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High strength and good electrical conductivity in Cu–Cr
alloysprocessed 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,
Russiab National University of Science and Technology “MISIS”,
Laboratory of Hybrid Nanostructured Materials, Leninskii Ave. 4,
119049 Moscow, Russiac Department of Materials Physics, Eotvos
Lorand University, Pazmany Peter Setany 1/A., Budapest, Hungary
a r t i c l e i n f o
Article history:Received 19 March 2015Accepted 29 March
2015Available online 6 April 2015
Keywords:Cu–Cr alloysHigh pressure torsionUltrafine-grained
structureHardnessElectrical propertiesX-ray techniques
a b s t r a c t
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 ofgrain boundaries. The effect of heat-treatment after
HPT-processing on the hardness and the electricalconductivity was
studied for different chromium contents (0.75, 9.85 and 27 wt%).
For low Crconcentration (0.75%) the electrical conductivity
increased considerably above 250 1C, however thehardness decreased
concomitantly. At the same time, for high Cr content (9.85% and
27%) the hardnesswas only slightly reduced even at 500 1C, while
the electrical conductivity increased to a similar level asbefore
HPT due to grain boundary relaxation and decomposition of Cu–Cr
solid solution. Our studydemonstrates the capability of
SPD-processing and subsequent heat-treatment to achieve a
combinationof high strength and good electrical conductivity.
& 2015 Elsevier B.V. All rights reserved.
1. Introduction
Grain refinement via severe plastic deformation (SPD) is
aneffective tool for improving the mechanical performance of
chro-mium, zirconium and hafnium bronzes [1–8]. The formation
ofultrafine-grained (UFG) microstructures in copper–chromiumalloys
resulted in an improvement in durability, wear resistanceand
fatigue strength [2,9] which are important service propertiesin
their applications for resistance welding electrodes and switch-ing
devices [10]. One of the most effective SPD method in
grainrefinement is high pressure torsion (HPT) [1]. Former
papers[11,12] have shown that the application of HPT can reduce
thegrain size down to 10 nm, thereby achieving significant
hardeningin high-chromium alloys. At the same time,
SPD-processingusually yields the reduction of electrical
conductivity due to thehigh density of lattice defects, such as
grain boundaries. Tailoringthe microstructure by an additional
heat-treatment after SPD mayyield an improvement of electrical
conductivity while reservingthe high strength. Most probably,
chromium content in Cu has aconsiderable influence on the
effectiveness of these procedures inthe achievement of the optimal
properties. In this work, we demo-nstrate the capability of
HPT-processing and subsequent heattreatment in obtaining a
combination of high strength and good
electrical conductivity in chromium bronzes with different
chro-mium 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
andCu–9.85%Cr samples were subjected to hot forging at 800 1C,
thenthe forged specimens were heat-treated by two different
ways:(i) annealing at 1000 1C for 2 h and cooling in air to
roomtemperature (RT), and (ii) annealing at 1000 1C for 2 h and
waterquenching to RT. The samples processed by the first and
secondroutes are referred to as “annealed” and “quenched”
specimens,respectively. The Cu–27%Cr alloy was studied in the
as-cast state. Allthe samples with 10 mm in diameter and 0.6 mm in
thickness wereseverely deformed by HPT at RT and a rate of 1 rpm
under a pressureof 4 GPa. The deformation was performed in a
“groove” with thedepth of 0.2 mm. The final sample thickness was
0.3 mm. Thenumber of turns was 5 which corresponds to a true strain
ofεE4.8 at the half-radius of the disks. The HPT-processed
sampleswere heat-treated at temperatures ranging from 50 to 600 1C
with astep of 50 1C and a holding time of 1 h at each
temperature.
The microhardness was measured using a 402 MVD InstronWolpert
Wilson Instruments tester. The measurements were madeat a distance
of 2.5 mm from the sample center (i.e. at the
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Materials Letters
http://dx.doi.org/10.1016/j.matlet.2015.03.1440167-577X/&
2015 Elsevier B.V. All rights reserved.
n Corresponding author.E-mail address: [email protected]
(D.V. Shangina).
Materials Letters 153 (2015) 5–9
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half-radius of the disks). The resistivity was measured using a
BSZ-010-2 micro-ohmmeter at RT. The resistivity was calculated
andtransformed into electrical conductivity according to
InternationalAnnealed Copper Standards (IACS). The microstructure
was obser-ved using JEM-2100 transmission electron microscope. Thin
foils forelectron microscopy were prepared by ion polishing with a
GATAN600 unit. The crystallite size, the dislocation density and
the twin-fault probability were obtained by X-ray line profiles
using a high-resolution rotating anode diffractometer (Nonius,
FR591) withCuKα1 radiation (wavelength: λ¼0.15406 nm). The line
profileswere evaluated by the Convolutional Multiple Whole
Profile(CMWP) fitting method [13].
3. Results and discussion
Fig. 1 shows that HPT-processing yields significant hardening
inall alloys and the hardness increases with increasing Cr content.
Thiseffect can be explained by the higher dislocation density and
thesmaller grain size due to the pinning effect of chromium on
latticedefects, either these atoms are in solid solution or they
are inprecipitates. The grain structure in some selected specimens
isshown in Fig. 2 and the grain size values are listed in Table 1.
Theeffect of the initial treatment before HPT on the grain size
wasmarginal. In quenched Cu–0.75%Cr, Cu–9.85%Cr and Cu–27%Cr
alloysprocessed 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) 5–96
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40 nm, respectively. X-ray line profile analysis also revealed
that thelattice defect structure only slightly depends on the
initial state(annealed or quenched), while it is very sensitive on
the alloyingelement concentration (see Table 1). With increasing Cr
contentfrom 0.75% to 27% the crystallite (subgrain) size decreased
fromabout 60 nm to 36 nm, while the dislocation density and the
twin-fault probability increased from 38�1014 m�2 to 163�1014
m�2and 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 (especiallyfor initially quenched states) and increase of
the amount of grainboundaries and another lettice defects. Chromium
atoms in solidsolution reduce the electrical conductivity much more
stronglythan in the form of precipitates [5]. This effect yields a
higherelectrical 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),
thereforein the quenched states the solute Cr concentration cannot
exceed thisvalue even if the nominal Cr content is higher (e.g.
9.85%).
Heat-treatments were applied in order to improve the
electricalconductivity in the HPT-processed Cu–Cr alloys. The
change of thehardness and the electrical conductivity as a function
of the heat-treament temperature is shown in Fig. 1. It can be seen
that thehardness decreased or remained at the same level while the
electricalconductivity rised with increasing temperature. For both
initiallyannealed and the quenched Cu–0.75%Cr alloys processed by
HPTthe hardness decrease is marginal up to about 250 1С. This can
beexplained by the lack of considerable grain growth, as revealed
by thecomparison of Fig. 2a and b. These images show that in the
quenchedand HPT-processed Cu-0.75%Cr alloy the grain size increased
only
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 at500 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).
S.V. Dobatkin et al. / Materials Letters 153 (2015) 5–9 7
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from �209 to �245 nm due to the heat-treatment at 250 1С. Fig.
1bindicates that above 250 1С a significant increment in the
electricalconductivity took place through the supersaturated solid
solutiondecomposition (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-processedCu–9.85%Cr alloys is better than that for the
Cu–0.75%Cr compositionsince the higher Cr content has a stronger
retarding effect on graingrowth. After the heat-treatment at 500 1C
the grain size onlymoderately increased from �143 nm to �229 nm
(see Fig. 2c and d).
Therefore, the decrease of the hardness is marginal even athigh
temperatures while the electrical conductivity is
considerablyincreased, as shown in Fig. 1c and d. The strongly
improvedelectrical conductivity (by 38% IACS) in initially quenched
alloyafter HPT can be explained by chromium precipitation and
grainboundary relaxation.
Fig. 2e reveals the appearance of a dispersed Cr particle with
thesize of about 20 nm. In the annealed and HPT-processed alloy
grainboundary relaxation during heat-treatments has a major effect
onelectrical conductivity, while it has only marginal influence
onhardness.
It has been shown that the specific resistance of grain
bound-aries in Cu decreases from 5.5�10�16Ωm2 to 2�10�16 Ωm2due to
grain boundary relaxation during annealing [14]. Consider-ing the
grain boundary relaxation and the slight grain growth at500 1C an
electrical conductivity increase of about 13%IACS ispredicted which
is in accordance with the experimental resultsfor annealed and
HPT-processed sample.
For HPT-processed Cu–27%Cr alloy considerable softening was
notobserved up to 500 1C (see Fig. 1e) since the grain structure
remainedvery fine. As revealed by the TEM images in Fig. 2f–h the
heating upto 500 1C leads to an increase in the grain size from 40
to 96 nm. Atthe same time, at 500 1C the electrical conductivity
increased to thelevel characteristic for the as-cast state (see
Fig. 1f).
Reducing the level of electrical conductivity down to
20%IACSduring HPT indicates a possible occurrence of
deformation-inducedsupersaturated solid solution. Thus, a further
increase in electricalconductivity upon heating occurs both due to
decomposition of solidsolution, as well as grain boundary
relaxation and moderate grain-growth.
Table 2 summarizes the hardness and the electrical conductiv-ity
values in the initial and the HPT-processed states, and
afterheat-treatments at 250 1C for the Cu–0.7%Cr alloy, and at 500
1Cfor the Cu–9.85%Cr and Cu–27%Cr alloys. It can be seen that
HPT-processing and subsequent heat-treatment at appropriate
tem-peratures in Cu-alloys with high Cr content yields better
hardnessand electrical conductivity than in the initial states.
4. Conclusions
1. HPT-processing in copper-chromium alloys leads to a
signifi-cant hardening due to the formation of UFG
microstructure.With increasing chromium content the microhardness
risesfrom about 1700 to 2700 MPa due to the reduction in
averagegrain size from �209 to �40 nm as well as the increase of
thechromium content, the dislocation density and the
twin-faultprobability. The electrical conductivity is reduced with
increas-ing Cr content due to the higher amount of grain
boundariesand Cr alloying atoms.
2. The heat-treatment after HPT results in a gradual decrease
ofthe hardness and an increase in electrical conductivity. For
highCr contents (9.85% and 27%) an appropriate selection of
theheat-treatment temperature enables the preservation of thehigh
hardness while the electrical conductivity increased con-siderably.
The significant improvement in the electrical con-ductivity can be
explained by Cr precipitation and grainboundary relaxation. Our
study demonstrates the capability ofHPT-processing and subsequent
heating for obtaining bothhigh hardness and electrical conductivity
in Cu–Cr alloys.
Acknowledgment
The work was supported by the Russian Foundation for
BasicResearch (Project 13-08-00102), the Ministry of Education
andScience of the Russian Federation (Project no. 14.A12.31.0001)
andthe Hungarian Scientific Research Fund, OTKA, Grant no.
K-109021.
References
[1] Valiev RZ, Zhilyaev AP, Langdon TG. Bulk nanostructured
materials: funda-mentals and applications. Hoboken: TMS, Wiley;
2014.
[2] Vinogradov A, Ishida T, Kitagawa K, Kopylov VI. Effect of
strain path onstructure and mechanical behavior of ultra-fine grain
Cu–Cr alloy produced byequal-channel angular pressing. Acta Mater
2005;53(8):2181–92.
Table 2Electrical conductivity and microhardness of the
alloys.
Alloy Treatment Initial state HPT HPT and heatingn
HV (MPa) %IACS
HV (MPa) %IACS
HV (MPa) %IACS
Cu–0.7%Cr
Quenching 804738 39 1740774 34 1830736 35Annealing 1000736 86
1612738 61 1435730 72
Cu–9.85%Cr
Quenching 1270765 37 2140722 29 1753744 67Annealing 12687104 63
2107787 54 18927137 76
Cu–27%Cr
As-cast 1402779 41 2698790 20 26387109 42
n The heating temperatures are 250 1C for the Cu–0.7%Cr alloy,
500 1C for theCu–9.85%Cr and Cu–27%Cr alloys.
Table 1The 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).
Alloy Treatment Grain size (nm) 〈x〉area (nm) ρ (1014 m�2) β,
(%)
Cu–0.7%Cr Quenching þHPT 209 5876 3874 070.1AnnealingþHPT – 6476
3874 070.1
Cu–9.85%Cr QuenchingþHPT 143 5277 7577 0.370.1AnnealingþHPT –
4775 7178 0.370.1
Cu–27%Cr HPT 40 3675 163710 1.270.1
S.V. Dobatkin et al. / Materials Letters 153 (2015) 5–98
http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref1http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref1http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref2http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref2http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref2
-
[3] Wang QJ, Xu CZ, Zheng MS, Zhu JW, Du ZZ. Fatigue crack
initiation lifeprediction of ultra-fine grain chromium–bronze
prepared by equal-channelangular pressing. Mater Sci Eng A
2008;496(1-2):434–8.
[4] Shangina DV, Bochvar NR, Dobatkin SV. The effect of alloying
with hafnium onthe thermal stability of chromium bronze after
severe plastic deformation. JMater Sci 2012;47(22):7764–9.
[5] Shangina DV, Gubicza J, Dodony E, Bochvar NR, Straumal PB,
NYu Tabachkova,et al. Improvement of strength and conductivity in
Cu-alloys with theapplication of high pressure torsion and
subsequent heat-treatments. J MaterSci 2014;49(19):6674–81.
[6] Dopita M, Janecek M, Kuzel R, Seifert HJ, Dobatkin S.
Microstructure evolutionof CuZr polycrystals processed by
high-pressure torsion. J Mater Sci 2010;45(17):4631–44.
[7] Mishnev R, Shakhova I, Belyakov A, Kaibyshev R. Deformation
microstructures,strengthening mechanisms, and electrical
conductivity in a Cu–Cr–Zr alloy.Mater Sci Eng A
2015;629:29–40.
[8] Dobatkin SV, Shangina DV, Bochvar NR, Janeček M. Effect of
deformationschedules and initial states on structure and properties
of Cu–0.18% Zr alloyafter high-pressure torsion and heating. Mater
Sci Eng A 2014;598:288–92.
[9] Purcek G, Yanar H, Saray O, Karaman I, Maier HJ. Effect of
precipitation onmechanical and wear properties of ultrafine-grained
Cu–Cr–Zr alloy. Wear2014;311(1–2):149–58.
[10] Lamperti A, Ossi PM, Rotshtein VP. Surface analytical
chemical imaging andmorphology of Cu–Cr alloy. Surf Coat Technol
2006;200(22–23):6373–7.
[11] Sauvage X, Jessner P, Vurpillot F, Pippan R. Nanostructure
and properties of aCu–Cr composite processed by severe plastic
deformation. Scr Mater 2008;58(12):1125–8.
[12] Bachmaier A, Rathmayr GB, Bartosik M, Apel D, Zhang Z,
Pippan R. Newinsights on the formation of supersaturated solid
solutions in the Cu–Crsystem deformed by high-pressure torsion.
Acta Mater 2014;69:301–13.
[13] Ribárik G, Gubicza J, Ungár T. Correlation between strength
and microstruc-ture of ball-milled Al–Mg alloys determined by X-ray
diffraction. Mater Sci EngA 2004;387–389:343–7.
[14] Qian LH, Lu QH, Kong WJ, Lu K. Electrical resistivity of
fully-relaxed grainboundaries in nanocrystalline Cu. Scr Mater
2004;50(11):1407–11.
S.V. Dobatkin et al. / Materials Letters 153 (2015) 5–9 9
http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref3http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref3http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref3http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref4http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref4http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref4http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref5http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref5http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref5http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref5http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref6http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref6http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref6http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref7http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref7http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref7http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref8http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref8http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref8http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref9http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref9http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref9http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref10http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref10http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref11http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref11http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref11http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref12http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref12http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref12http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref13http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref13http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref13http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref14http://refhub.elsevier.com/S0167-577X(15)00538-8/sbref14
High strength and good electrical conductivity in Cu–Cr alloys
processed by severe plastic deformationIntroductionMaterials and
methodsResults and
discussionConclusionsAcknowledgmentReferences