Materials Transactions, Vol. 56, No. 3 (2015) pp. 297 to 302
© 2015 Journal of Japan Institute of Copper
Precipitation Behavior and Properties of Cu-Ti Alloys with Added Nitrogen+1
Jun Ikeda1,+2, Satoshi Semboshi1,2, Akihiro Iwase1, Weilin Gao3 and Akira Sugawara3
1
Department of Materials Science, Osaka Prefecture University, Sakai 599-8531, Japan
Kansai Center, Institute for Materials Research, Tohoku University, Sakai 599-8531, Japan
3
Technology Center, Dowa Metaltech Co. Ltd., Iwata 438-0025, Japan
2
The effects of adding nitrogen (N) to age-hardenable Cu-Ti alloys on their microstructure, hardness, and conductivity have been
investigated. It was found that the aging of Cu-Ti-(0.06­0.6) mol% N alloys resulted in the continuous formation of finely precipitated needleshaped ¡-Cu4Ti grains and the discontinuous formation of coarse cellular components composed of a stable ¢-Cu4Ti and Cu solid solution at the
grain boundaries, in a manner similar to that in the case of conventional Cu-Ti alloys without any N. Furthermore, a small amount of granular
TiN particles was also formed in Cu-Ti-N alloys. The hardening behavior of Cu-Ti-N alloys was similar to that of Cu-Ti alloys without N. This
similarity was attributed to the finely dispersed precipitation of ¡-Cu4Ti that was similar between Cu-Ti alloys with and without N. The electrical
conductivity of Cu-Ti-N alloys increased more steeply than that of Cu-Ti alloys without N. This is because in the case of Cu-Ti-N alloys, the coprecipitation of ¡-Cu4Ti, ¢-Cu4Ti, and TiN efficiently reduced the Ti concentration in the matrix. Thus, the conductivity of peak-hardened Cu-TiN alloys can be improved by optimizing the N concentration and aging temperature. [doi:10.2320/matertrans.M2014216]
(Received August 1, 2014; Accepted December 3, 2014; Published February 25, 2015)
Keywords: copper-titanium alloy, nitrogen, aging, precipitate, hardness, electrical conductivity
1.
Introduction
A balance between the electrical conductivity and mechanical properties of Cu alloys used is required for
connector materials in electrical and electronic components.
Among Cu alloys used for connectors, age-hardenable Cu-Ti
alloys have high tensile strength, good workability, and good
stress relaxation properties.1­3) However, their conductivity
is approximately 10­15% IACS (% IACS: a measure of
electrical conductivity with the International Annealed
Copper Standard at 25°C being 100%IACS) which is less
than half that of typical alloys such as Cu-Be alloys and
Corson alloys.1­4) For this reason, the use of Cu-Ti alloys has
been limited to applications that place emphasis on mechanical properties.
The Ti concentration in typical age-precipitation-type
Cu-Ti alloys is about 3­6 mol%. These alloys are quenched
after a solution treatment at a primary Cu solid solution
range of 800°C or higher to transform it into a supersaturated
solid solution, and aging is performed at 400­500°C
thereafter. By this process, a metastable phase, ¡-Cu4Ti
(tetragonal; a = 0.5864 nm, c = 0.3649 nm), with a fine
nanoscale structure is produced in a Cu matrix phase.1­3,5)
In addition, a cellular component having a lamination of a Cu
solid solution phase and a stable ¢-Cu4Ti (orthorhombic
crystal; a = 0.4523 nm, b = 0.4341 nm, c = 1.2918 nm)
phase at the grain boundaries of the Cu matrix phase under
over-aging conditions is generated and grown.5­7) This alloy
has excellent strength, mainly owing to the precipitation
strengthening of fine ¡-Cu4Ti; whereas, over-aging causes the
cellular component to become coarse to the micron order,
significantly deteriorating the workability and strength of the
alloy.7) On the other hand, the conductivity of this alloy
gradually improves with aging. This is because the amount of
Ti solid solution in the Cu matrix phase decreases with the
+1
This Paper was Originally Published in Japanese in J. JICu 53 (2014) 62­
67.
+2
Graduate Student, Osaka Prefecture University
generation of a cellular component and ¡-Cu4Ti. However,
when aging is allowed to proceed so that the conductivity
reaches 20% IACS normally, it corresponds to a state of overaging and the cellular component will develop excessively,
thereby deteriorating the mechanical properties. Therefore,
the age-precipitation-type Cu-Ti alloys are often used in a
state where the hardness has been adjusted to a maximum by
peak-aging (i.e., before over-aging) even if the conductivity
is still less than 20% IACS. If it is possible to set the
conductivity at 20% IACS or higher while maintaining the
excellent mechanical properties of Cu-Ti alloys, their
application can be greatly expanded as they can be used as
alternative materials to Cu-Be alloys, which are harmful to
humans despite their high strength and conductivity.
The application of sever plastic deformation process8) or
the addition of a third element9) has been proposed as
alternative approaches to improve the balance between the
mechanical properties and conductivity of Cu-Ti alloys. In
recent years, Semboshi et al. found that when Cu-Ti alloys
are aged in a hydrogen (H) atmosphere, their conductivity
increases rapidly and significantly,10) showing that the
balance between strength and conductivity can be considerably improved by optimizing the aging conditions.11,12)
However, safety concerns arise during the manufacturing
process when utilizing H, so it has not been put into practical
use yet. Similar to H, nitrogen (N) has been identified as an
element that can improve the conductivity of Cu-Ti alloys.
This is because N has high affinity with Ti as in the case with
H; thus, it is expected to reduce the Ti solid solution that
remains in the matrix phase. Therefore, this study was aimed
at preparing age-hardenable Cu-Ti alloys with added N to
investigate changes in conductivity and hardness associated
with changes in its microstructure.
2.
Experimental Procedure
Specimens of compositionally controlled homogeneous
Cu-Ti-N alloys were prepared by the following processes.
Pure Ti (99.99%) was subjected to arc melting under an
298
J. Ikeda, S. Semboshi, A. Iwase, W. Gao and A. Sugawara
Fig. 1
FE-SEM images of solution-treated (a) Cu-Ti, (b) Cu-Ti-0.06N, (c) Cu-Ti-0.3N, and (d) Cu-Ti-0.6N alloys.
Table 1 Chemically analyzed Ti and N concentrations for the specimens
prepared in this study.
specimen
Ti, mol%
N, mol%
Cu
Cu-Ti
3.72
<0.001
bal.
Cu-Ti-0.06N
3.51
0.06
bal.
Cu-Ti-0.3N
3.64
0.32
bal.
Cu-Ti-0.6N
3.64
0.60
bal.
Ar + 10 vol% N gas atmosphere to prepare alloys having a
Ti-14.7 mol% N (melting point: ³2300°C) chemical composition. After weighing the Ti-14.7 mol% N alloy and O-free
Cu (purity 99.99%) to obtain a weight ratio of 1 : 1 in order
to lower its melting point so that it would dissolve easily, arc
melting was carried out in an Ar atmosphere to obtain a
matrix alloy. This was weighed with O-free Cu and pure Ti
and subjected to high-frequency melting to prepare four types
of alloy ingots having the chemical composition listed in
Table 1. The four alloys are hereby designated as Cu-Ti, CuTi-0.06N, Cu-Ti-0.3N, and Cu-Ti-0.6N. The alloys were
homogenized at 850°C for 24 h in vacuum; thereafter, they
were cold-rolled and plate-like specimens with the dimensions 40­55 mm length, 5­7 mm width, and 0.5 mm thickness
were cut. In order to obtain a supersaturated solid solution,
the specimens were quenched in water after being heated for
3 h at 850°C in vacuum. As an aging treatment, the test pieces
were kept at 420°C and 450°C for 0­450 h in vacuum.
The hardening behavior of the specimens due to aging
was evaluated on the basis of their Vickers hardness. The
hardness of each specimen was measured at 10 points or
more with a load of 300 g and a load time of 10 s. Their
electrical resistance at room temperature was measured by
the four-probe technique (measured current: 10 mA), and
their conductivity was calculated. The microstructure of
the specimens was observed with field emission scanning
electron microscopy (FE-SEM) and a transmission electron
microscopy (TEM). The precipitation phase in the specimens
was collected by a dissolution extraction separation method
carried out using a nitric acid aqueous solution.13,14) The
aging specimens and precipitate powder acquired by the
extraction and separation methods were subjected to X-ray
diffraction (XRD) to carry out a structural analysis.
3.
Results and Discussion
3.1 Microstructure
Figure 1 shows the FE-SEM images of solution-treated
Cu-Ti, Cu-Ti-0.06N, Cu-Ti-0.3N, and Cu-Ti-0.6N alloys.
The Cu-Ti and Cu-Ti-0.06N alloys were single-phase supersaturated solid solutions with a grain size of 50­100 µm
(Fig. 1(a), (b)). Massive inclusions of about 1 µm in diameter
Precipitation Behavior and Properties of Cu-Ti Alloys with Added Nitrogen
299
Fig. 2 FE-SEM and TEM images of Cu-Ti-0.6N alloys aged at 450°C for 24 h: (a), (b) low- and high-magnification FE-SEM images,
respectively. (c) Bright-field TEM image. (d), (e) Selected area diffraction patterns taken from areas marked as “d” and “e” in (c),
respectively. Weak spots marked in (d) are indexed to ¡-Cu4Ti.
were partially observed in the matrix phase of the Cu-Ti-0.3N
and Cu-Ti-0.6N alloys (Fig. 1(c), (d), indicated by circles).
Figure 2 shows the FE-SEM and TEM images of the CuTi-0.6N alloy aged at 450°C for 24 h. Cellular components
can be observed at several locations at the crystal grain
boundaries as marked by solid circles in Fig. 2(a). Furthermore, the massive inclusions in the grains and needle-shaped
continuous precipitates about 10 nm in length can also be
observed (Fig. 2(b), (c)). From the selected area electron
diffraction analysis, the needle-shaped precipitates were
identified as ¡-Cu4Ti, and the massive inclusions were
identified as TiN (cubical crystal; a = 0.4238 nm) (Fig. 2(d),
(e)). Here, it has been confirmed that the cellular components
were composed of lamination of the ¢-Cu4Ti and Cu solid
solution.5,6,14)
Figure 3 shows the FE-SEM images of the Cu-Ti-0.6N
alloy aged at 450°C for 450 h. The cellular components were
found to occupy almost the entire surface of the specimen
aged for 450 h. Needle-shaped ¡-Cu4Ti was also coarsened
owing to aging; however, because of the development of the
cellular components during the erosion of ¡-Cu4Ti, almost all
¡-Cu4Ti disappeared after 450 h. The size of TiN particles
increased to several microns, suggesting that they became
coarser than the solution material and initial aging material
(Fig. 1(d) and Fig. 2).
Fig. 3 FE-SEM images of Cu-Ti-0.6N alloys aged at 450°C for 450 h.
Granular particles of TiN are marked by solid circles.
Figure 4 shows the XRD patterns of the Cu-Ti-0.60 N
alloys aged at 450°C for 0, 12, and 450 h. In the alloys aged
for 0 h and 12 h, we found only peaks from Cu solid solution,
300
J. Ikeda, S. Semboshi, A. Iwase, W. Gao and A. Sugawara
Table 2 Constituent phases of precipitates in Cu-Ti and Cu-Ti-N alloys aged at 450°C for 450 h. Their mass fractions and lattice parameters are analyzed by
Rietveld fitting.
¡-Cu4Ti
specimen
¢-Cu4Ti
TiN
lattice constant (nm)
mass fraction (%)
a
b
c
Cu-Ti
0.0
0.0
®
100.0
0.4525
0.4340
1.2921
Cu-Ti-0.06N
0.0
3.0
0.4229
97.0
0.4525
0.4340
1.2919
Cu-Ti-0.3N
0.0
6.6
0.4229
93.4
0.4525
0.4341
1.2923
Cu-Ti-0.6N
0.0
26.8
0.4229
73.2
0.4524
0.4341
1.2920
a
mass fraction (%)
lattice constant (nm)
mass fraction (%)
simultaneously, a cellular component gradually generated and
developed at the crystal grain boundary, especially in overaging. Further, in Cu-Ti-N alloys, the generation and growth
of TiN occurred competitively. It is considered that the Ti and
N solid solutions in the Cu matrix phase were gradually
consumed with the growth of ¡-Cu4Ti, ¢-Cu4Ti, and TiN.
Fig. 4 XRD patterns of Cu-Ti-0.6N alloys solution-treated (a), and aged at
450°C for 12 h (b) and 450 h (c). XRD pattern of powder extracted from
alloy aged for 450 h is shown in (cA).
while in the alloys aged for 450 h, peaks from ¢-Cu4Ti as
well as Cu were detected. Extraction and separation were
carried out in order to identify the precipitates in the aged
alloys clearly. From the specimen aged for 450 h, cellular and
massive precipitates were collected, and diffraction peaks
of ¢-Cu4Ti and TiN were detected in the XRD patterns
(Fig. 4(cA)). The structure of the precipitates was then
evaluated by performing the Rietveld analysis. The constituent phases and volume fractions in Cu-Ti, Cu-Ti-0.06N, CuTi-0.3N, and Cu-Ti-0.6N alloys aged for 450 h are summarized in Table 2. From the results shown, it can be confirmed
that after 450 h of aging, ¡-Cu4Ti was eliminated from every
Cu-Ti-N alloy and ¢-Cu4Ti and TiN were formed. The
formation rate of TiN increased with the N concentration in
the Cu-Ti-N alloys.
Massive inclusions about 1 µm in diameter were observed
in the solution-treated Cu-Ti-0.3N and Cu-Ti-0.6N alloys
(Fig. 1(c), (d)). They were identified as TiN (Fig. 2(c), (e)).
Further, since TiN was not observed in the solution-treated
Cu-Ti-0.06N alloy, it can be said that the solubility limit of
solid N in the Cu-3.6 mol% Ti alloy was between 0.06 and
0.3 mol%. In every Cu-Ti-N alloy, similar to the case of the
Cu-Ti alloys, fine needle-shaped ¡-Cu4Ti was continuously
precipitated in the crystal grains during the initial aging, and
3.2 Hardness and electrical conductivity
Figure 5 shows the Vickers hardness and electrical
conductivity of Cu-Ti, Cu-Ti-0.06N, and Cu-Ti-0.6N alloys
aged at 450°C and 420°C. Every alloy had a hardness of
about 120 Hv at the time of solution treatment. When the
alloys were aged at 450°C, their hardness increased with
aging, reaching a maximum at 12­24 h, and decreased
thereafter. When the alloys were aged at 420°C, their
hardness reached the maximum value at 96 h. The hardness
of the alloys decreased as a whole as the N concentration in
the alloys increased at both the aging temperatures.
Regarding electrical conductivity, every alloy had about
5% IACS at the time of solution treatment. The conductivity
of the all alloys increased with the aging time. In the alloy
with higher N concentration, the increasing rate of the
conductivity with the aging time became more rapid. The
conductivity of the Cu-Ti-0.6N alloys at the aging time at the
maximum hardness was 18% IACS at the aging temperature
of 450°C and 22% IACS at the aging temperature of 420°C.
In other words, the conductivity was improved by 4% IACS
at the aging temperature of 450°C and 5% IACS at the aging
temperature of 420°C compared to Cu-Ti alloys.
The main factor responsible for the high hardness of Cu-Ti
alloys is the dispersion of fine needle-shaped ¡-Cu4Ti;1­3,5,14)
the amount, size, and number density of ¡-Cu4Ti are effective
for the strength. In the Cu-Ti-N alloys, a portion of the Ti
solid solution is spent for the generation and growth of TiN,
leading to relative decrease in the amount and number density
of ¡-Cu4Ti precipitates, although the size is similar in both
the alloys with and without N. Therefore, the hardness
decreases slightly with the N concentration of the aged
alloys.
Since the effect on the electrical resistance of the Ti
solid solution is extremely large with respect to Cu,10) the
conductivity of Cu-Ti alloys mainly increases with decrease
in the concentration of the Ti solid solution in the Cu matrix
phase. In the Cu-Ti-N alloys, the Ti solid solution is spent not
only in the formation of ¡-Cu4Ti and ¢-Cu4Ti but also for the
formation of TiN; therefore, the amount of Ti solid solution
decreases rather efficiently. For this reason, the rate of
Precipitation Behavior and Properties of Cu-Ti Alloys with Added Nitrogen
300
300
(b) 420 °C
(a) 450 °C
250
Vickers hardness,
Vickers hardness,
301
200
Cu-Ti
150
Cu-Ti-0.06N
250
200
Cu-Ti
Cu-Ti-0.06N
150
Cu-Ti-0.60N
Cu-Ti-0.60N
30
30
Cu-Ti-0.06N
Conductivity, % IACS
Conductivity, % IACS
Cu-Ti
25
Cu-Ti-0.60N
20
15
10
5
0
25
20
15
10
5
0
0
1
10
100
1000
Cu-Ti
Cu-Ti-0.06N
Cu-Ti-0.60N
0.1
1
Aging time, h
10
100
1000
Aging time, h
Fig. 5 Vickers hardness (upper) and electrical conductivity (lower) of Cu-Ti, Cu-Ti-0.06N, and Cu-Ti-0.6N alloys aged at (a) 450°C and
(b) 420°C.
conductivity increase on aging is higher in the Cu-Ti-N
alloys.
From a practical point of view, the maximum hardness and
corresponding conductivity are important in aged-hardenable
Cu-Ti alloys. This is because after peak-hardening condition
(i.e., under the over-aging condition) cellular components
develop significantly,5,14) which cause to degradation of not
only the strength but also workability and fatigue characteristics. In this study, aging conditions under which hardness
was highest (250 Hv or higher) and the conductivity
exceeded 20% IACS were present in both alloys: with or
without added N, when aging at 420°C: for the Cu-Ti-N
alloys, aging at 420°C resulted in the conductivity exceeding
20% IACS (22% IACS for a Cu-Ti-0.6N alloy) at the aging
time corresponding to the maximum hardness. On the other
hand, the conductivity of the Cu-Ti alloys without N did not
reach 20% IACS at the aging time corresponding to the
maximum hardness, but reached it after over-aging for 240 h.
Thus, in the case of Cu-Ti-N alloys, it is possible to realize
aging materials with cellular components not yet developed
having a Vickers hardness of 250 Hv or higher and a
conductivity of 20% IACS or higher. Decrease in the
maximum hardness due to N addition is an issue that needs
to be addressed. However, it can be said that one of the
effective additive elements is N that can improve the balance
between workability and conductivity, as well as provide
optimum structure control when an appropriate amount of N
is added in the Cu-Ti alloys.
4.
Conclusions
In this study, we prepared Cu-Ti-N alloys with constituents
controlled homogenously by using the dissolution method.
The precipitation behavior of Cu-Ti-N alloys was evaluated
by investigating the changes in their conductivity and
hardness, and observing the texture of aged Cu-Ti-N alloys
subjected to a solution treatment. The following conclusions
were obtained:
(1) The solid solubility limit of N for Cu-3.6 mol% Ti
alloys was between 0.06 and 0.3 mol%.
(2) In Cu-Ti-N alloys (similar to Cu-Ti alloys), a
continuous precipitation of fine needle-shaped ¡-Cu4Ti
and a discontinuous precipitation of coarse cellular
¢-Cu4Ti occurred with aging. In addition, in Cu-Ti-N
alloys, the generation and growth of TiN, with a
tetragonal structure, occurred.
(3) In Cu-Ti-N alloys, an age hardening curve similar to
that for Cu-Ti alloys was observed. However, the
overall hardness decreased as the amount of added N
increased. Furthermore, the electrical conductivity
improved as the amount of added N increased.
(4) If the amount of added N and the aging conditions are
selected, it is possible to obtain alloys having a
conductivity of more than 20% IACS with a Vickers
hardness of more than 250 Hv.
Acknowledgements
The authors are grateful to Prof. T. Takasugi and Prof. Y.
Kaneno of Osaka Prefecture University, and Prof. N.
Masahashi and Prof. S. Sato of the Institute for Materials
Research (IMR) of Tohoku University for their useful
discussions and comments. The authors also thank Mr. M.
Ishikuro, Mr. E. Aoyagi, and Mr. Y. Hayasaka of the IMR for
302
J. Ikeda, S. Semboshi, A. Iwase, W. Gao and A. Sugawara
their technical assistance. Financial support provided by the
Japan Society for the Promotion of Science (JSPS) via a
Grant-in-Aid for Young Research (A) (No. 236860104), and
by the Japan Copper and Brass Association is gratefully
acknowledged.
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