Materials Letters 153 (2015) 5–9
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Materials Letters
journal homepage: www.elsevier.com/locate/matlet
High strength and good electrical conductivity in Cu–Cr alloys
processed by severe plastic deformation
S.V. Dobatkin a,b, J. Gubicza c, D.V. Shangina a,b,n, N.R. Bochvar a, N.Y. Tabachkova b
a
A.A. Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii Ave. 49, 119991 Moscow, Russia
National University of Science and Technology “MISIS”, Laboratory of Hybrid Nanostructured Materials, Leninskii Ave. 4, 119049 Moscow, Russia
c
Department of Materials Physics, Eotvos Lorand University, Pazmany Peter Setany 1/A., Budapest, Hungary
b
art ic l e i nf o
a b s t r a c t
Article history:
Received 19 March 2015
Accepted 29 March 2015
Available online 6 April 2015
Ultrafine-grained (UFG) microstructures in Cu–Cr alloys were processed by high pressure torsion (HPT).
The improved hardness was accompanied by a reduced electrical conductivity due to the large amount of
grain boundaries. The effect of heat-treatment after HPT-processing on the hardness and the electrical
conductivity was studied for different chromium contents (0.75, 9.85 and 27 wt%). For low Cr
concentration (0.75%) the electrical conductivity increased considerably above 250 1C, however the
hardness decreased concomitantly. At the same time, for high Cr content (9.85% and 27%) the hardness
was only slightly reduced even at 500 1C, while the electrical conductivity increased to a similar level as
before HPT due to grain boundary relaxation and decomposition of Cu–Cr solid solution. Our study
demonstrates the capability of SPD-processing and subsequent heat-treatment to achieve a combination
of high strength and good electrical conductivity.
& 2015 Elsevier B.V. All rights reserved.
Keywords:
Cu–Cr alloys
High pressure torsion
Ultrafine-grained structure
Hardness
Electrical properties
X-ray techniques
1. Introduction
Grain refinement via severe plastic deformation (SPD) is an
effective tool for improving the mechanical performance of chromium, zirconium and hafnium bronzes [1–8]. The formation of
ultrafine-grained (UFG) microstructures in copper–chromium
alloys resulted in an improvement in durability, wear resistance
and fatigue strength [2,9] which are important service properties
in their applications for resistance welding electrodes and switching devices [10]. One of the most effective SPD method in grain
refinement is high pressure torsion (HPT) [1]. Former papers
[11,12] have shown that the application of HPT can reduce the
grain size down to 10 nm, thereby achieving significant hardening
in high-chromium alloys. At the same time, SPD-processing
usually yields the reduction of electrical conductivity due to the
high density of lattice defects, such as grain boundaries. Tailoring
the microstructure by an additional heat-treatment after SPD may
yield an improvement of electrical conductivity while reserving
the high strength. Most probably, chromium content in Cu has a
considerable influence on the effectiveness of these procedures in
the achievement of the optimal properties. In this work, we demonstrate the capability of HPT-processing and subsequent heat
treatment in obtaining a combination of high strength and good
n
Corresponding author.
E-mail address: shanginadaria@mail.ru (D.V. Shangina).
http://dx.doi.org/10.1016/j.matlet.2015.03.144
0167-577X/& 2015 Elsevier B.V. All rights reserved.
electrical conductivity in chromium bronzes with different chromium contents of 0.7%, 9.85% and 27%.
2. Materials and methods
Copper alloys containing 0.75%, 9.85% and 27% chromium (in wt
%) were chosen for the study. Before deformation Cu–0.75%Cr and
Cu–9.85%Cr samples were subjected to hot forging at 800 1C, then
the forged specimens were heat-treated by two different ways:
(i) annealing at 1000 1C for 2 h and cooling in air to room
temperature (RT), and (ii) annealing at 1000 1C for 2 h and water
quenching to RT. The samples processed by the first and second
routes are referred to as “annealed” and “quenched” specimens,
respectively. The Cu–27%Cr alloy was studied in the as-cast state. All
the samples with 10 mm in diameter and 0.6 mm in thickness were
severely deformed by HPT at RT and a rate of 1 rpm under a pressure
of 4 GPa. The deformation was performed in a “groove” with the
depth of 0.2 mm. The final sample thickness was 0.3 mm. The
number of turns was 5 which corresponds to a true strain of
ε E4.8 at the half-radius of the disks. The HPT-processed samples
were heat-treated at temperatures ranging from 50 to 600 1C with a
step of 50 1C and a holding time of 1 h at each temperature.
The microhardness was measured using a 402 MVD Instron
Wolpert Wilson Instruments tester. The measurements were made
at a distance of 2.5 mm from the sample center (i.e. at the
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S.V. Dobatkin et al. / Materials Letters 153 (2015) 5–9
half-radius of the disks). The resistivity was measured using a BSZ010-2 micro-ohmmeter at RT. The resistivity was calculated and
transformed into electrical conductivity according to International
Annealed Copper Standards (IACS). The microstructure was observed using JEM-2100 transmission electron microscope. Thin foils for
electron microscopy were prepared by ion polishing with a GATAN
600 unit. The crystallite size, the dislocation density and the twinfault probability were obtained by X-ray line profiles using a highresolution rotating anode diffractometer (Nonius, FR591) with
CuKα1 radiation (wavelength: λ ¼0.15406 nm). The line profiles
were evaluated by the Convolutional Multiple Whole Profile
(CMWP) fitting method [13].
3. Results and discussion
Fig. 1 shows that HPT-processing yields significant hardening in
all alloys and the hardness increases with increasing Cr content. This
effect can be explained by the higher dislocation density and the
smaller grain size due to the pinning effect of chromium on lattice
defects, either these atoms are in solid solution or they are in
precipitates. The grain structure in some selected specimens is
shown in Fig. 2 and the grain size values are listed in Table 1. The
effect of the initial treatment before HPT on the grain size was
marginal. In quenched Cu–0.75%Cr, Cu–9.85%Cr and Cu–27%Cr alloys
processed by HPT the average grain size values were 209, 143 and
Fig. 1. Temperature dependence of microhardness (a,c,e) and electrical conductivity (b, d, f) of Сu–0.7%Cr (a,b), Cu–9.85%Cr (c,d) and Cu–27%Cr (e,f) alloys.
S.V. Dobatkin et al. / Materials Letters 153 (2015) 5–9
7
Fig. 2. Microstructure of the quenched Сu–0.7%Cr alloy after HPT (а) and heat-treatment at 250 1С (b), quenched Сu–9.85%Cr alloy after HPT (c) and heat-treatment at
500 1С (d and e), as-cast Сu–27%Cr alloy after HPT (f and g) and heat-treatment at 500 1С (h). A nanosized Cr particle is shown in (e).
40 nm, respectively. X-ray line profile analysis also revealed that the
lattice defect structure only slightly depends on the initial state
(annealed or quenched), while it is very sensitive on the alloying
element concentration (see Table 1). With increasing Cr content
from 0.75% to 27% the crystallite (subgrain) size decreased from
about 60 nm to 36 nm, while the dislocation density and the twinfault probability increased from 38 1014 m 2 to 163 1014 m 2
and 0% to 1.2%, respectively.
The alloys after HPT have a reduced electrical conductivity,
mainly due to the presence of Cr atoms in solid solution (especially
for initially quenched states) and increase of the amount of grain
boundaries and another lettice defects. Chromium atoms in solid
solution reduce the electrical conductivity much more strongly
than in the form of precipitates [5]. This effect yields a higher
electrical conductivity of the annealed specimens compared to the
quenched samples. The solubility limit of Cr in Cu is about 0.75 wt%
at the temperature of the intial heat-treatment (1000 1C), therefore
in the quenched states the solute Cr concentration cannot exceed this
value even if the nominal Cr content is higher (e.g. 9.85%).
Heat-treatments were applied in order to improve the electrical
conductivity in the HPT-processed Cu–Cr alloys. The change of the
hardness and the electrical conductivity as a function of the heattreament temperature is shown in Fig. 1. It can be seen that the
hardness decreased or remained at the same level while the electrical
conductivity rised with increasing temperature. For both initially
annealed and the quenched Cu–0.75%Cr alloys processed by HPT
the hardness decrease is marginal up to about 250 1С. This can be
explained by the lack of considerable grain growth, as revealed by the
comparison of Fig. 2a and b. These images show that in the quenched
and HPT-processed Cu-0.75%Cr alloy the grain size increased only
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S.V. Dobatkin et al. / Materials Letters 153 (2015) 5–9
Table 1
The parameters of the microstructure obtained by TEM and X-ray line profile analysis: the grain size, the area-weigthed mean crystallite size (⟨x⟩area), the dislocation density
(ρ) and the twin boundary probability (β).
ρ (1014 m 2)
Alloy
Treatment
Grain size (nm)
⟨x⟩area (nm)
Cu–0.7%Cr
Quenching þHPT
Annealing þHPT
209
–
587 6
64 7 6
387 4
387 4
Cu–9.85%Cr
Quenching þ HPT
Annealing þHPT
HPT
143
–
40
527 7
477 5
367 5
757 7
717 8
1637 10
Cu–27%Cr
Table 2
Electrical conductivity and microhardness of the alloys.
Alloy
Treatment Initial state
HV (MPa)
Cu–0.7%
Cr
Cu–
9.85%
Cr
Cu–27%
Cr
Quenching 8047 38
Annealing 10007 36
Quenching 12707 65
Annealing 12687 104
As-cast
1402 7 79
HPT and heatingn
HPT
%
HV (MPa) %
HV (MPa)
IACS
IACS
39
86
37
63
1740 774
1612 738
2140 722
2107 787
41
2698 790 20
34
61
29
54
1830 7 36
14357 30
1753 7 44
1892 7 137
%
IACS
35
72
67
76
2638 7 109 42
β, (%)
07 0.1
07 0.1
0.3 7 0.1
0.3 7 0.1
1.2 7 0.1
Reducing the level of electrical conductivity down to 20%IACS
during HPT indicates a possible occurrence of deformation-induced
supersaturated solid solution. Thus, a further increase in electrical
conductivity upon heating occurs both due to decomposition of solid
solution, as well as grain boundary relaxation and moderate graingrowth.
Table 2 summarizes the hardness and the electrical conductivity values in the initial and the HPT-processed states, and after
heat-treatments at 250 1C for the Cu–0.7%Cr alloy, and at 500 1C
for the Cu–9.85%Cr and Cu–27%Cr alloys. It can be seen that HPTprocessing and subsequent heat-treatment at appropriate temperatures in Cu-alloys with high Cr content yields better hardness
and electrical conductivity than in the initial states.
n
The heating temperatures are 250 1C for the Cu–0.7%Cr alloy, 500 1C for the
Cu–9.85%Cr and Cu–27%Cr alloys.
4. Conclusions
from 209 to 245 nm due to the heat-treatment at 250 1С. Fig. 1b
indicates that above 250 1С a significant increment in the electrical
conductivity took place through the supersaturated solid solution
decomposition (for initially quenched state) and the grain growth
(for both initially annealed and the quenched states).
Fig. 1 shows that the thermal stability of the HPT-processed
Cu–9.85%Cr alloys is better than that for the Cu–0.75%Cr composition
since the higher Cr content has a stronger retarding effect on grain
growth. After the heat-treatment at 500 1C the grain size only
moderately increased from 143 nm to 229 nm (see Fig. 2c and d).
Therefore, the decrease of the hardness is marginal even at
high temperatures while the electrical conductivity is considerably
increased, as shown in Fig. 1c and d. The strongly improved
electrical conductivity (by 38% IACS) in initially quenched alloy
after HPT can be explained by chromium precipitation and grain
boundary relaxation.
Fig. 2e reveals the appearance of a dispersed Cr particle with the
size of about 20 nm. In the annealed and HPT-processed alloy grain
boundary relaxation during heat-treatments has a major effect on
electrical conductivity, while it has only marginal influence on
hardness.
It has been shown that the specific resistance of grain boundaries in Cu decreases from 5.5 10 16 Ω m2 to 2 10 16 Ω m2
due to grain boundary relaxation during annealing [14]. Considering the grain boundary relaxation and the slight grain growth at
500 1C an electrical conductivity increase of about 13%IACS is
predicted which is in accordance with the experimental results
for annealed and HPT-processed sample.
For HPT-processed Cu–27%Cr alloy considerable softening was not
observed up to 500 1C (see Fig. 1e) since the grain structure remained
very fine. As revealed by the TEM images in Fig. 2f–h the heating up
to 500 1C leads to an increase in the grain size from 40 to 96 nm. At
the same time, at 500 1C the electrical conductivity increased to the
level characteristic for the as-cast state (see Fig. 1f).
1. HPT-processing in copper-chromium alloys leads to a significant hardening due to the formation of UFG microstructure.
With increasing chromium content the microhardness rises
from about 1700 to 2700 MPa due to the reduction in average
grain size from 209 to 40 nm as well as the increase of the
chromium content, the dislocation density and the twin-fault
probability. The electrical conductivity is reduced with increasing Cr content due to the higher amount of grain boundaries
and Cr alloying atoms.
2. The heat-treatment after HPT results in a gradual decrease of
the hardness and an increase in electrical conductivity. For high
Cr contents (9.85% and 27%) an appropriate selection of the
heat-treatment temperature enables the preservation of the
high hardness while the electrical conductivity increased considerably. The significant improvement in the electrical conductivity can be explained by Cr precipitation and grain
boundary relaxation. Our study demonstrates the capability of
HPT-processing and subsequent heating for obtaining both
high hardness and electrical conductivity in Cu–Cr alloys.
Acknowledgment
The work was supported by the Russian Foundation for Basic
Research (Project 13-08-00102), the Ministry of Education and
Science of the Russian Federation (Project no. 14.A12.31.0001) and
the Hungarian Scientific Research Fund, OTKA, Grant no. K-109021.
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