〉〉 Electronics & Automotive Systems

Development of a High Stress Relaxation Resistance
Cu-Co-Si Alloy
Shunta Akiya*1, Shoichi Danjo*1, Takemi Isomatsu*2, Ryosuke Matsuo*1, Tatsuhiko Eguchi *1

The electrical contact materials such as connectors have been required to havea higher strength and a higher electrical conductivity because of the advance ofthe increase in current and the size reduction of electronic devices. In this study, we haveresearched the influence of the concentrations of Co and Si and the addition of Sn and Mg tothe strength, the electrical conductivity and the stress relaxation property of Cu-Co-Si alloys forthe development of an alloy with balanced properties including a stress relaxation property
which is important for connector materials. We have improved the balance of various properties
by increasing the concentrations of Co and Si and simultaneously adding Sn and Mg, thereby
developed a Cu-Co-Si based alloy with an excellent strength, electrical conductivity and stressrelaxation property.

ABSTRACTINTRODUCTION
1.1.INTRODUCTION
In recent years, the electric circuits in automobiles and inelectronic devices have been requiring an increase involtage and in current with a back ground where thereduction in size and in thickness of the connectors forautomobiles and electronic devices has been rapidly
advancing and also where the quick charge technology ofhybrid vehicles and electric vehicles has been spreadingand progress. Therefore, the copper alloys for connectors
highly require both high mechanical and high electric
properties in terms of the connection reliability and thesuppression of resistance heating.
In the past, Cu-Ni-Si alloys, i.e. Corson alloys havebeen used because they have an excellent balance of
mechanical and electrical properties. However, especiallyin the next generation connectors, the requirements forthe improvement of the electrical conductivity especiallybased on the uses are nearly exceeding the performanceof the Corson alloys. To meet such requirements, we havecommitted in improving the balance among the electricalconductivity, the strength and the bending workability,etc. by focusing on Cu-Co-Si alloys which are the promising copper alloys with a high strength and a high electrical conductivity1), 2).
However, the strength of the alloys has not exceededthe Corson alloys and need further improvements. To
improve the strength, an increase in the concentrations ofCo and Si are expected to be efficient based the priorresearch3). In addition, although a stress relaxation property is important in the operation of connectors, improvements in the stress relaxation property has not been
attempted in the research of the Cu-Co-Si alloys. On theother hand, while the stress relaxation properties improvein phosphor bronze and Corson alloys when the grains
become coarse4), 5), it is reported that the simultaneousachievement of the bending workability is difficult in
Cu-Co-Si alloys2). In the case of Corson alloys, the priorstudies show that the stress relaxation property improves
with the increase in the NiSi compound dispersed as asecond phase in the copper matrix or the addition of asmall amount of Sn or Mg5), 6).
Given the above, in order to develop a Cu-Co-Si based
alloy excellent in a stress relaxation property (development target: tensile strength≧700 MPa, in electricalconductivity≧50%IACS and in stress relaxation rate
(120 °C ×1000 hrs.) ≦10%), we have clarified the influence of the Co and Si concentrations and the addition ofa small amount of Si and Mg on various properties, anddeveloped a copper alloy with an excellent balance of theproperties, so we are reporting on these.

Automotive Products & Electronics Laboratories, R&D Division.	Electronics Component Material Division, Copper & High
Performance Material Production Division*1*2

Furukawa Electric Review, No. 50 201944〉〉 Electronics & Automotive Systems

Development of a High Stress Relaxation Resistance Cu-Co-Si Alloy

EXPERIMENTAL PROCEDURE
2.2. EXPERIMENTAL
PROCEDUREFirst, copper alloys with the compositions shown in Table 1were melted in the atmosphere using a high frequency
furnace and casted with a metallic mold. Next, they weresubjected to homogenization heat treatment at 1273 K for
30 minutes. After that, they were hot-rolled, subjected tomilling and cold-rolled. Finally, they were subjected tosolution heat treatment in a salt bath at 1148 to 1248 K
for 30 seconds and promptly cooled by water.
The solution heat treatment was conducted at the temperatures shown in Table 2 with reference to the solubility
curve temperature for each alloy stated in REFERENCES
7. Then, these alloys were subjected to an aging heat
treatment in an argon atmosphere at 798 K for two hours,
then cold-rolled (processing rate: Red.≧20) to be provided in the thin plate material with 0.16 mm of the plate
thickness. In addition, they were subjected to low temperature annealing in an argon atmosphere at 673 K for
30 minutes. Table 2 shows the experimental methods.
First, the grain structures of the thin plate material wereobserved with an optical microscope. After that, theseplates were thinned by electropolishing and the microstructures were observed with a transmission electron
microscope (TEM). The tensile strengths of the thin platematerial were measured using tensile tests (specimens ofTable 1

Chemical compositions of the samples.

JIS Z 2241-13B, rolling direction, crosshead speed
10 mm/m), and the electrical conductivities were measured (room temperature) with a four-terminal method(length of the measured terminal 100 mm). In addition,
strip samples of 10 mm in width and 70 mm in length andalso rolled in a longitudinal direction were processed sothat burrs on the end face do not appear, and they wereprovided as specimens. The stress relaxation tests werebased on the cantilever block method of the Japan
Copper and Brass Association Technical Standard (JCBA)
T309 (2004). The initial deflection displacements h0
shown in Figure 1 were calculated according to the formula (1) so that the load stress was 80% of the yieldstress which is the 0.2% of the yield strength, and theload was applied until time = maximum 1000 hours at theholding temperature Temp. = 393 K. The permanent
deflection displacements h1 at each time were measured
and the stress relaxation rates were obtained from theequation (2).
h0 = (σ × L)/(1.5 × E × t)(1)Stress relaxation rate = h1/h0 × 100(2)
(σ: load stress, E:Young’s modulus, t: plate thickness,
L: span length)

Table 2Elements
alloy

CoSi

Sn

mass% mass% mass%

Experimental method.

ProcessMg
at%mass%

at%

Cu

Alloy
123

456

Melting andcasting

－

789

1

0.95

0.24－

－－－

Bal.

2

0.95

0.24

0.15

0.08－

－

Bal.

Homogenization
heat treatment

1273 K × 30 min (In the atmosphere)

30.95

0.24

0.45

0.24－

－

Bal.

0.24－－0.95－

Hot rolling

40.10

0.26

Bal.

0.95

0.24－

－

5－

Milling

0.20

0.52

Bal.

Cold rolling

Red . ≧ 90%

60.95

0.24

0.15

0.08

0.10

0.26

Bal.

7

1.10

0.27

－

－

－

－

Bal.

Solution heat
treatment

8

1.10

0.27

0.15

0.08

0.10

0.26

Bal.

9

1.40

0.35

－

－

－

－

Bal.

10

1.40

0.35

0.15

0.08

0.10

0.26

Bal.

1198 K× 30 s1148 K × 30 s

101248 K
× 30 sAging heat
treatment

798 K × 2 hr (In an argon atmosphere)

Cold rolling

Red . ≧ 20%

Low temperatureannealing673 K × 30 min (In an argon atmosphere)

UnloadInitial Stress is 80%YSLTest piece

h0h1Condition
0-1000 hourat 393 KFigure 1 Schematic illustration of the stress relaxation test.

Furukawa Electric Review, No. 50 201945〉〉 Electronics & Automotive Systems

Development of a High Stress Relaxation Resistance Cu-Co-Si AlloyRESULT
ANDSTUDY
STUDY
3.3.RESULTAND
Effect of the Alloy Composition on the Strengthand the Electrical Conductivity
The thin plate material of the Alloys, having the compositions shown in Table 1, were manufactured through theprocesses from melting and casting to low temperatureannealing shown in the experimental method stated inTable2. The low temperature annealed material tensile
strengths and the electrical conductivities of the manufactured Alloy-1, -6, -7, -8, -9 and -10 were measured.
The measured data were plotted against the Co concentrations, respectively, and shown in Figure 2.

70Tensile strength,σB/ MPa

750
725

Alloy-6700

Alloy-1

625

Alloy-10

Alloy-1

Alloy-10

Alloy-7

Alloy-6600
0.91

Alloy-8

1.165
60

Alloy-8
Alloy-7675
650

Alloy-955

Alloy-9 50
As_σB
As+0.15Sn+0.1Mg_σB 45
As_EC
As+0.15Sn+0.1Mg_EC
40
1.2
1.3
1.4
1.5

Electrical conductivity, EC (%IACS)

3.1

Concentration of Co (mass%)
Figure 2 Changes in the tensile strengths and the electricalconductivities of the Alloy-1, -6, -7, -8, -9 and -10 after
annealing at 673 K vs the Co concentration.

Alloy-1

As the Figure 2 shows, the higher the Co concentration,
the higher the tensile strength of the low-temperatureannealed material. This is assumed to be because Coand Si added by the solution heat treatment are all dissolved in the matrix, and the added Co and Si are all in
the ideal states which can contribute to strengthening theprecipitation. Then, the increase in strength is caused dueto the fact that the larger the amount of Co and Si dissolved by the solution heat treatment, the larger theamount of the fine precipitates contributing to thestrength are generated at the aging heat treatment. Inaddition, the strength slightly increased due to the solid
solution strengthening of Sn and Mg.
In contrast, the electrical conductivity of the low temperature annealed material decreased as the amount ofthe added Co and Si increased. The assumed reason isthat the amount of Co and Si remaining in the matrix
increased after the aging heat treatment, in addition theelectrical conductivity decreased by adding Sn and Mg.
3.2 Effect of the Alloy Composition on theMicrostructure
Figure 3 shows the optical microscope images of the lowtemperature annealed materials. The average grain size ofthe samples created was about 10 to 20 µm. Here, Alloy3, -8 and -10 with Sn and Mg added also had almost thesame grain size. It is reported that the difference of thestress relaxation rates is about two to four percent within
the range of the grain size in a Cu-Ni-Si based alloy,
which is a precipitation type similar to a Cu-Co-Si basealloy5). Although it was assumed that the influence of thegrain diameter difference appeared in the stress relaxation property of the specimen used in this experiment,
the influence of the particle diameter was not taken into
consideration based on the assumption that the influence
was small.

Alloy-650 µm

Alloy-8

Alloy-750 µm

Alloy-950 µm

50 µm

Alloy-1050 µm

50 µmFigure 3 Optical microscope images of the specimens.

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Development of a High Stress Relaxation Resistance Cu-Co-Si Alloy

Figure 4 shows the TEM bright field images of the lowtemperature annealed Alloy-7 and -9. As a precipitation
concentration at aging heat treatment increases alongwith the increase in the amount of Co and Si solid solution, a large amount of the precipitation of the Co-Si compounds of about 10 nm2) were observed.
3.3 Effect of the Concentrations of Co and Si on theStress Relaxation Property
Figure 5 shows the results of the stress relaxation tests
for the low temperature annealed materials of the Alloy-1,-7 and -9. The stress relaxation rates increased with thetime duration in every case. Figure 6 is the graph plotting

the stress relaxation rates after testing for 25, 100 and
200 hours on the Co concentrations of the Alloy-1, -7 and-9. The stress relaxation rates decreased as the Co andSi concentrations increased. This is a kind of a creep
phenomenon caused by a stress relaxation phenomenon
generated by a movement of the dislocations6). And it isassumed, as shown in Figure 4, Alloy-9 which had a higher Co concentration than Alloy-7 had higher density ofthe precipitates in the Cu matrix. Then, as shown schematically in Figure 7, the stress relaxation property wasimproved because the precipitates inhibited the movement of the dislocation movements.

Alloy-7

Alloy-950 nm

50 nm
1800

400200Figure 4 TEM bright field images of the specimens andEDX spectrum of the arrowed compound inAlloy-9.

CuKa
CuKb600

CoKb800

CoKa

1000

SiKa

Counts1200

CuLl
CoLaCuLa

1400

Compounds at an arrow in Alloy-9.

CKa
OKa CoLl

1600

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
KeV4025201510
Alloy-1
Alloy-7
Alloy-9

5100

1000

Time, t / minFigure 5 Changes in the stress relaxation rates of the Cu-Co-Si
alloys vs the testing time.

Stress relaxation rate (%)

Stress relaxation rate (%)

300
1040Temp. = 393 K

3525 h100 h200 h

3530Temp. = 393 K

2520151050.8

0.9

11.1

1.2

1.3

1.4

1.5

Concentration of Co (mass%)
Figure 6 Changes in the stress relaxation rates after testing vsthe concentration of Co.

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Development of a High Stress Relaxation Resistance Cu-Co-Si Alloy

40
Stress relaxation rate (%)

Inhibition ofdislocation movement

Alloy-1

35

SnMg

30

Temp. = 393 K

25
20
15

Alloy-3

10

Alloy-4

Alloy-2

Alloy-5

500

0.1

0.2

0.3

0.4

0.5

0.6

Concentration of Sn or Mg (at%)
Figure 9Co-Si compound
Figure 7 Schematic illustration of the dislocation movement
inhibited by the Co-Si compounds.

3.4

Effect of the Addition of Sn and Mg on theStress Relaxation Property
Figure 8 shows the results of the stress relaxation tests
for the low temperature annealed materials of the Alloy-2,-3, -4, -5 and -10.
18
Stress relaxation rate (%)

16
14
1210

Alloy-2
Alloy-3
Alloy-4
Alloy-5
Alloy-10

Temp. = 393 K

864
2010

100
Time, t / min1000

Figure 8 Changes in the stress relaxation rates of the Cu-CoSi-Sn-Mg alloys vs the testing time.

Comparing the Alloy-1 in Figure 5 with the Alloy-2, -3,-4 and -5 in Figure 8, the addition of Sn or Mg onto theCu-Co-Si alloys lowered the stress relaxation rates andimproved the stress relaxation properties.
Figure 9 is a graph plotting the stress relaxation rates
after 200 hours of the stress relaxation tests on the Sn orMg concentrations.

Changes in the stress relaxation rates after testing for 200
hours vs the Sn or Mg concentrations.

The stress relaxation rate was remarkably lowered byadding 0.08at% (0.15mass%) in the case of Sn and
0.26at% (0.1mass%) in the case of Mg, but when theaddition was further increased, the changes in the stressrelaxation rates were little. In addition, when comparing at
the same atomic concentration, Mg has a higher effect ofimproving the stress relaxation property than Sn. It is presumed that in the Cu-Co-Si matrix, Mg forms a Cottrell
atmosphere, which will be described later, more than incase of Sn, but more detailed research is required. Inaddition, comparing the results of the Alloy-9 in Figure 5
and the Alloy-10 in Figure 8, simultaneous addition of Snand Mg improved the stress relaxation property morethan when they were separately added.
Regarding the influence of Sn and Mg on the stressrelaxation property as described above, a similar tendency has also been reported for a Cu-Ni-Si based Corsonalloy6). This is assumed to be because Sn (atomic radius:
1.41 Å) or Mg (1.60 Å)having a relatively large difference
in atomic radius from Cu (1.28 Å) as shown in Figure 10
has a large interaction with atomic vacancies and dislocations and inhibits dislocation movement by inhibiting
vacancy diffusion and forming a Cottrell atmosphere, andalso because Sn and Mg form clusters near the dislocation and their effect become larger6). It is presumed thatthe same mechanism works in the Cu-Co-Si based
alloys.

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Development of a High Stress Relaxation Resistance Cu-Co-Si Alloy

CONCLUSION
4.4.CONCLUSION

CuSnMgV⊥Inhibitionof vacancy diﬀusion
V: Vacancy

Inhibitionof dislocation movement⊥: Dislocation

Figure 10 Schematic illustration of the inhibition of vacancy
diffusion and dislocation movement.

Attempts were made to further improve the stressrelaxation property by combining the effects of increasing
the concentrations of Co and Si and adding Sn and Mg
as described in 3.3. Comparing the Alloy-9 in Figure 5 inwhich the Co and Si concentrations were increased withthe Alloy-10 in Figure 8 in which additional Sn and Mg
were added, the stress relaxation rate of the Alloy-10 waslower. The amounts of Sn and Mg added in the Alloy-10
were determined as Sn=0.15mass% and Mg=0.1mass%in consideration of the results shown in Figure 9. With thiscomponent amount (Alloy-10), 740 MPa of tensile
strength, 48%IACS of electrical conductivity and 5.7% ofstress relaxation rate, as shown in Table 3, were obtained.
Although the electrical conductivity of the specimen usedin this experiment was less than 50%IACS, it was suggested that the tensile strength of 700 MPa or more, theelectrical conductivity of 50%IACS or more and thestress relaxation rate of 10% or less were satisfied by
optimizing the composition and the process.
Table 3As a result of researching the influence of the Co and Si
concentrations and the Sn and Mg addition amount on
the strength, the electrical conductivity and the stressrelaxation property in Cu-Co-Si based alloys, we haveobtained the following findings and developed a Cu-CoSi based alloy which has a strength equivalent to that of
the existing Corson alloys, and also has an excellent
electrical conductivity and an stress relaxation property.
1) When Sn or Mg are added to a Cu-Co-Si alloy, theelectrical conductivity is lowered, but the rise in thetensile strength due to solution strengthening is little.
2) The stress relaxation property is improved by increasing the concentrations of Co and Si, appropriate solution heat treatment, aging heat treatment and low temperature annealing.
3) The stress relaxation property is improved by the addition of Sn and Mg, and it is further improved by thesimultaneous addition of Sn and Mg as well as Corsonalloys.
4) The Cu-1.4mass%Co-0.35mass%Si-0.15mass%Sn0.1mass%Mg alloy made in this experiment, having
740 MPa of the tensile strength, 48%IACS of the electrical conductivity and 5.7 of the stress relaxation rate
(120°C × 1000 hrs.), is the suitable material for highcurrent connectors.

REFERENCES
1) R. Matsuo, K. Hirose and T. Eguchi: J. JRICu, 49 (2010) 153-156.
2) S. Danjo, T. Isomatsu and T. Eguchi: J. JRICu, 56 (2017), 35-38.
3) H.Kanoh, T. Sato, K, Hirose and T. Eguchi: J. JRICu, 45 (2006), 115119.
4) J. Miyake: Journal of the Japan Copper and Brass Research
Association, 36 (1997), 73-77.
5) T. Isomatsu, M. Higuchi and T. Eguchi: J. JRICu, 53 (2014), 73-77.
6) T. Ogura: Journal of the Japan Copper and Brass Research
Association, 38 (1999), 274-280.
7) S. Danjo, T. Isomatsu, R. Matsuo and M. Higuchi: Furukawa DenkoJiho, 136 (2017), 46-51.

Properties of the new Alloy.

Cu-1.4mass%Co-0.35mass%Si-0.15mass%Sn-0.1mass%Mg
Tensile strength

Electrical conductivity

Stress relaxation rate*740 MPa

48%IACS

5.7%*Test condition：120℃× 1000 h

Furukawa Electric Review, No. 50 201949