sample, but a much lower electrical
conductivity 共51% IACS兲.7
It is known that alloying Cu with other elements, such as
Cr, Fe, Ni, etc., can elevate strength to a level of
500– 700 MPa but accompanied with a significant loss in
conductivity. The electrical conductivities of the corresponding Cu alloys range from 50% to 80% IACS. By plotting
yield strength versus electrical conductivity of the SPD Cu
and Cu alloys, as shown in Fig. 2, one may see that the data
points fall in the shade regime at the lower-left side. Distinctly, the DPD Cu sample stands out from this regime,
representing a superior strength-conductivity combination.
Comparison with Cu– Cr– X alloys 共widely used as conductors兲, the DPD Cu exhibits a similar strength but a much
higher conductivity.
The high strength of the DPD Cu is in part a consequence of the extremely refined nanograins. Previous investigations indicated that the classical Hall-Petch relation is
valid for nanograined Cu with grain size as small as 10 nm.8
In terms of this, a yield strength of about 455 MPa is expected for an average grain size of 66 nm. TBs are also effective in blocking dislocation motions, hence strengthening
the materials like conventional GBs.2,9,10 A yield strength of
about 831 MPa was estimated for the deformation twins with
an average T / M lamellar thickness of 44 nm.11 Following
the rule of mixture, a total yield strength of the DPD Cu
sample is estimated to be about 590 MPa, which agrees reasonably with our measurement strength 共610 MPa兲. Obviously, the strengthening effect of NTs is predominant in the
present sample relative to the grain size effect, although the
volume fraction of NTs is lower than that of the nanograins.
The slight drop in electrical conductivity of the DPD Cu
relative to the CG Cu originates from the presence of plenty
of interfaces including GBs and TBs, as indicated in the
nanocrystalline Cu and the Cu samples deformed at cryogenic temperatures.5,7 The intrinsic GB resistivity of Cu is
closely related to the GB structures and energy state and can
vary from 0.5⫻ 10−7 to 2.5⫻ 10−7 n⍀ m2 with GB misorientations increasing from low to high angles.12 The electrical
resistivity of coherent TB is usually taken as half of the
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211901-3
Appl. Phys. Lett. 91, 211901 共2007兲
Zhang et al.
specific stacking fault resistivity, i.e., 1.5⫻ 10−8 n⍀ m2 in
Cu,13 which is much lower than those of conventional GBs.
Recent systematical investigations14 showed that the numerous dislocations accumulated at the TBs have a negligible
influence on the specific TB resistivity. Consequently, the
contribution of deformation TBs to the overall electrical resistivity should be approximately in the order of 0.1 n⍀ m in
the present sample.
In the present sample, characteristics of the GBs are
complicated. First, GB misorientations in the present sample
vary from low to high angles, the volume fraction of which
is difficult to be determined due to the nanoscale grain size.
Second, parts of the nanograins are derived from the fragmentation of T / M lamellae, of which misorientations
slightly deviate from the exact ⌺3 relationship by a few
degrees.3 However, these parts of GBs are expected to still
possess a lower resistivity compared with the conventional
GBs.14 For simplicity, it is supposed that the specific GB
resistivity varies in the same range as the above mentioned,
then the total GB contribution to the electrical resistivity
should be between 0.89 and 4.4 n⍀ m. In comparison with
GBs, the contribution of TBs to the electrical resistivity is
negligibly small, though the TB area per unit volume is comparable to that of the GBs. In other words, the decrease of
electrical conductivity in the DPD Cu compared with the
bulk CG Cu primarily resulted from the presence of a large
number of GBs. In terms of the above analysis, the conductivity drop in the DPD Cu owing to GBs is estimated to be
about 2%–18%. Considering the measurement uncertainties
in grain sizes and the GB characteristics, the predicted value
is in an acceptable agreement with the experimental result.
In order to verify the contribution of NTs and nanograins
to strength and electrical conductivity, the DPD Cu sample
was subjected to an additional CR with a rolling strain of
50% at ambient temperature 共the rolling strain is defined as
␧ = 共Si − S f 兲 / S f ⫻ 100%, in which Si and S f are the crosssectional areas of the sample before and after CR兲, and details about the CR are in Ref. 11. After CR, the volume
fraction of NTs decreases to about 24% primarily due to the
fragmentation of NTs into nanograins. Meanwhile, the volume fraction of nanograins becomes larger, and the average
grain size increases slightly to about 90 nm. In addition, GB
relaxation occurred during rolling and some GBs exhibited
sharper contrasts. Tensile tests showed that the DPD+ CR Cu
exhibits a yield strength of 545 MPa, which is lower than
that of the DPD Cu. The electrical conductivity at room temperature is about 97% IACS. The yield strength decrease is
directly resulted from the reduction of the volume fraction of
NTs. In terms of the rule of mixture, the predicted strength
reduction is about 55 MPa, which is consistent with the mea-
sured data. The majority of the strength drop originates from
the decreasing volume fraction of twin bundles while the
slight increase in grain size plays a minor role.
Upon CR, the volume fraction of nanograins increases
with a slight increase in grain size while the volume fraction
of NTs decreases. Hence, the slight increase in electrical conductivity after CR can be reasonably attributed to GB relaxation. These results suggest that the electrical conductivity in
the DPD Cu is determined by the amounts and characteristics
of GBs while the TBs play a minor role.
In summary, a bulk nanograined Cu sample with embedded NTs was produced using the DPD processing at LNT,
which exhibits a unique combination of high strength and
high electrical conductivity. The high strength originates
from the extremely refined nanostructure 共NTs and nanograins兲, in which the NTs play a predominant role in
strengthening. The high electrical conductivity is mainly attributed to the high density of TBs with extremely high specific electrical conductivity. The present work demonstrates
clearly that introducing a high density of NTs into bulk Cu is
an effective approach to elevate strength without sacrificing
its high electrical conductivity.
Financial supports from the National Natural Science
Foundation of China 共Grant Nos. 50021101, 50431010,
50671106, and 50071061兲 and the Ministry of Science and
Technology of China are acknowledged.
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