9th International DAAAM Baltic Conference
"INDUSTRIAL ENGINEERING”
24-26 April 2014, Tallinn, Estonia
MICROMECHANICAL PROPERTIES AND ELECTRICAL
CONDUCTIVITY OF Cu AND Cu-0.7wt% Cr ALLOY
Kommel, L. Pardis, N. & Kimmari, E.
Abstract:
Microstructure,
electrical
conductivity
and
micromechanical
properties of industrial Cu and Cu0.7wt%Cr alloy after equal channel
angular pressing, hard cyclic viscoplastic
deformation and followed ageing treatment
were studied. The tensile strength of 430
MPa and hardness of 195HV0.05 were
arrived. The Young modulus was increased
to 125 GPa and the maximal electrical
conductivity for Cu was achieved 103%
IACS after hard cyclic viscoplastic
deformation and 94.5% IACS for Cu0.7wt% Cr alloy after ageing at 450 °C for
1 h. The Cu-0.7% Cr alloy have high
electrical conductivity at operation
temperatures from 260 °C to 550 °C while
the pure ultrafine grained Cu lost
hardness, wear resistance and conductivity
at 170 °C, respectively.
The fields for further development related
to UFG Cu and Cu-base alloys span a wide
range of applications [7-9] in the different
industries. Nevertheless, it is clear that for
electrical conduction the Cu-based alloys
with high electrical conduction properties,
suitable high wear resistance (WR) and
low coefficient of friction (COF) at
increased temperatures will be used [10]. To
date, the tribological properties of UFG Cu
[11, 12] have been studied. Unfortunately,
the results in [10-12] were inconsistent
because the wear tests were conducted
using different methods and as result the
properties are not comparable. The Cu-Cr
investigations are
described
alloys
hardening by annealing and softening by
deformation is studied [13-15]. In these
articles also the different mechanisms of
hardening
via
vacancy-assisted
deformation are presented. The goal of this
investigation is to study the mechanisms of
UFG or NC microstructures, mechanical,
physical and tribological properties
forming in pure Cu and Cu-0.7wt% Cr
alloy during ECAP and PCAP followed
HCV deformation [16] and ageing
treatment. In addition, we examine by
tension, nanoindentation [17, 18] and wear
testing the micromechanical properties in
the sample body and/or inside of wear
tracks.
Key words: Ultrafine grained
microstructure, Electrical conductivity,
Micromechanical properties, Young
modulus, Copper.
1. INTRODUCTION
By using of severe plastic deformation
(SPD) techniques like equal channel
angular pressing (ECAP), parallel channel
angular pressing (PCAP) or high pressure
torsion (HPT) is possible to change the
mechanical properties and microstructure
of plastically deformable metals and alloys
[1-3]. During the past two decades the
microstructure, mechanical and physical
properties stability of bulk ultrafine grained
(UFG) and nanocrystalline (NC) materials
are studied in large amount of papers [1-6].
2. EXPERIMENTAL
The commercially pure Cu and Cu-0.7wt%
Cr alloy were selected as test materials for
the study. The Cu samples were cut to
diameter of 15.6 mm and in length of 120
mm, and samples from Cu-Cr alloy were
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cut to square 12x12 mm and 120 mm in
length, respectively. The samples from Cu
were heat treated at 650 °C for 2h. The
samples from Cu were processed in PCAP
die with two intersection angles of 110°
and step-by-step decrease of channel
diameter from 16 to 15.5 and to 15 mm,
respectively. The samples were routed by
90º and pressing direction was changed to
180º after each pressing. The 12 passes
(ε vM ~ 22) were conducted at room
temperature. The Cu-0.7wt% Cr alloy was
cold water quenching from 1000 ºC (for
2h), respectively. The samples from Cu0.7wt% Cr alloy were subjected to ECAP
by the B c route with 6 passes (ε vM = 6.9) at
room temperature in the ECAP die with
channel intersection angle of Φ = 90 o and
Ψ = 0º in air with a pressing speed of 5 mm
s-1 [3]. From received rods were cut off
tension/compression test samples with test
part of 8 mm in diameter and 12 mm in
length. The extensometer base length was
10 mm. The samples were subjected to
HCV
deformation
for
20
tension/compression cycles at strain
amplitudes of ±0.05%, ±0.1%, ±0.5%,
±1%, ±1.5%, and ±2% at a low frequency
of 0.5 Hz, respectively. The Young
modulus was measured at least three-five
times between each series for 20 cycles at
constant
strain
amplitudes.
These
processed samples from Cu-Cr alloy were
subjected to heat treatment at temperatures
250-750 ºC (with step of 100 ºC) for 1 h
and some samples for 2h. The HCV
deformation [16, 18] technique was
conducted on the materials testing system
Instron-8516. The microhardness was
measured using a Mikromet-2001 tester
after holding for 12 s at a load of 50 and
100
g.
The
obtained
samples'
microstructure was studied by optical
(Nikon CX) and scanning electron (Zeiss
EVO MA-15) microscopy. For SEM
imaging
a
secondary
(SE)
and
backscattered (BE) electrons were used.
The accelerating voltage was 20 kV. The
micromechanical
properties
were
characterized using the nanoindentation
device of the NanoTest NTX testing center
(Micro
Materials
Ltd.).
The
nanoindentation was conducted for 49
indents on grinded and etched surface of
sample under load of 100 mN. The
tribological behavior under dry sliding
conditions was investigated before and
after ECAP, HCV deformation and heat
treatment to provide a comparison over a
range of material harness and to understand
their
influence
on
the
electrical
conductivity, COF and specific wear rate.
The dry sliding wear was studied using a
ball-on-plate tribometer (CETR, Bruker,
and UMT2) with a counterface ball of
alumina (Al 2 O 3 ) with a ball diameter of 3
mm. The tribological tests were conducted
at room temperature in air under normal
compression load of 100 g. The sliding
distance amplitude was 3 mm at a
frequency of 5 Hz, velocity of 20 mm/s,
testing time of 10 min and sliding distance
of 12 m for all samples. For wear volume
calculations, the cross-sectional area of the
wear tracks was measured by the Mahr
Pertohometer PGK 120 Concept 7.21. The
electrical conductivity was determined by
means of the Sigmatest 2.069 (Foerster),
according to NPL standards. The
measurements for different orientations at
frequency of 60 and 480 kHz on a
calibration area of 8 mm in diameter at
room temperature of 23.0±0.5°C and
humidity of 45±5 % with an uncertainty of
1% according to International Annealed
Copper Standard (IACS) in the National
Standard
Laboratory for Electrical
Quantities of Estonia were conducted.
3. RESULTS AND DISCUSSION
3.1. Microstructure
The SEM/EDS investigation of as-cast Cu0.7wt% Cr alloy show that it contain
porous and no-dissolved Cr particles with
measures not higher then one micrometer
(Fig 1a). During followed heat treatment at
1000 °C and ECAP for 6 passes by B c
route these inclusions measures were
decreased (Fig 1b) and after followed HCV
355
deformation dissolved via
diffusion into the Cu matrix.
Cr
atoms
The hardness of Cu-0.7wt% Cr alloy (in
as-cast condition and after heat treatment at
1000 ºC for 2h) was ~75HV 0.1 and
electrical conductivity was 40% IACS,
respectively (Fig 2). During ECAP the
hardness was increased up to 192HV 0.1 and
electrical conductivity to 74.16% IACS,
respectively. By this, during followed HCV
deformation the hardness was decreased
slightly to 187HV 0.1 and electrical
conductivity increased to 75.52% IACS,
respectively.
The optimal parameters of microhardness
and electrical conductivity were choosing
after heat treatment at temperatures in the
interval from 250 to 750 ºC with step of
100 ºC. The optimal ageing temperatures in
the interval of 250-500 °C for 1h were the
mean microhardness of 120±10HV 0.1 and
the mean electrical conductivity of
93.3±0.6% IACS. However, heating at
temperatures higher than 650 °C leads to
decreasing of the electrical conductivity
due to the decomposition of the
supersaturated solid solution [9]. The
increasing of heat treatment temperature to
650 ºC and 750 ºC leads to decrease in the
microhardness to 95HV 0.1 and to 78HV 0.1
and electrical conductivity to 87% IACS
and to 77% IACS, respectively.
The electrical conductivity of pure Cu was
decreased by increase of equivalent von
Mieses strain higher then ~4 εvM (Fig 3).
As is shown the electrical conductivity at
~11 εvM to ~14 εvM strain decrease
sharply and by cumulative strain increase
to ~22 εvM it was decreased to 58.95%
IACS measured at frequency of 60 kHz
and to 51.05% IACS measured at
frequency of 480 kHz, respectively. At the
frequency of 60 kHz the depth of measure
is 3-3.5 mm and at frequency of 480 kHz it
decreases to ~1 mm. This result shows that
dislocation density has very large influence
on the results of electrical conductivity
measure as the dislocations have obstacle
to electrons mowing in the material [6]. By
this, in other works [7, 9, 10] is shown that
the
electrical
resistivity/conductivity
depends on hardness of material and the
correlation is shown.
Fig 1. SEM/EDS micrograph of as-cast Cu0.7wt% Cr alloy with not-dissolved Cr
particles and porous (a), SEM (SE)
microstructures on cross-section of ECAP
processed by B c route for 6 passes (b) and
HCV deformed microstructure (c).
3.2. Mechanical and micromechanical
properties correlation with electrical
conductivity
356
cycle’s from 339.7 MPa to 265.3 MPa and
the Young modulus (Fig 4b) was sharply
decreased from 130 GPa to ~100 GPa at
these some strain amplitude. This result
depend on the test material grain size
before HCV deformation [16] and show the
phenomena of NC metals [14] as softening
by slight deformation. According to our
speculative opinion the Young modulus
was decreased (Fig 4b) as the interatomic
interaction was decreased by cumulative
strain or cycles number of HCV
deformation increase (see Fig 4a). But
these
standpoints
need
detailed
investigations in future.
The results of nanoindentation for UFG
Cu-0.7wt% Cr are presented in Fig 5. The
mean (from 49 indents measured under 100
mN load) nanohardness of sample N1 was
HN = 3.84 GPa and indentation modulus
Er = 538.5 GPA, respectively. The ageing
time increase to 2h (sample N2) leads to
decrease of hardness but increase of
indentation modulus.
Fig 2. Electrical conductivity and
microhardness of Cu-0.73wt% Cr alloy
dependence from ageing temperature.
Fig 3. Electrical conductivity of pure Cu
dependence from equivalent von Mieses
strain and testing frequency.
3.3. Strength and thermal stability of UFG
Cu and Cu-Cr alloy
The UFG pure Cu was lost your strength
properties at temperature of 170ºC. After
PCAP for 6 passes the strength was Rm =
430 MPa, after ageing treatment at
temperature of 170 ºC the Rm = 355 MPa
and at 200 ºC this parameter was decreased
to 264 MPa.
The results of HCV deformation show that
at strain amplitude increases (from ±0.1%
to ~± 1%) the stress amplitude and Young
module increase, respectively. At the
increased strain amplitudes of ±2% (Fig
4a) the stress amplitude decrease at 15
Fig 4. HCV deformation stress amplitude
curves of Cu-0.7wt.% Cr alloy for ±2% of
strain amplitude (a) and corresponding
curve show the Young modulus increase to
~133 GPa at lowest (up to ±1%) strain
amplitudes and decrease to ~100 GPa (b)
by strain amplitude increase up to ±2%.
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COF equal to 0.32 at the end of wear
testing. By this at the first stage of testing
the annealed Cu has slightly higher COF
equal to 0.38 and Cu-0.7wt% Cr alloy COF
was 0.57 at the end of wear testing. The
COF was higher for Cu-0.7wt% Cr alloys
as these have no dissolved hard Cr particles
in soft Cu matrix. By this in our
experiments the Cu-0.7wt% Cr alloys with
higher hardness have highest COF but
lowest specific wear rate.
Fig
5.
Nanohardness
(HN)
and
Young/elastic modulus (EM) of Cu-0.7Cr
alloy after 6 passes measured in crosssection of sample by nanoindentation.
Definitions: N1 - ageing at 450 °C for 1 h;
N2 – 450 °C for 2h; N3 – 450 °C for
2h+HCV; N4 – 550 °C for 1h; and N5 –
550 °C for 2h.
4. CONCLUSIONS
In this study the pure Cu and Cu-0.7wt%
Cr alloy the physical, mechanical and
micromechanical properties, electrical
conductivity, specific wear rate and COF
were studied in dependence on cumulative
strain and ageing temperature.
The UFG pure Cu (after ECAP+HCV
deformation)
have
best
electrical
conductivity (~103% IACS) and low COF
as well low specific wear rate but lowest
thermal stability by temperature increase
over 170 ºC.
The UFG Cu-0.7wt.% Cr alloy after ECAP
show highest microhardness, good wear
resistance but low electrical conductivity
(74.16% IACS). During followed HCV
deformation the electrical conductivity was
lightly increased (75.52% IACS).
The ageing treated at 250-550 ºC UFG Cu0.7wt% Cr alloy have highly stabile
mechanical, tribological and electrical
conduction (~94% IACS) properties and
will be used as material for electrical
energy industry.
The minimal indentation modulus Er = 437
GPa was measured for hardest HN 100 =
3.89 GPa Cu-0.7wt% Cr alloy (sample N3)
after HCV deformation. The ageing time
increase to 550 °C (sample N4 for 1h and
sample N5 for 2h) leads to increase of
hardness (from HN = 3.02 GPa to HV =
3.59 GPa) but decrease of indentation
modulus.
The specific wear rate and COF
measurements show their dependence from
chemical composition of sample material,
sample surface hardness as well material
surface softening/hardening during wear
testing. Results show that HCV deformed
sample in surface was hardened from
77HV 0.05 to 90HV 0.05 and on the wear
track surface from 115HV 0.05 to
126HV 0.05 , respectively. In this case the
wear track surface hardening was induced
by cyclic straining as result of sliding. The
specific wear rate was decrease from 0.3 to
0.07 mm3min-1, respectively. The minimal
specific wear rate has sample with
maximal hardness. By this, during cycling
(Fig 4a) at strain amplitude of ±2% the
stress amplitude and Young modulus (Fig.
4b) were sharply decreased as the
interatomic interaction was lowered [18]. At
that time the rate was increased. The
results show that the pure Cu has lowest
5. ACKNOWLEDGEMENTS
The work was supported by the Ministry of
Education and Science of the Estonia,
Project No. IUT19-29 and EU ERA.Net
RUS STPojects-219.
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7. CORRESPONDING ADDRESS
PhD. Lembit Kommel
Department of Materials Engineering,
Tallinn Univ of Technology, Estonia
Phone: +372 52 365 13
E-mail: lembit.kommel@ttu.ee
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