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Materials and Design 32 (2011) 192–197

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

Contribution of Ti addition to characteristics of extruded Cu40Zn brass alloy
prepared by powder metallurgy
Shufeng Li *, Hisashi Imai, Haruhiko Atsumi, Katsuyoshi Kondoh
Department of Composites Processing, Joining and Welding Research Institute, Osaka University, 567-0047 11-1, Mihogaoka, Ibaraki, Osaka, Japana r t i c l e

i n f oArticle history:
Received 15 February 2010
Accepted 7 June 2010
Available online 10 June 2010
Keywords:
Brass
Spark plasma sintering
Precipitation hardeninga b s t r a c t
The aim of this paper was to investigate the properties of Cu40ZnTi for the purpose of developing a new
high-strength, lead-free brass by powder metallurgy. The effect of Ti addition on precipitation hardening
behavior of Cu40Zn (denoted as BS40) brass was studied with respect to mechanical properties andmicrostructures. BS40 and Cu40Zn  1.0 wt.%Ti (denoted as BS40-A) brass powders were prepared bywater atomization process, and b phase was retained in the raw powders predominately. The BS40 powder and Ti powder were elementally mixed to prepare Cu40Zn + 0.5 wt.%Ti (denoted as BS40-B) and
Cu40Zn + 1.0 wt.%Ti (denoted as BS40-C) premixed powders. The alloy powders and premixed powders
were solidiﬁed at 1053 K for 600 s by spark plasma sintering (SPS) and extruded subsequently. It wasobserved that Cu2ZnTi intermetallic compound (IMC) and CuZnTi metastable phase resulted from thereaction between Ti and CuZn showed distinct grain reﬁnement effect on extruded Cu40Zn brass. Thus,
the excellent strengthening effect processes by precipitation hardening and deform working was
obtained, which responding to an yield strength of 345 MPa, and a ultimate tensile strength of
597 MPa, showed 65.9% and 30.4% higher than that of extruded Cu40Zn brass, respectively.
Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction
Brasses are widely used in many applications, such as leadframes, connectors and other electronic components, pipes, valves,and ﬁttings in potable water system and so on, because of their
excellent electrical and thermal conductivities, outstanding resistance to corrosion, ease of fabrication, and good strength and fatigue resistance [1]. In recent years, the development of highstrength and high-conductive brasses are urgent to satisfy the
new demands of the electronic industry, automotive and aerospace
applications, household appliances, etc. [2]. On the other hand, thestricter regulations for allowable lead content levels in products
provide the impetus for the development of lead-free brass [3–5].
These applications require alloys with unique combinations ofstrength and conductivity coupled with environment amiability.
Precipitation hardening is the primary method of developing
high-strength copper-based alloys, because it could effectively improve the mechanical properties of alloys. The binary phase diagrams of copper alloys [6] indicate that Cr, Fe, Ti, Co, Mg, Sn, etc.
could be served as candidate alloying elements for precipitation
strengthening, to develop high-strength and high-conductivity
copper alloys, because of the solid solubility of these alloying
elements in copper would decrease sharply with the decreasing
temperature.
* Corresponding author. Tel.: +81 080 5513 6888; fax: +81 06 6879 8669.
E-mail address: shufengli@hotmail.com (S. Li).

Cu–Ti binary alloys are precipitation strengthened by spinodal
decomposition mechanism [7,8] involving composition modulations and long range ordering in the initial stages of ageing. The
tensile strength value of 930 N/mm2 was obtained for Cu–
5.4 wt.%Ti alloy by the precipitation of a coherent and metastable0
ﬁne precipitation of Cu4Ti (b ) [9]. According to Kumar et al. [10],the phase diagram shows ﬁve intermediate phases in the Cu–Ti
system, i.e. Cu4Ti, Cu3Ti2, Cu4Ti3, CuTi and CuTi2, which indicates
Ti is an attractive candidate to develop high strength copper alloys
by precipitation hardening. However, few literatures available reported the inﬂuence of Ti addition on mechanical properties of
Cu40Zn brass, which is worth to be investigating. The research isa part of a project whose scope was to investigate the engineering
properties of new commercial alloy formulations base on the
Cu40Zn duplex phase brass. The primary purpose of this paper is
to explore the possibility that new high strength lead-free brass
could be developed by novel spark plasma sintering (SPS), employing water atomized powders as raw materials. The properties ofprecipitation hardening response of Ti were employed and themechanical properties and microstructure were investigated indetail.
2. Experimental
As-received water atomized BS40 and BS40-A alloy powders(Nihon atomized metal powders Co.) were used as raw materials.
The compositions of the raw powder were shown in Table 1. To

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S. Li et al. / Materials and Design 32 (2011) 192–197
Table 1
Particle size and chemical compositions of the BS40 and BS40-A alloy powders.
Powders

Particle size (lm)Median size

Mean size

Zn

Ti

O

Cu

BS40
BS40-A

248
106

279
150

40
41.19

–
0.99

0.05
0.23

Bal.
Bal.

Mass%

understand the effects of Ti contents on properties of brass, thepowder of BS40-B and BS40-C were also prepared, respectively,by premixing BS40 powder and Ti powder for 2 h using a ball milling machine. For sintering, 300 g powder was loaded into a cylindrical graphite die and sintered using a DR.Sinter/SPS-1030
system (Sumitomo Coal Mining, Japan). The sintering temperatureand pressure were set as 1053 K and 40 MPa. After holding at desired temperature for 600 s, the power was turned off and the sample was cooled in the chamber to less than 423 K. The sinteringparameters were optimized to obtain a full density sample, andto avoid zinc vaporization at higher temperature, referring to the
optimal sintering parameters for Cu40Zn brass at present experimental conditions in previous investigations [11].
The billets obtained with a diameter 42 mm were extruded
with a load of 2000 kN by the hydraulic press machine (SHP200-50, Shibayama Machine Co.). Before extrusion, the billets werepreheated at 928 K for 30 min in nitrogen gas atmosphere, to ensure the billet having sufﬁcient plastic deformation properties atpresent extrusion conditions. The ﬁnal diameter after extrusion
was of 7 mm, and the extrusion ratio was 37:1. The schematic
sketch of the experimental procedure used to preparing samples
is shown in Fig. 1. The extruded round bar were machined into test
samples with 3 mm diameter in accordance with ICS 59.100.01.
Tensile strength was conducted on a universal testing machine(Autograph AG-X 50 N, Shimadzu) with a strain rate of 5 
104 s1. The strain was recorded by a CCD camera accessorized
to the machine. Three samples at the same conditions were pre-193

pared for the tensile strength test in order to obtain the averagevalue.
The phase compositions in samples were identiﬁed by using Xray diffraction (Labx, XRD-6100, Shimadzu) referenced to the standard ICDD PDF cards available in the system software. In addition,
the microstructure evolution of the raw powder and samples withthe transversal and longitudinal cross-section to the extrusion
direction, including fracture surface were conducted by usingﬁeld-emission scanning electronic microscope (FE-SEM, JEM6500F, JEOL), phases were examined by the energy-dispersive Xray spectrometer (DES) equipped by the SEM.
3. Results and discussion
3.1. Characteristic of the as-received powders
The FE-SEM micrographs of the raw powders are depicted inFig. 2. The BS40 and BS40-A particles show irregular shapes which
had a typical morphology prepared by water atomization process.
Fig. 3 showed the XRD diffractography of the as-atomized powderof BS40 and BS40-A. It can be observed that the raw powders
showing predominantly b phase peak at 43.28°, 62.87° and
79.39°. The analysis result is agreeable with the status that theraw powder were prepared by water atomized rapid solidiﬁcation
method, rapid cooled from the b phase ﬁeld retains a single b phasein the powders, referring to the Cu–Zn binary phase diagram.
3.2. Mechanical properties
Fig. 4 shows the nominal stress versus nominal strain curve of
the extruded alloying and premixed samples with different Ti content. The photograph of the tensile sample employed at present paper is illustrated in the ﬁgure. It is found that the content of Ti
addition has an observable inﬂuence on mechanical properties ofbrass alloys. The yield strength and ultimate tensile strength show

Fig. 1. Schematic sketch of the experimental procedure used to preparing samples.

Fig. 2. FE-SEM microstructures of the water atomized alloy powders. (a) BS40; (b) BS40-A; (c) Ti.

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S. Li et al. / Materials and Design 32 (2011) 192–197

the extruded premixed BS40-C sample. Furthermore, the yieldstrength of the extruded BS40-A is 345 MPa, 65.9% higher thanthe extruded BS40, possessing the highest value which is higher
than 290 MPa of the extruded BS40 sample. With respect to theductility, the extruded BS40 showed highest value of 44.0%, maintained best ductility than the other samples. The extruded BS40-A
showed an elongation of 25.2%, lower than that of the extruded
BS40-C. Based on the results as aforementioned, Ti addition withdifferent processing methods, saying of premixing or alloying,showed signiﬁcant effects on mechanical properties of BS40 alloy.
3.3. X-ray diffraction analysis
Fig. 3. X-ray diffractography of BS40 and BS40-A raw powders.

The X-ray diffractography of the extruded BS40, BS40-A and
BS40-C samples prepared by SPS are shown in Fig. 5. It can be observed that peaks appeared at 42.32°, 49.27° and 72.24° besides bphase peaks, which are a phase peaks, suggesting phase transformation of b to a + b duplex phases structure after the raw powder
experiencing thermal histories such as sintering at 1073 K, preheating at 923 K and extrusion at 673 K, and pressing deformation.
In addition, the minor peak is detected at 42.54° by narrow scanmethod, which is considered to be the main peak of TiO. Furthermore, the others phases conceivable presenting in reaction between Ti and CuZn were hard to be detected, because of the very
low amount of these phases in samples, the phases should be conﬁrmed by qualitative morphological characterization by SEM/EDS
analysis.
3.4. Microstructure

Fig. 4. Nominal stress–nominal strain curves of the extruded alloyed and premixed
samples with different Ti content.

an increasing trend with the content of Ti addition increase. BS40-C
achieve high yield strength of 290 MPa and tensile strength of
520 MPa, which show 39.4% and 13.5% higher than that of BS40.
Unfortunately, the increase of Ti addition shows an deterioration
effect on ductility of brass, saying the elongation decrease from
44% to 32.9% when Ti addition increases from 0 wt.% to 1.0 wt.%.
The ultimate tensile strength of the extruded BS40-A sample is597 MPa, 30.4% higher than the extruded BS40, indicating thehighest value in all the samples, which is higher than 520 MPa ofFig. 6 shows of the longitudinal and transverse cross-section of
the extruded samples. The longitudinal cross-section shown in
Fig. 6a is extruded BS40 sample, indicating the morphology ofindiscrete coarse a-phase grain elongated along the extrusion
direction in b-phase matrix. The phase structure consists of aphase crystals precipitated in b-phase matrix, due to the nucleation of a-phase grains during cooling after sintered at 1073 K,
and subsequently hot extruded at 928 K. The raw powder ofBS40 and BS40–1.0Ti are completely constituted by b-phase (asshown in Fig. 3) before sintering and hot extrusion. The a + b duplex phase structure came into being by phase transformation of
the parent b-phase at high temperature of sintering and subsequently extrusion. The extrusion deformation in hot extrusion also
provided energy required during phase transformation from b

Fig. 5. X-ray diffractography of the extruded alloyed and premixed samples prepared by spark plasma sintering.

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S. Li et al. / Materials and Design 32 (2011) 192–197

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Fig. 6. FE-SEM micrographs of the longitudinal and transversal cross-section of the extruded samples. (a–a0 ) BS40, (b–b0 ) BS40-A and (c–c0 ) BS40-C.

phase to a + b duplex phase structure. The average equivalentdiameter of a-phase grain size is approximately 6 lm in the transversal cross-section as shown in Fig. 6a0 .
The morphological features of the extruded BS40-A compact are
shown in Fig. 6b, b0 , which show quite different microstructures
when compared with the BS40 sample. The continuous a-phasegrains are ﬁner and elongated along the extrusion direction in bphase matrix, comparing with the microstructures of extruded
BS40 shown in Fig. 6a, a0 , which indicates Ti is a beneﬁcial element
for grain reﬁnement. The maximum solubility of Ti in copper is 8
at.% at 1158 K, while at room temperature the solubility is <1.0
at.% [12]. The maximum solubility of Ti in Zn is 0.027 at.% at room
temperature [13]. These data implies the solid solubility of Ti in
Cu40Zn alloy less than that of Ti in copper, higher concentrationof Ti leads to precipitation.
The EDS analysis at present study exhibits the solubility of Ti is
1.13 at.% in as-atomized raw powder of BS40-A. After sintered andextruded, the solubility of Ti decrease to 0.20 at.% in b-phase, and0.20 at.% in a-phase. Remained Ti precipitates in form of ultraﬁne

Cu2TiZn intermetallic compound (IMC), distributing uniformly inthe matrix [14], it is not possible to detect the chemical compositions of the IMC because of the small size and quantity of thatphase. Furthermore, EDS analysis result also indicates a concentration of 0.67 at.% O in the raw BS40-A alloy powder. Fine particle
can be detected at point 2 and point 4 in Fig. 7a, which is considered to be the particles of TiO, the result is consistent with the
XRD analysis as shown in Fig. 5. Portion of the oxygen is originatedin the preparation of the raw powder by water atomization process
as shown in Table 1. Further oxidation resulted from the sintering
during SPS and preheating before extrusion (see Table 2). The TiO
particles are inferior in bonding with matrix (as shown in Fig. 7aand Table 3), which deteriorates the ductility of BS40-A, saying
25.2%, which is lower than that of extruded BS40 and BS40-C.
The extruded BS40-C sample shows quite different microstructural and morphological characteristics (see Fig. 6c, c0 ), comparedwith that of the extruded BS40 and BS40-A. The a-phase primarygrains elongated along the extrusion direction in b-phase matrix,
plentiful of ultraﬁne Cu2ZnTi intermetallic compound (IMC) andAuthor's personal copy

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S. Li et al. / Materials and Design 32 (2011) 192–197

Fig. 7. EDS analysis of the extruded samples. (a) BS40-A and (b) BS40-C.

Table 2The tensile test results of the extruded alloyed and premixed samples with different
Ti contents.
Materials

0.2%YS (MPa)UTS (MPa)

Elongation (%)

BS40
BS40-B
BS40-C
BS40-A

208
278.3
290.1
345.5

458
514.8
520.3
597.1

44
38.3
32.9
25.2

CuZnTi metastable phase particles can be observed dispersed in aphase secondary grain boundaries (see Fig. 7b and Table 3), whichsuggest the dispersoids of Cu2ZnTi and CuZnTi particles redound to
grain reﬁnement during the a-phase recrystallization. In additional, the EDS analysis shows that the a-phase has higher solubility of 0.38 at.% for Ti than the b-phase of 0.08 at.% in premixed
BS40-B.
4. Discussion
The extruded samples prepared for this work, using BS40 alloy
powder and BS40-A alloy powder as raw materials. BS40 and BS40A alloy powders were prepared by water atomized method withhigh cooling rate. To understand the effects of Ti contents on properties of brass, the powders of BS40-B and BS40-C were preparedby premixing BS40 powder and Ti powder.
The results suggest that the content of Ti addition has an observable inﬂuence on mechanical properties of brass. The yieldstrength and ultimate strength show an increasing trend withthe content of Ti addition increase. On the other hand, Ti additionshows adverse inﬂuence on ductility of brass, accordingly, the content of Ti addition should be elaborately weighted combining withthe yield strength and ultimate strength.

A high cooling rate can lead to the formation of a more reﬁned
microstructure and extended solid solubility and even metastablephases [15,16]. Rapid cooling rate retained single b phase in the
powders, and super-saturated solid solubility of 1.13 at.% Ti in
BS40-A powder are formed. The solid solubility of Ti in brass decreases rapidly after experiencing different thermal histories, theequilibrium solubility of Ti in brass are 0.2 at.% in both a phaseand b phase. The ternary phase diagram shows that the b phasehas higher solubility for Ti than the a phase at equilibrium conditions [17]. However, at the non-equilibrium conditions, the solid
solubility of Ti in a phase and b phase show different variation
trend at present study.
The difference in atomic radius between Ti, Cu and Zn provides
a strain ﬁeld, which is beneﬁcial to the solid solution strength. It isfound that Ti has higher solid solution in extruded BS40-A matrix
than that of extruded BS40-C (as shown in Table 3), which is beneﬁcial to the solid solution strength of extruded BS40-A. Furthermore, super-saturated Ti in the brass matrix creates a highchemical potential for precipitation reaction of Ti. The ultraﬁnedispersoids of Cu2ZnTi precipitated during sintering and consequent extrusion, served as precipitation hardening which increase
resistance to grain boundary slip, achieving yield strength of
597 MPa and tensile strength of 345 MPa, which are higher thanthose of BS40 and BS40-C.
In respect of the extruded BS40-C, which was prepared by premixing BS40 powder and Ti powder, showed different microstructures and mechanical properties although conceiving the same
nominal composition as that of BS40-A. Cu2ZnTi dispersoids and
CuZnTi metastable particles are distributed in the grain boundaries, attributing to the reaction between Ti and Cu40Zn at hightemperature, and acting as grain reﬁners, enhancing the mechanical properties. Furthermore, the diffusion between Ti and Cu40Zn
at high temperature results in the equilibrium solubility of Ti isTable 3Chemical compositions (at.%) of the phase present in the extruded samples detected by SEM/EDS analysis.
Alloy

Phase

% Cu

% Zn

% Ti

%O

(a) BS40-A

1 (a phase)
2 (TiO)3 (b phase)
4 (TiO)

65.09 ± 2.95
6.80 ± 3.24
56.30 ± 2.63
17.58 ± 2.53

34.71 ± 4.16
5.92 ± 4.56
43.01 ± 3.72
17.06 ± 3.57

0.20 ± 0.55
43.51 ± 0.57
0.20 ± 0.59
41.39 ± 0.46–
43.77 ± 0.69
0.5 ± 0.18
23.98 ± 0.41(b) BS40-C

1 (a phase)
2 (Cu2TiZn)
3 (CuZnTi)
4 (b phase)

64.46 ± 2.46
49.23 ± 1.72
30.55 ± 1.90
57.26 ± 2.50

35.16 ± 3.48
25.25 ± 2.42
31.19 ± 2.68
42.65 ± 3.54

0.38 ± 0.46
25.51 ± 0.32
38.26 ± 0.35
0.08 ± 0.47–––

0.35 ± 0.23

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S. Li et al. / Materials and Design 32 (2011) 192–197

0.38 at.% in a phase and 0.08 at.% in b phase, suggesting the aphase has higher solubility for Ti than the b phase in premixed
samples. Precipitations presence in Cu–Ti binary phase system,
0such as Cu4Ti (b ), was difﬁcult to be conﬁrmed because of the very
low amount of this phase in both extruded alloyed and premixed
samples used in the present study.
5. Conclusions
This paper reports on the effects of Ti addition on microstructures and mechanical properties of Cu40Zn brass. Ti additive
showed signiﬁcant precipitation hardening response on mechanicalproperties of Cu40Zn brass, which could improve the yield strengthand ultimate tensile strength distinctly to about 65.9% and 30.4%
higher than BS40, respectively. The results suggest the effect of Ti
addition on Cu40Zn brass was worthy of further investigation.
Acknowledgements
This work was ﬁnancially supported by Regional Research andDevelopment Resources Utilization Program, Japan Science andTechnology Agency (JST). The authors extend their thanks to Nihon
atomized metal powders corporation (Project Manager: Mr. Koji
Yamamoto, Mr. Motoi Takahashi and Mr. Eisuke Yotsuka), for providing powders used in this study. And also grateful to San-Etsu
metals Co. Ltd., (Project Manager: Mr. Yoshiharu Kosaka and Mr.
Akimichi Kojima), for their technical supports in this study. The
authors sincerely thank the researchers of the Joining and Welding
Research Institute (JWRI) in Osaka University, Mr. Yoshinori Muraki, Ms. Sachiyo Nakamura and Mr. Kyugo Inui for assistant carryingout extrusion experiment.

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References[1] Davis JR. Alloying, understanding the basics. Materials Park OH 440730002: ASM International; 2002. p. 364.
[2] Li S, Imai H, Atsumi H, Kondoh K. Characteristics of high strength extruded
BS40CrFeSn alloy prepared by spark plasma sintering and hot pressing. J Alloys
Compd 2010;493:128–33.
[3] La Fontaine A, Keast VJ. Compositional distributions in classical and lead-freebrasses. Mater Charact 2006;57:424–9.
[4] Xiao L, Shu X, Yi D, Zhang X, Qin J, Hu J. Microstructure and properties of
unleaded free-cutting brass containing stibium. Trans Nonferr Metal Soc China
2007;17:s1055–9.
[5] Vilarinho C, Davim JP, Soares D, Castro F, Barbosa J. Inﬂuence of the mechanicalcomposition on the machinability of brasses. J Mater Process Technol
2005;170:441–7.
[6] Metals handbook, alloy phase diagrams, ASM handbook, vol. 3, 10th ed.; 1990.
[7] Laughlin DE, Cahn JW. Spinodal decomposition in age hardening copper–
titanium alloys. Acta Metall 1975;23:329–39.
[8] Poter A, Thompson AW. On the mechanism of precipitation strengthening in
Cu–Ti alloys. Scripta Metall 1984;18:1185–8.
[9] Nagarjuna S, balasubramanian K, Sarma DS. Effect of prior cold work onmechanical properties, electrical conductivity and microstructure of aged Cu–
Ti alloys. J Mater Sci 1999;34:2929–42.
[10] Kumar KCH, Ansara I, Wollants P, Deliaaey L. Thermodynamic optimization of
the Cu–Ti system. Z Metall 1996;87:666–72.
[11] Imai H, Kosaka Y, Kojima A, Li S, Kondoh K. Characteristics and machinabilityof lead-free P/M Cu60–Zn40 brass alloys dispersed with graphite. Powder
Technol 2010;198:417–21.
[12] Binary alloy phase diagrams, Metals handbook, vol. 1, Metals Park, OH, ASM;
1986. p. 820.
[13] Murray JL. Phase diagram of binary titanium alloys. ASM Int.; 1987.
[14] Heine W, Zwecker U. Phases of B2-structure type (CsCl-type) in ternary
systems with copper and nickel (in German). Naturewissenschaften
1962;17:391–5.
[15] Jones H. Cooling rates during rapid solidiﬁcation from a chill surface. Mater
Lett 1996;26:133–6.
[16] Katgerman L, Dom F. Rapid solidiﬁed aluminum alloys by melt spinning. Mater
Sci Eng A 2004;375–377:1212–6.
[17] Soares D, Vilarinho C, Castro F. Contribution to the knowledge of the Cu–Zn–Ti
system for compositions close to brass alloys. Scand J Metall 2001;30:254–7.