Effects of Thermo-mechanical Treatment on Microstructure and Properties of Cu-Cr-Zr Alloys kermajani2013

Materials and Design 51 (2013) 688–694

Contents lists available at SciVerse ScienceDirect

Materials and Design
journal homepage: www.elsevier.com/locate/matdes

Influence of thermomechanical treatment on microstructure
and properties of electroslag remelted Cu–Cr–Zr alloy
M. Kermajani a,⇑, Sh. Raygan b, K. Hanayi c, H. Ghaffari a
a

Department of Research and Development, MAPNA Generator (PARS) Company, Karaj, Iran
School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran 1439957131, Iran
c
Research Group of Metal and Materials, Iranian Academic Center for Education, Culture and Research (ACECR), Tehran, Iran
b

a r t i c l e

i n f o

Article history:
Received 23 January 2013
Accepted 17 April 2013
Available online 9 May 2013
Keywords:
Thermomechanical treatment
Hardness
Tensile strength
Electrical conductivity

Copper alloys
Electroslag remelting

a b s t r a c t
Effect of thermomechanical treatment (TMT) on aging behavior of electroslag remelted Cu–Cr–Zr alloy
was investigated. The relationship between microstructure, mechanical and electrical properties was
clarified using hardness, tensile and electrical conductivity testing methods and optical and scanning
electron microscopy techniques. The results showed that an appropriate processing and aging treatment
may improve the properties of the alloy due to the formation of fine, dispersive and coherent precipitates
within the matrix. Indeed, the optimum condition for electrical conductivity and mechanical properties
was obtained after cold working of 40% followed by aging at 500 °C for 150 min.
Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction
Different types of high electrical conductivity alloys have been
developed in research centers for a variety of applications.
Ehsanian et al. [1] reported Cu–Cr–Zr alloys as an excellent candidate for getting fine combination of mechanical properties and
electrical conductivity. As demonstrated by Li et al. [2], these properties made this alloy a good choice to be applied in lead frame,
bullet train, long-distance wire, heat sink material for International
Thermonuclear Experimental Reactor (ITER), etc.
Eich et al. [3] concluded smelting and casting in controlled
atmospheres like vacuum or a protective high purity noble gas as
common methods for producing these alloys. Electroslag remelting
(hereinafter, this may be referred to as ESR process) is an attractive
process which has been used for producing ingots of metals with
high reaction activity, as described by Kelkar et al. [4]. Ahmadi
et al. [5] evaluated this process for a stainless steel and concluded
that the ingots and materials manufactured this way could possess
structures with uniform density and a high degree of homogeneity,
no segregation or shrinkage cavities and no undesired impurities

or oxide inclusions. Prasad and Rao [6] worked on recycling light
scrap of oxygen free high conductivity copper using ESR technique

⇑ Corresponding author. Tel.: +98 9122649783; fax: +98 2636618354.
E-mail address: (M. Kermajani).
0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
/>
and stated that the same process was used for producing certain
copper alloys like Cu–Cr, Cu–P and Cu–Ti from copper scrap. However, less is known about the effect of this process on Cu–Cr–Zr alloys. Jovanovic and Rajkovic [7] stated that the chemical
composition and especially processing technology of Cu–Cr–Zr alloys were protected by patents or classified documents.
Durashevich et al. [8] reported that appropriate thermomechanical treatment can raise strength and hardness and electrical
conductivity of these alloys produced by induction vacuum
method. Li et al. [2] concluded that due to very low solubility of
Cr and Zr in Cu at room temperature, Cu–Cr–Zr alloys possessed
high conductivity after proper aging treatment. Moreover, they
claimed that the strengthening mechanism was related to the
combined effect of precipitation hardening (Cr and intermetallic
compounds of Cu and Zr precipitates), work hardening and texture
strengthening. Liu et al. [9] revealed the way solidification rate
could affect the initial microstructure and subsequently aging
behavior of the alloy.
In the present work, ESR technique was conducted on an ingot
of Cu–Cr–Zr alloy prepared using an induction furnace and effects
of this process on microstructure and precipitation behavior of the
alloy was studied. The purpose of this paper was to design ESR
method to produce Cu–Cr–Zr alloy in a large scale and obtain the
proposed mechanical and electrical properties according to the
standard [10] through casting and subsequent deformation and
aging processes.

M. Kermajani et al. / Materials and Design 51 (2013) 688–694

2. Experimental procedure
Oxygen free high conductivity copper (OFHC), Cu–4.8%Cr and
Cu–32%Zr (KBM AFFLIPS) master alloys were used for alloy preparation in an induction furnace (AEG, medium frequency, capacity
120 kg). Oxygen free copper was produced using scrap copper, coal
and CaB6 (Dalian Co.) as deoxidizer in an induction furnace.
Cu–4.8%Cr master alloy was produced using oxygen free copper
and electrolytic chromium in a similar process. Surface of melt
was covered with coal and small amounts of magnesium (200 g)
to avoid oxygen absorption from the air. Then, the melt at
1300 °C was poured into a cast mold with graphite liner. Diameter
and height of the mold were 110 and 750 mm, respectively. The
solidified ingot weighting about 85 kg was refined by ESR method
using a water-cooling mold and slag with composition of 40%CaF2–
30%NaF–20%Na3AlF6–6.7%ZrO2–3.3%SiO2 to eliminate casting defects and unwanted elements like P, S and Mg [11]. Schematic of
ESR furnace used in this research is given in Fig. 1. The furnace
was operated at 40 V and 4500 A whereas the melt temperature
was about 1600 °C. Cu–0.65Cr–0.147Zr alloy was obtained as a result of melting and refining process.
The produced ingot (610 mm length, 130 mm diameter) was
extruded at 950 °C with the reduction of about 400% using a
1250 ton machine, solution annealed at the same temperature in
an electrical furnace (Azar furnace 1250) and water quenched
[12]. The samples of this alloy were then 20%, 40% and 60% cold
rolled. Afterward, the cold rolled samples were aged at 500 °C for
60, 90, 120, 150 and 180 min in the same furnace [7–9,13,14].
For optical microscopy studies, the selected samples of the produced alloy before and after aging were polished and etched in a
solution of HCl, FeCl3 and alcohol. Scanning electron microscopy

689

(SEM) investigations, as well as energy dispersive X-ray spectroscopy (EDS) studies were done using a MIRA//TESCAN scanning
electron microscope operating at 15 kV.
Hardness of the samples was tested in an Instron Wolpert
GmbH testing unit using the HB10 (HBW 2.5/62.5) method
(according to ASTM: E10-12 standard test) after being grounded
and polished. The selected samples of the alloy were cold worked,
aged and then machined to a diameter of about 6 mm and gage
length of about 30 mm (according to the standard test method
[15]) and employed for the uniaxial tensile test using an HEKERT
machine.
In order to find the best aging condition, electrical conductivity
of the samples was measured and expressed in %IACS (International Annealed Copper Standard) using SIGMASCOPEÒ SMP10
Fischer conductomer after polishing. In both mechanical tests as
well as electrical test three measurements were done for each sample and the average was reported.
3. Experimental results
Macro and microstructure of the alloy after ESR process are
shown in Fig. 2a and b. As seen in these figures, the microstructure
mostly consisted of fine and columnar grains with few amounts of
small precipitates in the matrix and at the grain boundaries. This
microstructure was the result of higher solidification rate in ESR
technique.
Fig. 3a–d illustrates X-ray maps of the microstructure of the
produced alloy. It can be observed that Zr dispersed rather uniformly throughout the matrix while Cr concentrated on special
locations and formed fine metallic precipitates. The high temperature of the molten slag could be the reason for this behavior.

Fig. 1. Schematic of ESR furnace used in this research.

Fig. 2. (a) Macrostructure and (b) microstructure of the alloy after ESR process.

690

M. Kermajani et al. / Materials and Design 51 (2013) 688–694

Fig. 3. X-ray maps of the microstructure after ESR process; (b) Cu Ka, (c) Cr Ka, (d) Zr La.

Fig. 4. Microstructure of a sample aged at 500 °C for 150 min after 40% cold work in (a) parallel and (b) transverse directions.

Fig. 5. Variation of (a) hardness and (b) electrical conductivity with aging time.


M. Kermajani et al. / Materials and Design 51 (2013) 688–694
Table 1
Influence of thermomechanical treatment on mechanical strength of Cu–Cr–Zr alloy
samples.
Cold work and aging time (min)

20% + 180
40% + 60
40% + 90
40% + 120
40% + 150
40% + 180
60% + 60
60% + 120
60% + 180

Mechanical strength (MPa)

rYS

rUTS

360
372
354
356
385
355
381
358
377

421
437
404
420
431
414
433
417
437

Fig. 4a and b shows the micrograph of the alloy after 40% cold
work and aging at 500 °C for 150 min in parallel and transverse
directions to mechanical work. Once again, the fine precipitates
distributed uniformly through the matrix with high volume fraction could be seen in the matrix. Also, the elongation of these particles and grains in the direction of cold work was obvious.
Fig. 5a and b summarizes the results of hardness and electrical

conductivity tests on a number of samples exposed to different
thermomechanical treatments to investigate the influence of prior
cold deformation and aging treatments on these properties. Obviously, the hardness of the alloy increased with the increment of
cold work. Furthermore, it could be seen that at each aging time
higher electrical conductivities corresponded to the lower hardness values. The best condition for high hardness about 131 HB
and electrical conductivity about 81% IACS seemed to occur in a
sample being aged at 500 °C for 150 min after 40% cold rolling.

691

Yield and ultimate strengths values of the samples are tabulated
in Table 1. Considering the test error range, the results showed that
changes in yield and ultimate strengths for different samples were
not significant. Yield and ultimate strengths of the sample with
40% cold work and aging time of about 150 min were about 385
and 431 MPa, which were suitable for the proposed applications,
as well.
SEM images presented in Fig. 6a–c shows detailed microstructure from aging this alloy at 500 °C for 180 min with 60% cold
deformation. According to the EDS analyses of the specified points
in Fig. 6, the uniformly distributed fine precipitates were chromium and zirconium enriched phases (Fig. 7 and Table 2),
respectively.
These fine dispersed particles in the matrix were necessary to
obtain the desired strengthening effect. According to these results,
B zone indicated the presence of Cr and Cu atoms which was not
combined. Regarding the ratio of Cu and Zr atoms in EDS analysis
of zone C, it seems that the intermetallic compound of Cu5Zr was
formed along with the metallic phase of Cr. In zone D, the metallic
phase of Cu could be seen.
4. Discussion
The results obtained from this investigation demonstrated that

a process including electroslag remelting of casting alloy, solution
treatment, cold work and optimally aged conditions could combine high hardness and strength with high electrical conductivity
in Cu–0.65Cr–0.147Zr alloy. Indeed, according to the Cu–Cr and
Cu–Zr phase diagrams demonstrated by Zeng et al. [16], rapid
solidification in ESR method and high temperature of molten slag
could lead to obvious extension of the solid solubility of Cr and Zr

Fig. 6. SEM micrograph of a sample aged at 500 °C for 150 min after 60% cold work.


692

M. Kermajani et al. / Materials and Design 51 (2013) 688–694

Fig. 7. EDS analyses of three zones indicated in Fig. 6: (a) spectrum of zone B, (b) spectrum of zone C and (c) spectrum of zone D.

Table 2
EDS analyses of the shown particles in Fig. 6.
Zone

Copper (at.%)

Chromium (at.%)

Zirconium (at.%)

Zone B
Zone C
Zone D

42.08
61.08
99.52

57.73
26.68
0.2

0.2
12.25
0.28

in Cu matrix and remarkable refinement of grain size. Since this
alloy was a precipitate hardenable one, as long as Cr and Zr were
dissolved in the matrix, more and better distribution of precipitates would be formed and also the strength of the alloy would
be higher. Another aspect of ESR method with rapid solidification

characteristic was precipitation progress through diffusion of solute atoms with the aid of vacancies formed with high concentration in rapidly solidified alloy. The same behavior was observed
by Liu et al. [9]. Upon aging, very fine dispersion of second phase
was precipitated in the matrix, because Cr in the supersaturated
Cu matrix created a high degree of thermodynamically meta-stable particles; thus, providing a high chemical potential for the
precipitation reaction which was activated by the process of subsequent aging. Through cold working, a high density of dislocations was introduced into the matrix (Durashevich et al. [8]). At
this time, precipitation of fine, dispersive and coherent precipitates could occur heterogeneously at the dislocations and sub
cells. This phenomenon created a large volume fraction of precipitates which increased the hardness and strength of the alloy.


M. Kermajani et al. / Materials and Design 51 (2013) 688–694

Others have confirmed these claims in Cu–Cr alloys [17–19]. Definitely, higher initial cold work imparted more structural defects
like vacancies, dislocations in the matrix and reinforced the precipitation response of the alloy, as shown in the figures (Figs. 4

and 6). The hardness and strength increase followed the empirical
Orowan relationship:

Ds ¼ kf

1=2 1

R

ð1Þ

presented by Dieter [20], where Ds is increase in shear stress; k is a
constant; f is volume fraction of precipitates and R is the diameter of
precipitates. The more volume fraction of precipitates and the smaller diameter of precipitates, the more increase in Ds and the higher
hardness and strength of the alloy would be.
According to literature [21], precipitates not only give rise to
high strength, but also reduce the solute content in the matrix,
maintaining good conductivity. Among various alloying strengthening mechanisms, namely, solid solution hardening, precipitation hardening, and dispersion hardening, solid solution
hardening has the most detrimental effects on the conductivity
and is the least favored mechanism to obtain high conductivity,
high-strength copper alloys. Variation in electrical properties as
explained by Liu et al. [9] could be attributed to the presence of
solute atoms in the solid solution matrix which can act as obstacles to the movement of electrons and increase the density of
electron scattering centers-lattice imperfections. Then, formation
and growth of precipitates reduced the contents of solute atoms
in the matrix and resulted in contiguous increase in electrical
conductivity during aging. Li and Zinkle [21] believed that the increase of electrical resistance in Cu–Cr–Zr alloys is considered to
be caused by lattice parameter deviation of the alloy matrix due
to the solid solution of foreign atoms. However, during aging
decomposition of Cr in supersaturated solid solution to soluterich clusters and then to metastable fcc ordered phase and ordered bcc precipitates happens, respectively [2]. It is also believed

that besides the early stage of decomposition, some of the Cr solute-rich clusters have already formed even during the casting
stage. At the early stage of aging treatment, some G.P. zones begin to transform into bcc Cr while new solute-rich clusters appear, unceasingly [2].
They also demonstrated that cold work can significantly increase the strength of pure copper while has a relatively moderate
effect on conductivity. The decrease in electrical conductivity with
increasing rolling ratio from 20% to 60% is ascribed to the effect of
the defects like dislocations and interfaces. The increases of coldworking ratio causes the increase of electron scattering ascribed
to the lattice distortion or internal fault structure, because the
phase interfaces act as the walls for scattering of electrons, thereby, the electrical conductivity is presumed to decrease with refinement of the microstructure [21].
Moreover, in order to control the microstructure and improve
the properties of the alloy, it is of great value to identify the composition of precipitates. Although there has been no unanimous
agreement on the chemical composition of precipitated phases in
the alloy, the results presented in this article showed the composition of the precipitates as Cu5Zr and metallic Cr-enriched phases.
These results were consistent with the experimental and computational work by Zeng et al. [16] and Ellis [22].
Electrical conductivities of samples with 20% prior cold work
were better than other samples while samples with 60% cold work
had higher mechanical strengths. Considering these facts, the hardness and electrical conductivity on the order of 131 HB and 81%
IACS, respectively were obtained after aging of 40% cold worked
samples at 500 °C for 150 min. The yield and ultimate strengths
of this sample would be 385 and 431 MPa, respectively, in this
condition.

693

5. Conclusions
In this study, the effects of thermomechanical treatment on
microstructure and properties of electroslag remelted Cu–Cr–Zr Alloy were investigated. The main outcomes of this study can be
highlighted in following statements:
(1) An optimized ESR process was suitable to produce homogeneous, segregation free Cu–Cr–Zr alloy with fine structure,
increased solubility of alloying elements and small size precipitates. The ingot was free from undesired inclusions and
other structural imperfections, as well.

(2) The microstructural features of the alloy after ESR like fine
grains and uniform distribution of alloying elements were
due to rapid solidification in this process.
(3) Fine dispersed precipitates inside the grains and along the
grain boundary made the Cu–Cr–Zr alloy possess higher
hardness and electrical conductivity after aging. These particles seemed to be Cu5Zr and metallic Cr-enriched phases.
(4) After 40% cold working and aging for 180 min, electrical conductivity could reach about 83% IACS.
(5) Hardness and strength values of aged samples with 60% cold
working were generally higher than those of other samples
while electrical conductivity of these samples was lower.
(6) Optimum of mechanical properties and electrical conductivity values were obtained after about 40% cold working and
aging at 500 °C for 150 min.

Acknowledgements
The authors gratefully acknowledge MAPNA Company for providing financial support for this research. University of Tehran is
also acknowledged for its advanced technologies and equipments.
References
[1] Ehsanian MH, Raygan Sh, Foroughi BAM, Hanaei K, Ahadi FK. Effect of coldworking and aging process on mechanical properties and electrical
conductivity of Cu–13.5%Mn–4%Ni–1.2%Ti alloy. Mater Des 2012;41:182–91.
[2] Li H, Xie S, Mi X, Wu P. Phase and microstructure analysis of Cu–Cr–Zr alloys. J
Mater Sci Technol 2007;23:795–800.
[3] Eich A, Franz H, Kemmer HJ. Vacuum metallurgy – production of special copper
alloys under vacuum. ALD Vacuum technology GmbH. In: Barcelona, IWCC
technical seminar melting and casting; 2009.
[4] Kelkar KM, Mok J, Patankar SV, Mitchell A. Computational modeling of ESR
process used for the production of ingots of high-performance alloy. J Phys IV
France 2004;120:421–8.
[5] Ahmadi S, Arabi H, Shokuhfar A, Rezaei A. Evaluation of electroslag remelting
process in medical grade 316LC stainless steel. J Mater Sci Technol
2009;25:592–6.

[6] Prasad SVV, Rao SA. Electroslag melting for recycling scrap of valuable metals
and alloys. Defence metallurgical research laboratory of India. In: Pittsburgh,
fourth international symposium on recycling of metals and engineered
materials; 2000. p. 503–16.
[7] Jovanovic MT, Rajkovic V. High electrical conductivity Cu-based alloys. MJoM
2009;15:125–33.
[8] Durashevich G, Cvetkovski V, Jovanovich V. Effect of thermomechanical
treatment on mechanical properties and electrical conductivity of a CuCrZr
alloy. Bull Mater Sci 2002;25:59–62.
[9] Liu P, Su J, Dong Q, Li H. Microstructure and properties of Cu–Cr–Zr alloy after
rapidly solidified aging and solid solution aging. J Mater Sci Technol
2005;21:475–8.
[10] DIN 17666 (2.1293). Low alloy wrough copper alloys-composition. Berlin,
Germany: Deutches Institut für Normung e.V.; 1983.
[11] Nafziger RH. The electroslag melting process, Bulletin, 1st ed., vol.
669. Washington: U.S. Dept. of the interior, Bereau of Mines; 1976. p. 131–3
and 55–72.
[12] Laue K, Stenger H. Extrusion processes, machinery, tooling. Metals Park, Ohio
44073: ASM; 1981. p. 156–179.
[13] Batra IS, Dey GK, Kulkarni UD, Banerjee S. Microstructure and properties of a
Cu–Cr–Zr alloy. Nucl Mater 2001;299:91–100.
[14] Tu JP, Qi WX, Yang YZ, Liu F, Zhang JT, Gan GY, et al. Effect of aging treatment
on the electrical sliding wear behavior of Cu–Cr–Zr alloy. Wear
2002;249:1021–7.


694

M. Kermajani et al. / Materials and Design 51 (2013) 688–694

[15] DIN EN 10002-1. Metallic materials-tensile testing – part 1: method of testing
at ambient temperature. Berlin, Germany: Deutches Institut für Normung e.V.;
2001.
[16] Zeng KJ, Haemaelaeinen M, Lilius K. Phase relationships in Cu-rich corner of
the Cu–Cr–Zr phase diagram. Scripta Metal Mater 1995;32:2009–14.
[17] Su JH, Liu P, Dong QM, Li HJ, Ren FZ, Tian BH. Recrystallization and
precipitation behavior of Cu–Cr–Zr alloy. J Mater Eng Perform 2007;16:490–3.
[18] Liu P, Kang BX, Cao XG. Aging precipitation and recrystallization of solidified
Cu–Cr–Zr–Mg alloy. J Mater Sci A 1999;265:262–7.

[19] Gao N, Huttunen-Saarivirta E. Influence of deformation on the age hardening
of a phosphorus containing Cu–0.61 wt.% Cr alloy. J Mater Sci A
2003;342:270–8.
[20] Dieter G. Mechanical metallurgy. 3rd ed. New York: McGraw-Hill; 1986. p.
137–148.
[21] Li M, Zinkle SJ. Physical and mechanical properties of copper and copper alloys.
Compr Nucl Mater 2012;4:667–90.
[22] Ellis DL. Observations of a Cast Cu–Cr–Zr Alloy. NASA/TM 213968; 2006.