Microstructure evolution and thermal physical properties ofCuCr alloy after high pressure treatment

Yu-Quan Ma*, Hong-ju Lin, Dong-dong Song

Received: 17 February 2014 / Revised: 28 February 2014 / Accepted: 21 March 2014 / Published online: 23 April 2014

� The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2014

Abstract The thermal diffusion coefficient, thermal con-

ductivity, and thermal expansion coefficient of CuCr alloy

prepared by infiltration were measured by thermal constant

tester and dilatometer before and after high pressure heat

treatment, at the same time, the effect of high pressure

treatment on the thermal physical properties of CuCr alloy

was discussed by the analysis of its microstructure. The

experimental results show that high pressure heat treatment

can increase the thermal diffusion coefficient and thermal

conductivity of CuCr alloy, but it changes slightly in the

pressure range of 1–6 GPa. As for thermal expansion

coefficient, when the temperature is higher than 130 �C, it is

obviously higher than that of the alloy without high pressure

treatment after 1 GPa pressure treatment, and the higher the

temperature is, the larger their differences are.

Keywords CuCr alloy; High pressure treatment; Thermal

diffusion coefficient; Thermal conductivity; Thermal

expansion coefficient

1 Introduction

The CuCr alloy is widely used for resistance welding

electrode, power converter switch, etc., owing to its supe-

rior mechanical strength, good electrical conductivity, and

thermal conductivity, so it obtains evident economic and

social benefits [1–3]. With the rapid development of the

technology of electrical engineering, the higher properties

are put forward for CuCr alloy. Therefore, it is very

important practical sense to study the properties of CuCr

alloy. At present, methods to improve the properties of

CuCr alloy include adding alloy element, heat treatment,

and deformation heat treatment [4, 5]. The preparation

methods are sintering, vacuum melting, smelting, infiltra-

tion process, consumable electrode, hot isostatic pressing,

and flash set [6, 7]. The infiltration process is often used in

the preparation of alloy, but the compactness of CuCr is

poorer [8], and the electrical conductivity (about 14

MS�m-1) and hardness (about HB 100) are lower, which

affect its performance. In general, the density of high

quality CuCr50 alloy should be higher than 98 % [9],

electrical conductivity and hardness are greater than

18 MS�m-1 and HB 110, respectively. Therefore,

improving the density of CuCr alloy is the key to produce

high performance CuCr alloy. According to the reports in

the literatures, high pressure treatment can refine the

microstructure and improve the compactness of metal

materials [10–12]. Therefore, domestic and abroad

researchers pay attention to improve the microstructures

and mechanical properties of the materials with high

pressure treatment [13–15]. In recent years, some resear-

ches on the microstructure and mechanical properties of Cu

alloy after high pressure treatment have been reported [16,

17], but the reports on the thermal physical properties of

Cu alloy after high pressure treatment were seldom

involved. While they directly have influences on alloy

service life [9]. Accordingly, it is necessary to study it. The

thermal diffusion coefficient, thermal conductivity, and

thermal expansion coefficient of CuCr alloy prepared by

infiltration were measured before and after high pressure

treatment, and the effects of the high pressure treatment on

the thermal physical properties of CuCr alloy were also

discussed in the paper.

Y.-Q. Ma*, H. Lin, D. Song

Mechanical and Electrical Engineering College, Hebei Normal

University of Science and Technology, Qinhuangdao 066004,

China

e-mail: mayuquan2004@126.com

123

Rare Met. (2014) 33(3):293–298 RARE METALSDOI 10.1007/s12598-014-0269-4 www.editorialmanager.com/rmet

2 Experimental

The experimental material is composed of oxygen-free

copper (99.97 % Cu) and chromium powder (99.9 % Cr).

The right amounts of copper powder (average particle

diameter of 50 lm) and chromium powder (average particle

diameter of 75 lm) were pressed to shape after mixing well,

sintered into a porous chromium skeleton at 1,085 �C, and

immersed into molten copper at 1,200 �C in vacuum for 2 h.

Then, the copper was penetrated into the porous chromium

skeleton, and finally the CuCr alloy was produced. Its

chemical composition is 50.21 wt%Cu, 49.66 wt%Cr, and

the other 0.13 wt%. Firstly, the samples of CuCr alloy were

sealed in graphite sleeve which was put into pyrophyllite

mold. High heat treatment was done on CS-IB type six-anvil

high pressure equipment under high pressure of 0.6, 1.0, 3.0,

and 6.0 GPa heated by electrical resistance. The samples

were heating at 900 �C and lasting for 20 min, then shutting

off power of electrical resistance and cooling to room

temperature on holding up pressure. Secondly, the samples

before and after high pressure heat treatment were processed

into U10 mm 9 1.5 mm and polished by 1,200 grit emery

paper. Then, the thermal diffusion coefficient and specific

heat capacity were measured by a TC-7,000 thermal con-

stant tester. They were measured before and after 1 GPa

high pressure treatment at 25, 50, 100, 200, 300, and 400 �C,

respectively, with precision of ±2 %. The thermal expan-

sion coefficient of the samples before and after 1 GPa high

pressure treatment with size of U8 mm 9 15 mm was

measured continually by DIL402C dilatometer (with pre-

cision of ±3 %) in the range of 25–600 �C when the heating

rate was 5 �C�min-1, and Al2O3 was reference. Its density

was tested by ESJ120-4 electronic balance by drainage

method. Finally, the microstructure of the CuCr alloy before

and after high pressure heat treatment was observed and

analyzed by Axiovert 200MAT metallographic microscope,

scanning electron microscope (SEM, S-3400 N SEM-BSE),

and Jeol-2010 transmission electron microscope (TEM).

3 Results and discussion

3.1 Microstructure

The microstructure of the CuCr alloy before and after high

pressure heat treatment is shown in Fig. 1. As can be seen,

the structure characteristic change of alloy before and after

high pressure heat treatment is not obvious, and they are

composed of Cu matrix and irregular granular Cr phases

such as diameters for 50–100 and 10–20 lm. TEM

observation (Fig. 2) shows that there are dislocations in

many micro areas, and the dislocation quantity in CuCr

alloy structures increases obviously after high pressure

treatment. Figure 3 is CuCr alloy SEM backscatter images

before and after high pressure treatment. As can be seen,

there are more microscopic holes in microstructure of

casted CuCr alloy, its compactness is worse. After high

pressure treatment, the number of microholes of matrix of

CuCr alloy and the size of the holes are obviously reduced.

According to the test results, average values of microholes

in matrix of CuCr before and after high pressure treatment

are about 0.45 and 0.16 lm, respectively. It is proved that

high pressure treatment can increase compactness of CuCr.

Table 1 also shows that the high pressure treatment can

increase the density of CuCr alloy. It is owing to the high

pressure that can cause high strain in CuCr alloy and

induce distortion of lattice, which lead to the increase of

dislocation. At the same time, it also makes internal mi-

cropores of the alloys to bridge and then causes reduction

of the number of microscopic holes and the increase of

alloy compactness.

Fig. 1 OM images of CuCr alloy: a original and b 1 GPa treatment

Fig. 2 TEM images of CuCr alloy: a original and b 1 GPa treatment

Fig. 3 SEM-BSE images of CuCr alloy: a original and b 1 GPa

treatment

294 Y.-Q. Ma et al.

123 Rare Met. (2014) 33(3):293–298

3.2 Thermal conduction properties

In the temperature range of 25–400 �C, the thermal diffu-

sivity coefficient and the specific heat capacity of the

sample can be directly measured by TC-7000 thermal

constant tester with laser pulse method and can calculate

the heat conductivity. Its principle is as follows. The front

surface of sample was heated by laser pulse instanta-

neously. At the same time, the change of temperature at the

back surface of sample was measured. According to the

size of sample and data tested by the instrument, the

thermal diffusivity coefficient and the heat conductivity

can be obtained. Thermal diffusion coefficient a is given by

Eq. (1):

a ¼ 1:38L2

p2t1=2

; ð1Þ

where L is the thickness of the sample, and t1/2 is half of the

time that is needed for the temperature of the back surface

of sample to reach the maximum. Then, heat conductivity kcan be calculated by Eq. (2)

k ¼ a � q � Cp; ð2Þ

where a, q, and Cp are thermal diffusivity coefficient,

density, and specific heat capacity of the sample, respec-

tively. The relation curve between the thermal diffusivity

coefficient and the pressure of the CuCr alloy before and

after high pressure treatment is shown in Fig. 4, and the

relation curve between heat conductivity and the pressure

is shown in Fig. 5. It can be seen that high pressure

treatment can increase the thermal diffusivity and the heat

conductivity of the CuCr alloy. When the pressure is 1

GPa and the temperature is 25 �C, their values are

0.4218 cm2�s-1 and 149.76 W�m-1�K-1, respectively,

increased by 11.44 % and 3.68 % than that of alloys

without treatment. When the pressure is over 1 GPa, the

variations of thermal diffusion coefficient and heat con-

ductivity of CuCr alloy change slightly with the increase of

pressure.

The relation curve between the heat conductivity con-

stant of CuCr alloy and the temperature before and after

1 GPa pressure heat treatment is shown in Fig. 6. It can be

seen that the heat conductivity of the CuCr alloy after 1

GPa pressure treatment is higher than that of alloy without

treatment in the temperature range of 25–400 �C, but both

trends of the thermal conductivity variation with the

increase of temperature are almost the same.

Table 1 Density of CuCr alloy before and after high pressure

treatment

Samples Original 0.6 GPa 1.0

GPa

3.0

GPa

6.0

GPa

Density/(g�cm-3) 8.044 8.051 8.058 8.062 8.062

Fig. 4 Relationship between thermal diffusivity coefficient of CuCr

alloy and pressure at 25 �C

Fig. 5 Relationship between thermal conductivity of CuCr alloy and

pressure at 25 �C

Fig. 6 Relationship between temperature and thermal conductivity of

CuCr alloy before and after 1 GPa pressure heat treatment

Properties of CuCr alloy after high pressure treatment 295

123Rare Met. (2014) 33(3):293–298

According to the physical nature of the metal heat

conduction, the flow of heat in metal material is mainly

transferred by the flow of free electrons. The microscopic

holes, the dislocation, and other lattice defects existed in

the metal material matrix will cause the scattering of

electrons [18] and block the heat conduction. And more-

over, the heat conductivity of the microscopic holes itself is

zero [19], so the microscopic holes, the dislocation, and

other lattice defects will reduce the heat conductivity of

metal material. The microstructures of the CuCr alloy are

composed of Cu matrix and irregular granular Cr phases.

Therefore, heat conduction of CuCr alloy is composed of

two parts: one is the heat conduction of Cu matrix, and the

other is the heat conduction of Cr phase. The high pressure

heat treatment cannot generate any new phase and cannot

change composition of each phase in CuCr alloy, but on the

one hand, it can increase defects quantity such as the lattice

distortion and dislocation in the structure of the alloy,

enlarge the electronic scattering, and reduce the heat con-

ductivity and thermal diffusivity. Moreover, high pressure

treatment increases the compactness of alloy, reduces the

quantity of microscopic holes in matrix, and increases the

thermal conductivity of the CuCr alloy. In addition, the

CuCr alloy was prepared by infiltration process which is

supersaturated part of the Cr atoms in Cu matrix. Thus, it

causes the effect of Cr atoms on the electron scattering, so

the thermal conductivity of the alloy prepared by infiltra-

tion process is poorer. In the process of high pressure

processing, the high pressure subsequently leads to the

increase in the number of dislocations of alloy. It provides

more nucleation sites for the precipitation of supersaturated

solid solution Cr atoms and results in the large number of

dispersed Cr particle precipitation in CuCr alloy (Fig. 7)

after high pressure treatment, which reduces the effect of

Cr atoms on the electronic scattering and causes the

increases in the heat conductivity and the diffusion coef-

ficient of CuCr alloy.

Under different pressure treatments, the two effects

above all play different roles, bring different results. As

shown in Figs. 4 and 5, the latter plays a dominant role in

the pressure range of 0–1 GPa. High pressure treatment

reduces the quantity of microscopic holes in matrix fast,

increases the compactness of alloy quickly, precipitates

large number of supersaturated solid solution Cr atoms,

which is in favor of the electron heat diffusion, and then the

thermal diffusion coefficient and heat conductivity of CuCr

alloy increase. But in the pressure range of 1–6 GPa, with

the pressure increasing, the number of dislocations in the

structure of CuCr alloy increases, which increases the

effect on the electronic scattering, so the increase of heat

conductivity and thermal diffusivity coefficient is inhibited

and change slightly.

3.3 Thermal expansion properties

The relation curve between the thermal expansion coeffi-

cients of the CuCr alloy and temperature is shown in

Fig. 8. The relation curve between the linear expansivity

and temperature is shown in Fig. 9. It can be seen that the

thermal expansion coefficient of CuCr alloy before and

after 1 GPa pressure treatment increases with the increase

of temperature. But when the temperature is below 130 �C,

the thermal expansion coefficients of the CuCr alloy before

and after 1 GPa pressure treatment are almost the same.

Just when the temperature is over 130 �C, the thermal

expansion coefficient after 1 GPa pressure treatment

increases more fastly than that of untreated alloy, and the

higher the temperature is, the larger it is. At 200 and

500 �C, the thermal expansion coefficients before and after

1 GPa pressure treatment are 12.228 9 10-6 and

13.793 9 10-6 �C-1, and compared with that of the state

without high pressure treatment increased by 2.27 % and

6.085 % at same temperature, respectively. It can be seen

in Fig. 9, the corresponding linear expansivity also shows

the same trend. Usually, the thermal motion of atoms will

increase when solid material is heated. Thus, the lattice

vibration inside the solid material is exacerbated and leads

to expansion of the solid material volume. The higher the

temperature is, the faster the lattice vibrating is, the more

the energy absorbs, the greater the expansion of the

material volume is.

The reasons of the above phenomena are as follows. The

microscopic holes of material can be considered as a zero-

expansion phase [20] since there is no expansion during

heating. When it rises to a certain temperature, the

microscopic holes of material can offset part of volume

expansion caused by thermal expansion. But high pressure

heat treatment can improve the compactness of CuCr alloy,

reduce the microscopic holes of the material, thus, reduce

the amount of expansion offset by the microscopic holes of

Fig. 7 TEM image of Cr particles precipitate in CuCr alloy after

1 GPa treatment

296 Y.-Q. Ma et al.

123 Rare Met. (2014) 33(3):293–298

material. Therefore, when the temperature is over 130 �C,

the thermal expansion coefficient of CuCr alloy after

1 GPa pressure treatment is higher than that of without

treatment.

4 Conclusion

High pressure heat treatment can increase the thermal

diffusion coefficient and heat conductivity of CuCr alloy in

the pressure range of 0.6–6.0 GPa. When the pressure

exceeds 1 GPa, the heat conductivity CuCr alloy changes

more slightly with pressure increasing. The thermal

expansion coefficient of CuCr alloy increases with tem-

perature increasing before and after 1 GPa high pressure

heat treatment. When the temperature is lower than 130 �C,

the effect of 1 GPa pressure treatment is not obvious on the

thermal expansion coefficient of CuCr alloys, but when the

temperature is over 130 �C, the thermal expansion

coefficient of CuCr alloy after 1 GPa pressure treatment

increases. The increase of the compactness of CuCr alloy

after high pressure treatment is the main reason for the

increase of the thermal diffusion coefficient, heat conduc-

tivity, and thermal expansion coefficient.

Acknowledgments This study was financially supported by the

Natural Science Foundation of Hebei Province (CHN) (No.

E2010001174).

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