Materials Science & Engineering A 558 (2012) 616–622

Contents lists available at SciVerse ScienceDirect

Materials Science & Engineering A
journal homepage: www.elsevier.com/locate/msea

Dependence of microstructure and mechanical properties on hot-extrusion
temperatures of the developed high-strength Cu40Zn–CrFeTiSn brassby powder metallurgy
Shufeng Li a,n, Hisashi Imai a, Katsuyoshi Kondoh a, Akimichi Kojima b, Yoshiharu Kosaka b,
Koji Yamamoto c, Motoi Takahashi cabcJoining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 5670047, Japan
San-Etsu Metals Co. Ltd., 1892, OHTA, Tonami, Toyama, Japan
Nippon Atomized Metal Powders Corporation, 87-16 Nishi-Sangao, Noda, Chiba, Japana r t i c l e i n f o

a b s t r a c t

Article history:
Received 16 April 2012
Received in revised form
10 August 2012
Accepted 14 August 2012
Available online 22 August 2012

Effect of extrusion temperature on the microstructure and mechanical properties of the powder
metallurgy Cu40Zn–CrFeTiSn alloy were investigated, through analysis on the solid solution of alloying
elements, phase transformation and precipitation behavior. The extrusion was carried out between
500 1C and 650 1C using an extrusion ratio of 37:1. After extrusion, the microstructure was foundconsisting of the dual a þ b phase structure and precipitates dispersing uniformly in the matrix. The
tensile strength gradually decreased when the extrusion preheating temperature was increased. Solidsolubility of Ti and Cr decreased steadily and grain coarsening occurred with increase in the extrusion
temperature. Consequently, the lower extrusion preheating temperature was necessary to obtain
excellent ﬁnal mechanical properties, thus the extrusion temperature of 500 1C was enough to produce
good ductility while maintaining the promising values of mechanical properties after extrusion.
& 2012 Elsevier B.V. All rights reserved.

Keywords:
Brass
Extrusion
Mechanical properties
Solid solution
Precipitation
Powder metallurgy

1. Introduction
The widely used 60/40 brass is a dual phase alloy possessing
the best combination of ductility and strength, these features
enable 60/40 brass easily be casted to shape or fabricated by
extrusion, rolling, drawing, hot stamping and cold forming. On theother hand, apart from the major alloying element Zn, small
additions (less than 5 wt%) of other alloying elements are made tobrasses to modify their properties for a given purpose [1], such as
high strength, corrosion resistance, machinability and so on. In
recent years, the impetus for the development of lead-free brasses
and the development of the electronics industry has led to anumber of new applications for brasses including lead frames,
connectors and other electronic components [2,3]. These applications require brasses with unique combinations of mechanicalstrength and conductivity coupled with environment amiability.
Solid solution strengthening and precipitation hardening are
the primary methods used for developing high-strength brasses,
by introducing alloying elements and impurities to improvethe mechanical properties of brasses [4–6]. The binary phasenCorresponding author. Tel./fax: þ 81 6 6879 8669.
E-mail address: shufenglimail@gmail.com (S. Li).

0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.msea.2012.08.062

diagrams of copper alloys [7] indicate that Cr, Fe, Ti, P, Co, Mg, Sn,etc. could be effectively used as candidate alloying elements fordevelopment of high-strength copper alloys. This is because thesolid solubility of these alloying elements would decrease sharply
with decreasing the temperature of copper. In order to obtainhigher strength, more than one alloying elements are needed toform intermetallic precipitates, such as CrFe [8], Fe2P [4], FeTi and
Cu4Ti [5], Cu3Zr [6], etc. during aging process.
In powder metallurgy, it is possible to fabricate components
which otherwise would decompose or disintegrate in direct
alloying of melted materials. Powder processes are more ﬂexible
than casting and forging techniques, they are used in a wide range
of industries, from automotive and aerospace applications topower tools and household appliances. The strengthening effectin conventional solid solution aging is limited due to low
solubility of Cr, Fe and Ti in brass with coarse grain structure at
the solid solution temperature. Rapid solidiﬁcation in powder
metallurgy process, e.g. high cooling rate can lead to supersaturated solid solution of Cr, Fe and Ti in brass matrix. Upon
heat treatment, very ﬁne dispersoids of second phase are precipitated in the matrix, as a result, the precipitation hardening
effect is greatly enhanced. Additionally, during fabrication ofhigh-strength brass alloys, powder-making process, consolidationand forming are the critical steps. The forming process after

S. Li et al. / Materials Science & Engineering A 558 (2012) 616–622

consolidation is so complicated to be put under control, so it
becomes the main factor that determines the ﬁnal properties ofthe brass alloys. In order to obtain good mechanical properties, it
should be taken into account that the super-saturated solid
solution brass will decompose and the precipitates will coarsen
during thermal histories in powder consolidation and hot extrusion. The purpose of the present paper is to investigate the effectof the extrusion temperature on microstructure and mechanicalproperties of the developed brass. Solid solution strengthening
and precipitation hardening response of Cr, Fe and Ti alloying
element additions on 60/40 brass matrix at different extrusion
temperature are studied in detail.

2. Experimental
The nominal composition of Cu—40 wt%-Zn—0.7 wt%Cr—0.2 wt%-Fe—0.6 wt%-Sn—0.6 wt%-Ti. (denoted as BS40-X)
was designed as experimental alloy in the present research. The
BS40-X brass alloy was prepared by melting the mixtures of pureCu, Ti, Cr, Fe, Sn and Zn metals ( 499.9 wt%) in an electricalinduction furnace with an argon atmosphere and kept at 1673 K
for several minutes to make the melt homogeneous. The molten
alloy was poured into a heated tundish at 1673 K. In the water
atomization process, high pressure water jet was directed against
the molten stream to prepare rapidly solidiﬁed BS40-X alloy
powder. The atomized powder was collected, dried and sieved.
The chemical composition and particle size analysis were carriedout, and the actual chemical composition was shown in Table 1.
As-atomized BS40-X brass powder was used as the raw
material. 300 g powder was loaded into a cylindrical graphite
die and consolidated by spark plasma sintering (DR.Sinter/SPS1030; Sumitomo Coal Mining, Japan) at a pressure of 40 MPa
under vacuum of 6 Pa. After sintered at 400 1C for 1.8 ks, thesample was cooled in the chamber to less than 150 1C. The
obtained billet with diameter of 41 mm was extruded using
hydraulic pressing with a load of 2000 kN (SHP-200-450;
Shibayama Machine). For hot extrusion, the billets were preheated

Table 1
Particle size and chemical composition of as-atomized BS40-X powder.
Powder

BS40-X

Particle size (mm)mass (%)

Median size

Mean size

Zn

Cr

Fe

Sn

Ti

Cu

184.3

206.8

39.78

0.76

0.17

0.60

0.66

Bal.

617

within the temperature range from 500 1C to 650 1C, stepping up
the temperature every 50 1C in nitrogen gas atmosphere. The ﬁnal
diameter after extrusion was 7 mm. The extruded round bar wasmachined into tensile test sample with 3 mm diameter in accordance with ICS 59.100.01. Tensile test was conducted on auniversal testing machine (Autograph AG-X 50 kN; Shimadzu)
with a strain rate of 5  10  4 s  1. The strain was recorded usinga CCD camera accessorized to the machine. Three samples wereprepared in identical conditions for the tensile strength test to
obtain the average value. The schematic sketch of the water
atomized powder procedure and the samples processing procedure are shown in Fig. 1.
The phase compositions in samples were identiﬁed by using
X-ray diffraction (Labx, XRD-6100, Shimadzu) referenced to thestandard ICDD PDF cards available in the system software.
In addition, the microstructure evolution of the raw powder andsamples were conducted by using a ﬁeld-emission scan electronic
microscope (FE-SEM, JEM-6500 F, JEOL), phases were examinedby the energy-dispersive X-ray spectrometer (EDS) equipped by
the SEM. Crystal orientation containing grain size and phase
information was analyzed by electron backscatter diffraction(EBSD) technique, which was performed using TSL instrument
attached to the FE-SEM operating at 20 kV. The longitudinalcross-section of the extruded sample was ground to 5 mm siliconcarbide paper ﬁnish, and vibratory polished for 30 min to remove
any remaining surface damage. The electron beam moved in
0.1 mm steps in an area 50 mm  50 mm.

3. Result and discussion
3.1. Characteristics of raw powder
The XRD pattern of the as-atomized powder sample wasshown in Fig. 2. It clearly revealed that the powder contains
mainly the b phase. The FE-SEM micrographs of the raw powder
showed an irregular shape with a mean diameter of 206 mm asshown in Fig. 2(a), which was the typical morphology prepared bywater-atomization process. The process resulted in the formationof super-saturated alloying elements in the matrix of b-brass
phase through rapid solidiﬁcation. Networks of small amount of
ultraﬁne, nano-scale-sized precipitate particles existing along thegrain boundaries were observed in the cross-section of the raw
powder as shown in Fig. 2(b). The structure with tiny precipitate
particles dispersed uniformly in matrix is expected to improve theproperties of the extruded materials as it improves the grain
reﬁnement and is beneﬁcial to work hardening.

Fig. 1. Schematic sketch of experimental procedure used to prepare samples.

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Fig. 2. XRD pattern and microstructure of as-atomized BS40-X raw powder. (a) Morphology of raw powder, and (b) cross-section of raw powder.

Fig. 3. True stress–strain curves of extruded BS40-X alloy at different extrusion
temperature.

3.2. Mechanical properties
Fig. 3 presented the true strain–stress curves of the extruded
BS40-X alloy at different extrusion temperature. A photograph ofthe tensile sample used to evaluate the mechanical properties
was presented in Fig. 3. The results of the tensile properties havebeen summarized in Table 1. Accordingly, the dependence oftensile properties on the extrusion temperatures was plotted inFig. 4. The yield strength (sYS) and tensile strength (sUTS) showed
higher values at the lower extrusion temperature, saying, sYS was
540 MPa and sUTS was 719 MPa when it was extruded at 500 1C.
The variations tendency of the yield and tensile strength were
continue to decline when the extrusion temperature wasincreased. The higher tensile strength at lower extrusion temperature could be attributed to the strain hardening that occurred
during plastic deformation in the extrusion process, which wasincreased in the case of extrusion at lower temperature. This led
to the formation of ﬁner microstructure that ﬁnally contributed tothe improvement in the mechanical properties. It could also be
shown from Fig. 4 that the elongation of the sample extruded at
higher temperature showed limited improvement, from 19.8% at
500 1C increased to 22.1% at 650 1C. This result showed that theFig. 4. Tensile properties of extruded BS40-X alloy as a function of extrusion
temperature.

Table 2Tensile properties of extruded BS40-X alloy at different extrusion temperature.
Temperature
(1C)

YS
(MPa)

Deviation UTS(MPa)

Deviation Elongation(%)

Deviation

500
550
600
650

540.4
502.0
429.5
381.2

7.4
8.9
1.6
3.8

4.9
2.1
1.4
5.3

0.6
2.5
1.7
1.9

719.2
710.4
681.2
667.6

19.8
19.9
22.0
22.1

extrusion temperature at 500 1C was enough to produce goodbonding between BS40-X powder particles after extrusion while
maintained promising values of mechanical properties (Table 2).
3.3. Microstructures of extruded materials
Fig. 5 showed the observed FE-SEM microstructure of BS40-X
compact sintered at 400 1C using SPS. The raw powders were
produced by water atomization consisted of fundamentallyequiaxed grains of the b0 phase as shown in Fig. 2(b). After
sintering at 400 1C, the a phase was observed to distribute along

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619

Fig. 5. FE-SEM microstructure of SPS compact. (a) x3000, (b) x10,000.

Fig. 6. Microstructures of extruded BS40-X samples along extrusion direction within temperature range from 500 1C to 650 1C.

the boundaries of the initial b0 phase, which was surrounded by amatrix of deeper etching b0 phase in Fig. 5(b).
Fig. 6 showed the microstructure of BS40-X brass extruded
within temperature range from 500 1C to 650 1C. Generally, thepowder produced by water atomization could result in very ﬁne
and uniform grain sizes in the order of 0.5 1.0 mm as shown in
Fig. 2(b). In the case of wrought brass with an extrusion ratio of
37 at 500 1C, a microstructure of minute recrystallized grains was
obtained. The a phase is hardly to be distinguished from b phasematrix (Fig. 6(a)). Ultraﬁne precipitate particles were uniformly
dispersed throughout the structure. Such a microstructure is
expected to be helpful in increasing the mechanical propertiesof brass alloys. The samples extruded at 600 1C and 650 1C
showed quite different morphology from the sample extruded
at lower temperatures (Fig. 6(c)). The a phase became progressively coarse and elongates along the extrusion direction. Additionally, the precipitate particles grew coarser and distributed

along the plastic ﬂow line between the a/b-phase bands. This
microstructure was explained by the tendency of precipitates tosegregate at the primary particle boundaries when the green
compact was preheated at higher temperature, which consequently weakened the constraints of the grain growth by precipitates [9]. Precipitates were redistributed with the ﬂow of thelarge plastic deformation of the soft brass matrix by the highextrusion pressure during the subsequent hot extrusion. Fig. 7(aand b) depicted the TEM micrographs of the BS40-X compact
sintered at 400 1C by SPS. Precipitate particles with a size of
50 nm were observed in the interiors of grains and at grainboundaries, as shown in the bright ﬁeld (BF) image of Fig. 7(a).
These precipitates were also discernible in the dark-ﬁeld (DF)image of Fig. 7(b).
The atom concentration variations of the alloying elements ina phase and b phase after extruded at different temperature wereshown in Fig. 8. It could be seen that atom concentrations of Ti

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Fig. 7. Transmission electron micrographs of extruded BS40-X alloy consolidated at 500 1C by SPS (a) bright-ﬁeld image; (b) dark-ﬁeld image.

Fig. 8. Atom concentrations of alloying elements in (a) alpha phase and (b) beta phase as functions of extrusion temperature.

and Cr in a phase decreased steadily with increase of extrusion
temperature, as well as in b phase. It was reported that the
maximum solubility of Ti in copper is 8.0 at% at 885 1C, while atroom temperature the solubility is o1.0 at% [10]. The maximum
solubility of Ti in Zn is 0.027 at% at room temperature [11]. It
implies the equilibrium solid solubility of Ti in Cu40Zn brass lessthan that of Ti in copper. As for the results at present study, when
the preheating temperature of the billet was 500 1C, the atom
concentration of Ti in a phase and b phase were 0.96 at% and0.61 at%, and their values decreased to 0.36 at% and 0.31 at% as
preheat temperature increased to 650 1C. At the same time, theatom concentration of Cr continuously decreased when extrusion
preheat temperature was increased. Especially, Cr concentration
dropped off dramatically in both a and b phase when temperature
was over 550 1C. Cu–Cr alloys have a large precipitation hardening response because the super- saturation of Cr in the Cu
matrix creates a high degree of thermo-dynamic meta-stability inthe solution treated condition, thus providing a high chemical
potential for the precipitation reaction of Cr [8,12]. Atom concentrations of Sn and Fe in a phase continued to decline when
extrusion temperature was increased. However, a contrary tendency was observed in b phase for variations of atom concentration of Sn and Fe.
Fig. 9 presented the image quality (IQ) map of the longitudinalcross-section micrographs of the extruded BS40-X as detected by

the EBSD (electron backscatter diffraction) technique. The grainboundaries were identiﬁed by the lines, and the phase boundaries
were identiﬁed by colors. The green area represented a-phasegrains, and the red area represented b-phase grains. At the lower
extrusion temperatures, the a phase was deﬁned as ﬁne globules
distributed along a different band after extrusion. Recrystallization occurred in both the a phase and the b phase after hot
extrusion, and the b phase exhibited a much ﬁner grain size thandoes the a phase. Twins could be observed in the a phase, which
are a typical feature for dual brass (Fig. 9(a)). Grains in both the aand b phases grew moderately at 500 1C. The precipitate particles(black dots) appeared as tiny second particles not only dispersed
along the plastic ﬂow line between a/b phase bands, but also
dispersed in grain boundaries (Fig. 9(b)). The morphology of the
extrusion sample at 600 1C and 650 1C differed from the morphologies of samples extruded at lower temperatures. The a phasebecame coarser and elongated successively along the extrusion
direction (Fig. 9(c and d)).
Phase volume fraction in the samples detected by EBSD phaseanalysis was plotted as a function of extrusion temperature(Fig. 10). The volume fraction of a phase decreases from 52% to
49% as the extrusion temperature increases from 500 1C to 650 1C.
The a phase volume fraction has an maximum value of 52% at
500 1C, which also accorded with the Cu–Zn binary phase diagram [2]. The volume fraction of b phase showed correspondingS. Li et al. / Materials Science & Engineering A 558 (2012) 616–622

621

Fig. 9. Image quality (IQ) map of longitudinal cross-section of extruded BS40-X alloy at different extrusion temperature.

Fig. 10. Phase volume fractions in extruded BS40-X and BS40 brasses as functionsof extrusion temperature.

balance variation with that of a phase: increased from 48% to 51%
within temperature range of 500 1C–650 1C. The increase volume
fraction of the harder b phase is effective to enhance themechanical strength in dual phase brass [13]. However, becauseof the less variation ranges 3% in phase ratio, the limited effects ofphase volume fraction on tensile strength at different extrusion
temperature could be observed at present conditions. Comparedwith the phase volume fraction of BS40, BS40-X showed muchhigher b phase ratio because of alloying elements such as Sn cansigniﬁcantly increase the b phase ratio.
The grain sizes in the longitudinal and transversal directions of
the extruded samples as functions of extrusion temperature
within range of 500 1C–650 1C were plotted in Fig. 11. The sample
extruded at lower temperature exhibited much smaller grain size
than does the sample extruded at higher temperature in both ofthe longitudinal and transversal extrusion directions. According
to the grain size in the longitudinal direction of the extruded
samples detected by EBSD, the relationship curves between YS
and grain size, the so-called Hall–Petch relationship curves wereplotted in Fig. 12. The correlation between YS and grain size forthe extruded BS40-X showed good linear-ﬁt relationship, that isFig. 11. Grain size in longitudinal and transversal directions of extruded BS40-X
alloy as function of extrusion temperature.

Fig. 12. Hall–Petch relationship between yield strength and grain size inthe longitudinal direction of extruded BS40-X brass at different extrusion
temperature.

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S. Li et al. / Materials Science & Engineering A 558 (2012) 616–622

to say, the yield strength of BS40-X brass increased with reduction in grain size. On the other hand, compared to that of BS40,
BS40-X brass showed higher slope value, which means that BS40X brass illustrates higher strengthening effect. YS of 540 MPa andUTS of 719 MPa for BS40-X after extruded at 500 1C showed 62.7%
and 45.3% higher values than that of 60/40 brass. The strengthening effect decreased 84 MPa (part 1) with increasing extrusion
temperature, which from the decrease of atom concentrations of
Cr and Ti with increase of extrusion temperature from 500 1C to
650 1C. In addition, BS40-X extruded at 650 1C retained 80 MPa(part 2) higher yield strength than that of BS40, this strengthening
value resulted from the higher b phase ratio and remained solid
solutions of Cr, Fe, Ti and Sn after extruded at 650 1C.
3.4. Discussion
From a viewpoint of the alloy design and materials processing,the difference in atomic radii between Ti, Cr, Fe, Sn, Cu, and Zn
provides a strain ﬁeld, which is favorable for the solid solution
strengthening. A high cooling rate can lead to the formation of amore reﬁned microstructure, extend the solid solubility and evenresult in meta-stable phases [14,15]. Rapid solidiﬁcation retains
the single b0 phase, and the super-saturated solid solution of Cr,
Fe and Ti in BS40-X raw powder is formed. Virtually insoluble Cr
and Ti show large precipitation-hardening response in 60/40
brass matrix, because of the super-saturation of Ti and Cr in the
60/40 brass matrix creates a high degree of thermo-dynamic
meta-stability. This meta-stability provides a high chemical
potential for the precipitation reaction of Ti and Cr in 60/40
brass. As cooling down from the high temperature during powderconsolidation or forming, solid solution of Ti and Cr in brassdecreases rapidly and presents in form of ﬁne precipitates.
Rapid cooling from high temperatures is one method for applying
stress to cause dislocation motion [16] and increase atomic vacancies
thereby creating a high precipitation potential in the raw powder. The
higher dislocation density and atomic vacancies provide more sites
for precipitation, which provide a thermo-kinetic advantage for the
precipitation reaction [17]. Super-saturated solution of Cr and Ti inthe brass matrix creates a high chemical potential for the precipitation reaction. Additionally, the dislocation density increases graduallywith increasing deformation strain during hot extrusion, which
supplies more potential nucleation sites for Cr and Ti precipitation.
The precipitates distribute in grain boundaries and act as a pinningeffect, which increase resistance to grain boundary slip and produce
marked work-hardening effects. As a result, BS40-X exhibits signiﬁcantly stronger mechanical properties at low extrusion temperaturethan that of 60/40 brass.
A higher preheating temperature is appreciated with regard to
the process of the materials, that is to say, rising the preheating
temperature can decrease the deformation resistance during hotextrusion. However, the rise of the preheating temperature will
result in the ripening of the precipitates. The precipitate ripening
rate (dr/dt) is given as follows [18]:dr=dt ¼ Dcsr 2

ð1Þ

where r is the precipitate size; dr/dt is the interfacial energy
between the matrix and the precipitate; c is the equilibrium
solubility. According to the above relation, the increase of the
diffusion coefﬁcient D and the equilibrium solubility c due to theincrease of the temperature will lead to the increase the precipitate
ripening rate and the degradation of the mechanical properties of
the alloys. It is appreciated that a lower preheating temperature isnecessary for excellent properties of the wrought alloys.
Additionally, the possible recovery or dynamic recrystallizationinduced by hot deformation at higher temperatures might reducethe dislocation density [19,20]. The recrystallization that occurs

after hot-extrusion decreases the dislocation density, whichstrongly affects the mechanical behaviors of BS40-X. Consequently,
the mechanical properties of BS40-X continue to weaken due to thecoarsening and clearing away of Ti and Cr precipitates in the grainboundaries at elevated extrusion temperatures.

4. Conclusion
The microstructure, phase-transformation behavior, precipitation
behavior and mechanical properties of BS40-X brass were studied at
different extrusion temperature by powder metallurgy processing.
(1) Super-saturated solid solution of Cr, Fe and Ti in water-atomized
60/40 brass powder created a high chemical potential for the
precipitation reaction. Atom concentrations of Ti and Cr reduced
rapidly by increasing the extrusion temperature, Ti and Cr
elements presented in form of ﬁne precipitates in the brass
matrix, the ripening of the precipitates consequently weakened
the constraints of the grain growth, thus resulted in degrade ofmechanical properties by decrease of solid solution and grain
growth as rise of extrusion preheating temperature.
(2) The lower extrusion preheating temperature is necessary toobtain the excellent ﬁnal mechanical properties, thus the
extrusion temperature at 500 1C was enough to produce goodductility after extrusion while maintaining the promisingvalues of mechanical properties.

Acknowledgment
This work was supported as a project of the Japan Science andTechnology Agency (JST). The authors sincerely thank theresearchers of the Joining and Welding Research Institute (JWRI)
at Osaka University as well as Mr. Yoshinori Muraki and
Mr. Kyugo Inui for assistance in carrying out the extrusion
experiments and tensile tests.
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