L. FEI et al.: FORMATION MECHANISM OF DIFFUSION-REACTION LAYER FOR A Cu/Ti ...
1025–1029
FORMATION MECHANISM OF DIFFUSION-REACTION LAYER
FOR A Cu/Ti DIFFUSION COUPLE UNDER DIFFERENT HEATING
METHODS
OBLIKOVANJE MEHANIZMA DIFUZIJSKO REAKCIJSKE PLASTI
NA Cu/Ti POVR[INI Z RAZLI^NIMI METODAMI SEGREVANJA
Liu Fei1,2, Wu Mingfang1, Pu Juan1
1Provincial Key Laboratory of Advanced Welding Technology, Jiangsu University of Science and Technology, Zhenjiang 212003, China
2Zhenjiang Technician Institute, Zhenjiang 212000, China
pu_juan84@163.com
Prejem rokopisa – received: 2017-05-23; sprejem za objavo – accepted for publication: 2017-06-22
doi:10.17222/mit.2017.062
Diffusion experiments with a Cu/Ti diffusion couple were conducted using the pulse-current-heating method and the
conventional-heating method. The interfacial microstructure and the growth behavior of the diffusion-reaction layer were
investigated with a scanning electron microscope (SEM), electron probe micro-analyzer (EPMA), energy dispersive
spectrometer (EDS) and X-ray diffraction (XRD). The results showed that the pulse-current heating could accelerate the
diffusion of Ti atoms into a Cu matrix, significantly improve the growth rate of the diffusion-action layer, located at the side of
the Cu matrix and make the growth of the interfacial reaction layer follow the parabolic rule. Meanwhile, under the
pulse-current heating, the stratification of the diffusion-reaction layer was not obvious. Correspondingly, under the conventional
heating method, the stratification of the diffusion-reaction layer was obvious and each reaction layer had a single
microstructure, which was consistent with the reactant from the corresponding Cu-Ti binary-alloy phase diagram.
Keywords: Cu/Ti diffusion couple, pulse-current heating, diffusion, interfacial microstructure, interfacial-layer growth
Avtorji prispevka so izvedli difuzijske eksperimente na paru Cu/Ti. Pri tem so difuzijski par ogrevali na dva razli~na na~ina; z
uporabo postopka impulznega tokovnega ogrevanja in konvencionalnega postopka ogrevanja. Medfazno mikrostrukturo in rast
difuzijsko-reakcijske plasti so analizirali z vrsti~nim elektronskim mikroskopom (SEM), mikroanalizatorjem z elektronsko
sondo (EPMA), energijskim disperzijskim spektrometrom (EDS) in rentgensko difrakcijsko analizo (XRD). Rezultati analiz so
pokazali, da impulzno tokovno ogrevanje lahko pospe{i difuzijo atomov Ti v matriko Cu in o~itno izbolj{a hitrost rasti
difuzijske plasti, ki se nahaja na strani Cu matrike. Pri tem kinetika rasti difuzijsko-reakcijske plasti sledi paraboli~nemu
zakonu. Niso pa opazili izrazitega plastenja difuzijsko- reakcijske plasti, ki je bilo o~itno pri konvencionalnem na~inu ogrevanja
difuzijskega para. V tem primeru je imela vsaka reakcijska plast enovito mikrostrukturo, skladno z reaktantoma v binarnem
faznem diagramu Cu-Ti.
Klju~ne besede: difuzijski par Cu/Ti, impulzno tokovno ogrevanje, difuzija, medfazna mikrostruktura, rast medfazne plasti
1 INTRODUCTION
Titanium alloys have been widely used in aerospace,
national defense, medical treatment, etc. since 1950s due
to their high strength, good corrosion resistance and
excellent heat resistance. However, their application is
restricted due to the high price. Therefore, it is of great
importance to study the joining technologies applicable
for titanium alloys and other materials. Currently, the
main joining technologies include brazing1,2, solid-phase
diffusion welding,3,4 superplastic forming and diffusion-
bonding composite welding,5 friction welding6 and laser
welding.7 Diffusion welding has been widely used in the
manufacture of titanium alloys that have a complex and
thin-walled structure.8
A titanium alloy and copper alloy were joined with
brazing and diffusion welding, respectively.9,10 A compa-
rative study showed that the microstructure and the
strength of welded joints joined with two different me-
thods show large differences. According to the Cu-Ti
binary-alloy phase diagram,11 titanium and copper gener-
ally cannot dissolve with each other. The solid solubility
of titanium dissolved in copper is very small and the
maximum solid solubility is ~5 % at 890 °C. Corres-
pondingly, the solid solubility of copper dissolved in
titanium is smaller and the maximum solid solubility is
less than ~3 % at 798 °C. Several different kinds of inter-
phases existed in a Cu-Ti binary-alloy system, and most
of the interphases are intermetallic compounds.
The welding thermal process is a non-equilibrium
thermodynamic process. It is difficult to simply judge the
welding microstructure of titanium alloy and copper
alloy joined by brazing and diffusion welding based on a
Cu-Ti binary-alloy phase diagram. In this paper,
diffusion experiments with a Cu/Ti diffusion couple were
carried out using pulse-current heating and conventional
heating at different heating temperatures and holding
times. The effects of different heating methods and pro-
cess parameters on the structure of an interfacial diffu-
sion-reaction layer, together with the growth behavior of
the diffusion-reaction layer under the condition of
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1025–1029 1025
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 620.1:669.295:621.3.017.7 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(6)1025(2017)
pulse-current heating were carefully investigated. The
related research is aimed to provide the necessary expe-
rimental data and theoretical analysis for predicting the
structural type and growth characteristics of an inter-
facial reaction layer.
2 EXPERIMENTAL PART
2.1 Materials and methods
Cylindrical samples of pure copper and pure titanium
(more than 99.99 %) with dimensions of 20 × 3 mm
were used as the diffusion matrix. The samples were first
mechanically polished, then polished with sand papers
and finally cleaned with ultrasonic vibration. The sam-
ples were assembled in the form of pure copper/pure
titanium/pure copper, then diffused and joined in a heat-
ing furnace using different heating methods. When the
pulse-current heating process was used, the vacuum was
not less than 13 Pa, the axial pressure load was 1 MPa
and the duty cycle of direct-current pulse was 16/2
(on/off). Meanwhile, the heating rate was set as
100 °C/min, the heating temperature was 700 °C and the
holding time was chosen in a range of 3–10 min. The
samples were cooled inside the furnace to room tempe-
rature after diffuse joining. For comparison, the diffusion
experiment under conventional heating was conducted
using the same diffusion couple and the related process-
ing parameters were set as follows: the thermal vacuum
degree was not less than 1 × 10–2 Pa, the axial pressure
load was 1 MPa, the heating temperature was 800 °C
with holding times of 10–60 min and a heating speed of
100 °C /min.
The analysis samples were cut by wire cutting along
the centerline, and the surfaces of the analysis samples
were polished with different-grade sand papers with
Nos.1–5. The diffusion-reaction areas of the copper/tita-
nium samples were observed with a scanning electron
microscope (SEM). The diffusion behavior between the
Cu atoms and Ti atoms was analyzed with an electron-
probe micro-analyzer (EPMA). The chemical composi-
tions of feature points in the microstructure were
detected with an energy dispersive spectrometer (EDS).
The microstructure of the diffusion-reaction zone was
characterized with X-ray diffraction (XRD).
3 RESULTS AND ANALYSIS
3.1 Structure analysis of the diffusion-reaction layer
under conventional heating
The diffusion-reaction experiments were carried on a
Cu/Ti diffusion couple with a heating temperature of
800 °C, holding times of 10–60 min and axial pressure
load of 1 MPa. Figure 1 shows the morphology and
elemental distribution of the diffusion-reaction layer at
holding times of 30 min and 60 min.
Figures 1a and 1b present SEM images and elemen-
tal-line scanning results for the sample with the holding
time of 30 min. The results indicate that the inter-diffu-
sion between the Cu atoms and Ti atoms occurred in the
interface of the Cu/Ti diffusion couple. The deep-gray
area on the left side is the Ti matrix, the gray region on
the right side is the Cu matrix, and the middle region bet-
ween them is the diffusion-reaction layer. The diffusion-
reaction area is divided into five layers due to different
contents of Ti atoms, namely, zones I–V. The width of
zone I at the side of the Ti matrix is about 4–6 μm,
formed due to the reaction of the Cu atoms and Ti atoms
when the Cu atoms diffused into the Ti matrix (Fig-
ure 1b). When the Ti atoms diffused into the Cu matrix,
zone II was formed at the side of the Cu matrix. Its width
is about 1–2 μm. In addition, zone III, zone IV and zone
V were formed along the depth direction of the Cu
matrix. The widths of these three zones are (8–10, 2–3
and 5–7) μm, respectively. The total width of the diffu-
sion-reaction layers is about 22–25 μm after the holding
time of 30 min.
Figure 1c shows a secondary electronic image of the
diffusion-reaction layer after the holding time of 60 min.
Figure 1d displays the elemental-line scanning result for
the corresponding diffusion-reaction layer (Figure 1c).
This diffusion-reaction layer also has five layers and the
total width of the diffusion-reaction layer is increased to
30–34 μm (Figure 1c), as compared with Figure 1a. The
inter-diffusion behavior of the Cu atoms and Ti atoms in
the Cu/Ti diffusion couple is more remarkable under the
condition of conventional heating with the longer
holding time of 60 min. Meanwhile, the rate of diffusion
of the Ti atoms into the Cu matrix is higher than that of
the Cu atoms into the Ti matrix. In order to determine
the chemical composition of each reaction layer, EDS
was employed to analyze the samples and the results are
shown in Table 1. The compounds can be preliminary
L. FEI et al.: FORMATION MECHANISM OF DIFFUSION-REACTION LAYER FOR A Cu/Ti ...
1026 Materiali in tehnologije / Materials and technology 51 (2017) 6, 1025–1029
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Morphology and elemental distribution of diffusion-reaction
layer in the case of conventional heating: a) morphology image for the
holding time of 30 min; b) EPMA line-scanning result for the holding
time of 30 min; c) morphology image for the holding time of 60 min;
d) EPMA line-scanning result for the holding time of 60 min
determined based on the contents of the Ti atoms and Cu
atoms for the feature points.
Table 1: EDS results for the interfacial reaction layer for conventional
heating
Feature points Ti Cu Compounds
I 95.78 4.22 (-Ti)+Ti2Cu
II 72.06 27.94 Ti2Cu
III 51.46 48.54 TiCu
IV 23.23 76.77 TiCu4
V 9.26 90.74 (Cu)+TiCu4
In order to realize the diffusion of atoms, a driving
force that can be either a concentration difference or a
chemical potential difference is necessary. From the
dynamic point of view, the inter-diffusion rate between
the Cu atoms and Ti atoms was increased by increasing
the temperature for the Cu/Ti diffusion couple. The
driving force of diffusion was derived from the concen-
tration difference for both sides of the interface.
Meanwhile, as the diffusion rate of the Ti atoms was
higher than that of the Cu atoms, the thickness of the
diffusion-reaction layer at the side of the Cu matrix was
increased significantly. From the view of thermodyna-
mics, atoms always transferred spontaneously from the
area of high chemical potential to the area of low
chemical potential. As the chemical potential of Ti atoms
was higher than that of Cu atoms, the gradient of che-
mical potential can also be deemed as a driving force
that promoted the diffusion of the Ti atoms into the Cu
matrix. According to the Cu-Ti binary-alloy phase dia-
gram and reference12, there is a variety of intermediate
phases between these two elements. Most of them are
intermetallic compounds, including TiCu4, Ti2Cu3,
Ti3Cu4, TiCu and Ti2Cu. Based on the experimental
results and the existing related researches, we can say
that when the heating temperature was set as 800 °C and
the holding time was 30 min, the reaction products of the
Cu/Ti diffusion couple were (-Ti)+Ti2Cu in zone I,
Ti2Cu in zone II, TiCu in zone III, TiCu4 in zone IV and
(Cu)+TiCu4 in zone V. Under the conventional-heating
conditions with sufficient holding time, practical
structures of the diffusion-reaction layers were consistent
with the compounds from the Cu-Ti binary-alloy phase
diagram. However, it should be noted that it was hard to
form a continuous and compact interfacial reaction layer
in the Cu/Ti diffusion couple under the condition of the
present conventional heating. In addition, microvoids can
be found in the initial interface.
3.2 The structure of diffusion reaction layer under
pulse current heating
For comparison, the diffusion experiment with the
Cu/Ti diffusion couple was carried out under pulse-
current heating at a temperature of 700 °C, holding times
of 3–10 min and pulse-duty ratio of 16/2. Figure 2
shows the morphology and elemental line-scanning
results for the diffusion-reaction layer between the Cu
matrix and the Ti matrix at holding times of 6 min and
3 min. Table 2 lists EDS analysis results for feature
points A, B, C and D.
Table 2: EDS results for the interfacial reaction layer for pulse-current
heating
Feature points Ti Cu Compounds
A 25.24 74.76 (Cu)+TiCu4
B 30.42 69.58 (Cu)+TiCu4
C 69.61 30.39 Ti2Cu
D 97.53 2.47 (-Ti)+Ti2Cu
As shown in Figure 2, the Cu/Ti diffusion-reaction
layer can be formed at different holding times. The width
of the diffusion-reaction layer was 4–6 μm at the holding
time of 3 min (Figure 2a) while it grew up to 10–12 μm
at the holding time of 6 min (Figure 2c). When com-
pared with the conventional heating method, a conti-
nuous and dense diffusion-reaction layer can be formed
in a short holding time under the condition of pulse-
current heating, proving that pulse-current heating can
accelerate the process of interfacial reaction. Figure 2b
shows the results of the elemental line-scanning analysis
for the corresponding diffusion-reaction layer (Figure
2a). The diffusion of the Ti atoms into the Cu matrix can
be clearly seen from Figure 2b. The depth of the
titanium diffusion-reaction layer can be up to 10 μm.
However, the diffusion of the Cu atoms into the Ti ma-
trix is invisible. These results confirmed that under the
effects of pulse-current heating, the diffusion of the Ti
atoms into the Cu matrix was promoted while the diffu-
sion of the Cu atoms into the Ti matrix was suppressed.
Although the heating temperature was relatively low and
the holding time was very short, the diffusion-reaction
layer between the Cu matrix and Ti matrix was formed.
L. FEI et al.: FORMATION MECHANISM OF DIFFUSION-REACTION LAYER FOR A Cu/Ti ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1025–1029 1027
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Morphology and element distribution of diffusion-reaction
layer in the case of pulse-current heating: a) morphology image for the
holding time of 6 min, b) EPMA line-scanning result for the holding
time of 6 min, c) morphology image for the holding time of 3 min, d)
EPMA line-scanning result for the holding time of 3 min
pulse-current heating were carefully investigated. The
related research is aimed to provide the necessary expe-
rimental data and theoretical analysis for predicting the
structural type and growth characteristics of an inter-
facial reaction layer.
2 EXPERIMENTAL PART
2.1 Materials and methods
Cylindrical samples of pure copper and pure titanium
(more than 99.99 %) with dimensions of 20 × 3 mm
were used as the diffusion matrix. The samples were first
mechanically polished, then polished with sand papers
and finally cleaned with ultrasonic vibration. The sam-
ples were assembled in the form of pure copper/pure
titanium/pure copper, then diffused and joined in a heat-
ing furnace using different heating methods. When the
pulse-current heating process was used, the vacuum was
not less than 13 Pa, the axial pressure load was 1 MPa
and the duty cycle of direct-current pulse was 16/2
(on/off). Meanwhile, the heating rate was set as
100 °C/min, the heating temperature was 700 °C and the
holding time was chosen in a range of 3–10 min. The
samples were cooled inside the furnace to room tempe-
rature after diffuse joining. For comparison, the diffusion
experiment under conventional heating was conducted
using the same diffusion couple and the related process-
ing parameters were set as follows: the thermal vacuum
degree was not less than 1 × 10–2 Pa, the axial pressure
load was 1 MPa, the heating temperature was 800 °C
with holding times of 10–60 min and a heating speed of
100 °C /min.
The analysis samples were cut by wire cutting along
the centerline, and the surfaces of the analysis samples
were polished with different-grade sand papers with
Nos.1–5. The diffusion-reaction areas of the copper/tita-
nium samples were observed with a scanning electron
microscope (SEM). The diffusion behavior between the
Cu atoms and Ti atoms was analyzed with an electron-
probe micro-analyzer (EPMA). The chemical composi-
tions of feature points in the microstructure were
detected with an energy dispersive spectrometer (EDS).
The microstructure of the diffusion-reaction zone was
characterized with X-ray diffraction (XRD).
3 RESULTS AND ANALYSIS
3.1 Structure analysis of the diffusion-reaction layer
under conventional heating
The diffusion-reaction experiments were carried on a
Cu/Ti diffusion couple with a heating temperature of
800 °C, holding times of 10–60 min and axial pressure
load of 1 MPa. Figure 1 shows the morphology and
elemental distribution of the diffusion-reaction layer at
holding times of 30 min and 60 min.
Figures 1a and 1b present SEM images and elemen-
tal-line scanning results for the sample with the holding
time of 30 min. The results indicate that the inter-diffu-
sion between the Cu atoms and Ti atoms occurred in the
interface of the Cu/Ti diffusion couple. The deep-gray
area on the left side is the Ti matrix, the gray region on
the right side is the Cu matrix, and the middle region bet-
ween them is the diffusion-reaction layer. The diffusion-
reaction area is divided into five layers due to different
contents of Ti atoms, namely, zones I–V. The width of
zone I at the side of the Ti matrix is about 4–6 μm,
formed due to the reaction of the Cu atoms and Ti atoms
when the Cu atoms diffused into the Ti matrix (Fig-
ure 1b). When the Ti atoms diffused into the Cu matrix,
zone II was formed at the side of the Cu matrix. Its width
is about 1–2 μm. In addition, zone III, zone IV and zone
V were formed along the depth direction of the Cu
matrix. The widths of these three zones are (8–10, 2–3
and 5–7) μm, respectively. The total width of the diffu-
sion-reaction layers is about 22–25 μm after the holding
time of 30 min.
Figure 1c shows a secondary electronic image of the
diffusion-reaction layer after the holding time of 60 min.
Figure 1d displays the elemental-line scanning result for
the corresponding diffusion-reaction layer (Figure 1c).
This diffusion-reaction layer also has five layers and the
total width of the diffusion-reaction layer is increased to
30–34 μm (Figure 1c), as compared with Figure 1a. The
inter-diffusion behavior of the Cu atoms and Ti atoms in
the Cu/Ti diffusion couple is more remarkable under the
condition of conventional heating with the longer
holding time of 60 min. Meanwhile, the rate of diffusion
of the Ti atoms into the Cu matrix is higher than that of
the Cu atoms into the Ti matrix. In order to determine
the chemical composition of each reaction layer, EDS
was employed to analyze the samples and the results are
shown in Table 1. The compounds can be preliminary
L. FEI et al.: FORMATION MECHANISM OF DIFFUSION-REACTION LAYER FOR A Cu/Ti ...
1026 Materiali in tehnologije / Materials and technology 51 (2017) 6, 1025–1029
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Morphology and elemental distribution of diffusion-reaction
layer in the case of conventional heating: a) morphology image for the
holding time of 30 min; b) EPMA line-scanning result for the holding
time of 30 min; c) morphology image for the holding time of 60 min;
d) EPMA line-scanning result for the holding time of 60 min
determined based on the contents of the Ti atoms and Cu
atoms for the feature points.
Table 1: EDS results for the interfacial reaction layer for conventional
heating
Feature points Ti Cu Compounds
I 95.78 4.22 (-Ti)+Ti2Cu
II 72.06 27.94 Ti2Cu
III 51.46 48.54 TiCu
IV 23.23 76.77 TiCu4
V 9.26 90.74 (Cu)+TiCu4
In order to realize the diffusion of atoms, a driving
force that can be either a concentration difference or a
chemical potential difference is necessary. From the
dynamic point of view, the inter-diffusion rate between
the Cu atoms and Ti atoms was increased by increasing
the temperature for the Cu/Ti diffusion couple. The
driving force of diffusion was derived from the concen-
tration difference for both sides of the interface.
Meanwhile, as the diffusion rate of the Ti atoms was
higher than that of the Cu atoms, the thickness of the
diffusion-reaction layer at the side of the Cu matrix was
increased significantly. From the view of thermodyna-
mics, atoms always transferred spontaneously from the
area of high chemical potential to the area of low
chemical potential. As the chemical potential of Ti atoms
was higher than that of Cu atoms, the gradient of che-
mical potential can also be deemed as a driving force
that promoted the diffusion of the Ti atoms into the Cu
matrix. According to the Cu-Ti binary-alloy phase dia-
gram and reference12, there is a variety of intermediate
phases between these two elements. Most of them are
intermetallic compounds, including TiCu4, Ti2Cu3,
Ti3Cu4, TiCu and Ti2Cu. Based on the experimental
results and the existing related researches, we can say
that when the heating temperature was set as 800 °C and
the holding time was 30 min, the reaction products of the
Cu/Ti diffusion couple were (-Ti)+Ti2Cu in zone I,
Ti2Cu in zone II, TiCu in zone III, TiCu4 in zone IV and
(Cu)+TiCu4 in zone V. Under the conventional-heating
conditions with sufficient holding time, practical
structures of the diffusion-reaction layers were consistent
with the compounds from the Cu-Ti binary-alloy phase
diagram. However, it should be noted that it was hard to
form a continuous and compact interfacial reaction layer
in the Cu/Ti diffusion couple under the condition of the
present conventional heating. In addition, microvoids can
be found in the initial interface.
3.2 The structure of diffusion reaction layer under
pulse current heating
For comparison, the diffusion experiment with the
Cu/Ti diffusion couple was carried out under pulse-
current heating at a temperature of 700 °C, holding times
of 3–10 min and pulse-duty ratio of 16/2. Figure 2
shows the morphology and elemental line-scanning
results for the diffusion-reaction layer between the Cu
matrix and the Ti matrix at holding times of 6 min and
3 min. Table 2 lists EDS analysis results for feature
points A, B, C and D.
Table 2: EDS results for the interfacial reaction layer for pulse-current
heating
Feature points Ti Cu Compounds
A 25.24 74.76 (Cu)+TiCu4
B 30.42 69.58 (Cu)+TiCu4
C 69.61 30.39 Ti2Cu
D 97.53 2.47 (-Ti)+Ti2Cu
As shown in Figure 2, the Cu/Ti diffusion-reaction
layer can be formed at different holding times. The width
of the diffusion-reaction layer was 4–6 μm at the holding
time of 3 min (Figure 2a) while it grew up to 10–12 μm
at the holding time of 6 min (Figure 2c). When com-
pared with the conventional heating method, a conti-
nuous and dense diffusion-reaction layer can be formed
in a short holding time under the condition of pulse-
current heating, proving that pulse-current heating can
accelerate the process of interfacial reaction. Figure 2b
shows the results of the elemental line-scanning analysis
for the corresponding diffusion-reaction layer (Figure
2a). The diffusion of the Ti atoms into the Cu matrix can
be clearly seen from Figure 2b. The depth of the
titanium diffusion-reaction layer can be up to 10 μm.
However, the diffusion of the Cu atoms into the Ti ma-
trix is invisible. These results confirmed that under the
effects of pulse-current heating, the diffusion of the Ti
atoms into the Cu matrix was promoted while the diffu-
sion of the Cu atoms into the Ti matrix was suppressed.
Although the heating temperature was relatively low and
the holding time was very short, the diffusion-reaction
layer between the Cu matrix and Ti matrix was formed.
L. FEI et al.: FORMATION MECHANISM OF DIFFUSION-REACTION LAYER FOR A Cu/Ti ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1025–1029 1027
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Morphology and element distribution of diffusion-reaction
layer in the case of pulse-current heating: a) morphology image for the
holding time of 6 min, b) EPMA line-scanning result for the holding
time of 6 min, c) morphology image for the holding time of 3 min, d)
EPMA line-scanning result for the holding time of 3 min
When combining the line-scanning results with the
EDS analysis, the interface reaction layer can be gener-
ally divided into two zones: the initial interface reaction
layer I for the Cu/Ti diffusion couple (a width of about 1
μm) and the reaction layer II along the depth direction of
the Cu matrix (a width of about 10 μm). According to the
data in Table 2, the contents of Cu and Ti in points A
and B were almost equal. When combining the Cu-Ti
binary-alloy phase diagram with the related researches, it
can be inferred that the compounds in points A and B
were composed of a Cu solid solution and TiCu4 com-
pound. Meanwhile, point C was composed of a Ti2Cu
compound and point D was composed of (-Ti) and
Ti2Cu compounds. That is to say, the Ti2Cu phase was
formed in the reaction layer I, while the TiCu4 phase was
formed in the reaction layer II. Therefore, even though
the heating temperature was only 700 °C and the holding
time was only 6 min, the diffusion-reaction layer bet-
ween the Cu matrix and the Ti matrix could still be
formed under the condition of pulse current. When the
holding time was prolonged, there was no obvious vari-
ation in the width of the reaction zone I, but the width of
reaction zone II was increased remarkably.
Figure 2d shows the elemental line scanning of the
diffusion-reaction layer between the Cu matrix and the
Ti matrix at the holding time of 3 min. The elemental
diffusion trends of the diffusion-reaction layer were
approximately consistent with the results shown in Fig-
ure 2b. Namely, the diffusion rate of the Ti atoms into
the Cu atoms was higher than that of the Cu atoms into
the Ti atoms. The initial interfacial reaction layer I at the
side of the Cu matrix was clearly visible, while the diffu-
sion reaction layer II along the depth direction of the Cu
matrix was invisible.
To further clarify the structural constitution of the
interfacial reaction layer for the Cu/Ti diffusion couple
under pulse current, the samples were grinded layer by
layer, and then the interfacial reaction layer was tested
with XRD. The XRD results are shown in Figure 3. It
can be seen that the compounds were mainly composed
of TiCu4 and Ti2Cu at 700 °C, the holding time of 6 min
and pulse-duty ratio of 16/2 (on/off).
Based on the above analysis, it can be concluded that
the width of the Cu/Ti diffusion-reaction layer increased
with the increasing holding time for either conventional-
heating diffusion or pulse-current-heating diffusion. By
contrast, pulse current could accelerate the diffusion rate
of the Ti atoms into the Cu matrix and also promote the
formation of the interface reaction layer. Meanwhile,
pulse current could also suppress the diffusion of the Cu
atoms into the Ti matrix. In the case of conventional-
heating diffusion, each reaction layer had only one single
phase. The phase of each reaction layer was consistent
with the phase from the Cu-Ti binary-alloy phase dia-
gram. However, for pulse-current-heating diffusion, the
stratification phenomenon of the whole reaction layer
was not obvious. The Ti2Cu compound was formed at the
initial interface, while the TiCu4 compound was formed
at the reaction layer along the depth direction of the Cu
matrix. Although the diffusion of the Ti atoms into the
Cu matrix was fast, no concentration gradient was
formed in the whole reaction layer. It was inferred that
pulse-current heating could change the structural con-
stituent of the diffusion-reaction layer.
3.3 Growth of the interfacial reaction layer under
pulse-current heating
According to the above analysis, the growth process
of the interfacial reaction layer for the Cu/Ti diffusion
couple under pulse-current heating could be divided into
two stages, namely, (i) the nucleation and growth of a
Ti2Cu compound at the initial interface of the Cu/Ti
diffusion couple, (ii) the nucleation and rapid growth of a
Ti4Cu compound at the side of the Cu matrix. During the
first stage, a pure Cu/Ti diffusion couple was placed in a
graphite heating body and pressure was pre-loaded with
a pressure rod; the graphite heating body and the Cu/Ti
diffusion couple were heated by pulsed direct current.
The temperature at the initial interface was rapidly
increased due to the heating of contact resistance. The
inter-diffusion of the Ti atoms and Cu atoms was
promoted; finally, the Ti2Cu compound was formed.
L. FEI et al.: FORMATION MECHANISM OF DIFFUSION-REACTION LAYER FOR A Cu/Ti ...
1028 Materiali in tehnologije / Materials and technology 51 (2017) 6, 1025–1029
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: XRD test results for diffusion-reaction layer at the holding
time of 6 min: phases of initial interface reaction layer I, b) phases of
reaction layer II
During the second stage, when the holding time was
prolonged, free-state Ti atoms in the Ti matrix went
across the Ti2Cu compound layer, then they diffused into
the Cu matrix and reacted with the Cu atoms; finally, the
TiCu4 compound was formed. The TiCu4 reaction layer
grew fast due to the higher diffusion rate of the Ti atoms.
Meanwhile, the Cu matrix could provide enough free-
state Cu atoms for the formation of a TiCu4 diffusion-
reaction layer. However, the diffusion rate of the Cu
atoms into the Ti matrix was slow, and it took more time
for the Cu atoms to pass through the Ti2Cu reaction
layer, resulting in a relatively slow growth rate of the
Ti2Cu diffusion-reaction layer. The growth rate of the
whole reaction layer depends on the growth rate of the
TiCu4 compound.
According to Figure 2 and the other experimental
data, at the same heating temperature of 700 °C, axial
pressure of 1MPa and pulse duty of 16/2, the average
thickness values of the interfacial reaction layer of the
Cu/Ti diffusion couple were (5, 11, 14 and 16) μm,
corresponding with the holding times of 3, 6, 8 and 10
min. Figure 4 shows the relationship between the
holding time and the thickness of the diffusion reaction
layer. The Y axis represents the thickness of the reaction
layer and the X axis shows the square root of the holding
time. It can be seen from Figure 4 that the growth of the
interfacial reaction layer followed the parabolic law for
the Cu/Ti diffusion couple under pulse-current heating.
4 CONCLUSIONS
Under the condition of pulse-current heating, the
diffusion rate of the Ti atoms into the Cu matrix was
accelerated, the growth rate of the interfacial reaction
layer was promoted, and the growth of the interfacial
reaction layer followed the parabolic law.
For pulse-current-heating diffusion, the formation of
the stratification of the whole reaction layer was not
obvious. A Ti2Cu compound was formed at the initial
interfacial layer while a single Ti4Cu compound was
formed in the depth direction of the Cu matrix.
For conventional-heating diffusion, a stratification of
the whole reaction layer was obvious. If the holding time
was long enough, the phase of each reaction layer was
consistent with the phase from the Cu-Ti binary-alloy
phase diagram.
Acknowledgements
This research was funded by the National Natural
Science Foundation of China (No.51175239). The
authors would like to express their gratitude to the
technical stuff of the Provincial Key Laboratory of
Advanced Welding Technology for various assistances.
5 REFERENCES
1 X. P. Xu, H. Wang, J. S. Zou, C. Z. Xia, Interfacial structure and
properties of Si3N4 ceramic and TiAl alloys brazed with Ti/Ag-
Cu/Cu interlayers, Transactions of the China Welding Institution, 37
(2016) 12, 91–94, doi:10.3321/j.issn:0253-360X.2016.12.023
2 M. F. Wu, Z. S. Yu, R. F. Li, A study of Ti/Fe contact reaction
structures, Materials Science and Technology, 20 (2004) 5, 658–660,
doi:10.1179/026708304225012080
3 S. Kundu, S. Chatterjee, Interface microstructure and strength pro-
perties of diffusion bonded joints of Titanium-Al interlayer-18Cr-8Ni
stainless steel, Materials Science and Engineering A, 527 (2010)
10–11, 2714–2719, doi:10.1016/j.msea.2009.12.042
4 S. Kundu, S. Sam, S. Chatterjee, Interface microstructure and
strength properties of Ti-6Al-4V and microduplex stainless steel
diffusion bonded joints, Materials and Design, 32 (2011) 5,
2997–3003, doi:10.1016/j.matdes.2010.12.052
5 Z. R. Lin, Z. Y. Zhang, W. D. Huang, An investigation of diffusion
bonding under superplastic condition for Ti-6A-4V titanium alloys,
Acta Aeronautica et Astronautica Sinica, 13 (1992) 5, 288–295,
doi:10.3321/j.issn:1000-6893.1992.05.010
6 J. Y. Li, L. H. Ni, X. Jin, Continuous drive friction welding proce-
dures of TC4/T2 and microstructure and performance of dissimilar
metal joints, Transactions of the China Welding Institution, 37
(2016) 9, 115–118, doi:10.3321/j.issn:0253-360X.2016.09.026
7 R. F. Li, Z. G. Li, Y. Y. Zhu, L. Rong, A comparative study of laser
beam welding and laser-MIG hybrid welding of Ti-Al-Zr-Fe titanium
alloy, Materials Science and Engineering A, 528 (2011) 3,
1138–1142, doi:10.1016/j.msea.2010.09.084
8 R. Yang, Y. S. Zhu, F. Y. Gao, Diffusion bonding for titanium alloy,
Development and Application of Materials, 28 (2013) 5, 109–115,
doi:10.3969/j.issn.1003-1545.2013.05.023
9 R. K. Shiue, S. K. Wu, C. H. Chan, The interfacial reactions of infra-
red brazing Cu and Ti with two silver-based braze alloys, Journal of
Alloys and Compounds, 372 (2004) 1–2, 148–157, doi:10.1016/
j.jallcom.2003.09.155
10 M. F. Wu, C. Yu, Z. S. Yu, K. Qi, R. F. Li, Dissolution behaviour of
Ti/Cu contact reaction, Materials Science and Technology, 21 (2005)
2, 250–254, doi:10.1179/174328405X18683
11 V. N. Eremenko, Y. I. Buyanov, S. B. Prima, Phase diagram of the
system titanium-copper, Powder Metallurgy and Metal Ceramics, 5
(1966) 6, 494–502, doi:10.1007/BF00775543
12 J. L. Murray, The Cu-Ti (Copper-Titanium) system, Journal of Phase
Equilibria, 4 (1983) 1, 81–95, doi:10.1007/BF02880329
L. FEI et al.: FORMATION MECHANISM OF DIFFUSION-REACTION LAYER FOR A Cu/Ti ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1025–1029 1029
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 4: Relationship between the thickness of the interfacial
reaction layer and the holding time under pulse-current heating
When combining the line-scanning results with the
EDS analysis, the interface reaction layer can be gener-
ally divided into two zones: the initial interface reaction
layer I for the Cu/Ti diffusion couple (a width of about 1
μm) and the reaction layer II along the depth direction of
the Cu matrix (a width of about 10 μm). According to the
data in Table 2, the contents of Cu and Ti in points A
and B were almost equal. When combining the Cu-Ti
binary-alloy phase diagram with the related researches, it
can be inferred that the compounds in points A and B
were composed of a Cu solid solution and TiCu4 com-
pound. Meanwhile, point C was composed of a Ti2Cu
compound and point D was composed of (-Ti) and
Ti2Cu compounds. That is to say, the Ti2Cu phase was
formed in the reaction layer I, while the TiCu4 phase was
formed in the reaction layer II. Therefore, even though
the heating temperature was only 700 °C and the holding
time was only 6 min, the diffusion-reaction layer bet-
ween the Cu matrix and the Ti matrix could still be
formed under the condition of pulse current. When the
holding time was prolonged, there was no obvious vari-
ation in the width of the reaction zone I, but the width of
reaction zone II was increased remarkably.
Figure 2d shows the elemental line scanning of the
diffusion-reaction layer between the Cu matrix and the
Ti matrix at the holding time of 3 min. The elemental
diffusion trends of the diffusion-reaction layer were
approximately consistent with the results shown in Fig-
ure 2b. Namely, the diffusion rate of the Ti atoms into
the Cu atoms was higher than that of the Cu atoms into
the Ti atoms. The initial interfacial reaction layer I at the
side of the Cu matrix was clearly visible, while the diffu-
sion reaction layer II along the depth direction of the Cu
matrix was invisible.
To further clarify the structural constitution of the
interfacial reaction layer for the Cu/Ti diffusion couple
under pulse current, the samples were grinded layer by
layer, and then the interfacial reaction layer was tested
with XRD. The XRD results are shown in Figure 3. It
can be seen that the compounds were mainly composed
of TiCu4 and Ti2Cu at 700 °C, the holding time of 6 min
and pulse-duty ratio of 16/2 (on/off).
Based on the above analysis, it can be concluded that
the width of the Cu/Ti diffusion-reaction layer increased
with the increasing holding time for either conventional-
heating diffusion or pulse-current-heating diffusion. By
contrast, pulse current could accelerate the diffusion rate
of the Ti atoms into the Cu matrix and also promote the
formation of the interface reaction layer. Meanwhile,
pulse current could also suppress the diffusion of the Cu
atoms into the Ti matrix. In the case of conventional-
heating diffusion, each reaction layer had only one single
phase. The phase of each reaction layer was consistent
with the phase from the Cu-Ti binary-alloy phase dia-
gram. However, for pulse-current-heating diffusion, the
stratification phenomenon of the whole reaction layer
was not obvious. The Ti2Cu compound was formed at the
initial interface, while the TiCu4 compound was formed
at the reaction layer along the depth direction of the Cu
matrix. Although the diffusion of the Ti atoms into the
Cu matrix was fast, no concentration gradient was
formed in the whole reaction layer. It was inferred that
pulse-current heating could change the structural con-
stituent of the diffusion-reaction layer.
3.3 Growth of the interfacial reaction layer under
pulse-current heating
According to the above analysis, the growth process
of the interfacial reaction layer for the Cu/Ti diffusion
couple under pulse-current heating could be divided into
two stages, namely, (i) the nucleation and growth of a
Ti2Cu compound at the initial interface of the Cu/Ti
diffusion couple, (ii) the nucleation and rapid growth of a
Ti4Cu compound at the side of the Cu matrix. During the
first stage, a pure Cu/Ti diffusion couple was placed in a
graphite heating body and pressure was pre-loaded with
a pressure rod; the graphite heating body and the Cu/Ti
diffusion couple were heated by pulsed direct current.
The temperature at the initial interface was rapidly
increased due to the heating of contact resistance. The
inter-diffusion of the Ti atoms and Cu atoms was
promoted; finally, the Ti2Cu compound was formed.
L. FEI et al.: FORMATION MECHANISM OF DIFFUSION-REACTION LAYER FOR A Cu/Ti ...
1028 Materiali in tehnologije / Materials and technology 51 (2017) 6, 1025–1029
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: XRD test results for diffusion-reaction layer at the holding
time of 6 min: phases of initial interface reaction layer I, b) phases of
reaction layer II
During the second stage, when the holding time was
prolonged, free-state Ti atoms in the Ti matrix went
across the Ti2Cu compound layer, then they diffused into
the Cu matrix and reacted with the Cu atoms; finally, the
TiCu4 compound was formed. The TiCu4 reaction layer
grew fast due to the higher diffusion rate of the Ti atoms.
Meanwhile, the Cu matrix could provide enough free-
state Cu atoms for the formation of a TiCu4 diffusion-
reaction layer. However, the diffusion rate of the Cu
atoms into the Ti matrix was slow, and it took more time
for the Cu atoms to pass through the Ti2Cu reaction
layer, resulting in a relatively slow growth rate of the
Ti2Cu diffusion-reaction layer. The growth rate of the
whole reaction layer depends on the growth rate of the
TiCu4 compound.
According to Figure 2 and the other experimental
data, at the same heating temperature of 700 °C, axial
pressure of 1MPa and pulse duty of 16/2, the average
thickness values of the interfacial reaction layer of the
Cu/Ti diffusion couple were (5, 11, 14 and 16) μm,
corresponding with the holding times of 3, 6, 8 and 10
min. Figure 4 shows the relationship between the
holding time and the thickness of the diffusion reaction
layer. The Y axis represents the thickness of the reaction
layer and the X axis shows the square root of the holding
time. It can be seen from Figure 4 that the growth of the
interfacial reaction layer followed the parabolic law for
the Cu/Ti diffusion couple under pulse-current heating.
4 CONCLUSIONS
Under the condition of pulse-current heating, the
diffusion rate of the Ti atoms into the Cu matrix was
accelerated, the growth rate of the interfacial reaction
layer was promoted, and the growth of the interfacial
reaction layer followed the parabolic law.
For pulse-current-heating diffusion, the formation of
the stratification of the whole reaction layer was not
obvious. A Ti2Cu compound was formed at the initial
interfacial layer while a single Ti4Cu compound was
formed in the depth direction of the Cu matrix.
For conventional-heating diffusion, a stratification of
the whole reaction layer was obvious. If the holding time
was long enough, the phase of each reaction layer was
consistent with the phase from the Cu-Ti binary-alloy
phase diagram.
Acknowledgements
This research was funded by the National Natural
Science Foundation of China (No.51175239). The
authors would like to express their gratitude to the
technical stuff of the Provincial Key Laboratory of
Advanced Welding Technology for various assistances.
5 REFERENCES
1 X. P. Xu, H. Wang, J. S. Zou, C. Z. Xia, Interfacial structure and
properties of Si3N4 ceramic and TiAl alloys brazed with Ti/Ag-
Cu/Cu interlayers, Transactions of the China Welding Institution, 37
(2016) 12, 91–94, doi:10.3321/j.issn:0253-360X.2016.12.023
2 M. F. Wu, Z. S. Yu, R. F. Li, A study of Ti/Fe contact reaction
structures, Materials Science and Technology, 20 (2004) 5, 658–660,
doi:10.1179/026708304225012080
3 S. Kundu, S. Chatterjee, Interface microstructure and strength pro-
perties of diffusion bonded joints of Titanium-Al interlayer-18Cr-8Ni
stainless steel, Materials Science and Engineering A, 527 (2010)
10–11, 2714–2719, doi:10.1016/j.msea.2009.12.042
4 S. Kundu, S. Sam, S. Chatterjee, Interface microstructure and
strength properties of Ti-6Al-4V and microduplex stainless steel
diffusion bonded joints, Materials and Design, 32 (2011) 5,
2997–3003, doi:10.1016/j.matdes.2010.12.052
5 Z. R. Lin, Z. Y. Zhang, W. D. Huang, An investigation of diffusion
bonding under superplastic condition for Ti-6A-4V titanium alloys,
Acta Aeronautica et Astronautica Sinica, 13 (1992) 5, 288–295,
doi:10.3321/j.issn:1000-6893.1992.05.010
6 J. Y. Li, L. H. Ni, X. Jin, Continuous drive friction welding proce-
dures of TC4/T2 and microstructure and performance of dissimilar
metal joints, Transactions of the China Welding Institution, 37
(2016) 9, 115–118, doi:10.3321/j.issn:0253-360X.2016.09.026
7 R. F. Li, Z. G. Li, Y. Y. Zhu, L. Rong, A comparative study of laser
beam welding and laser-MIG hybrid welding of Ti-Al-Zr-Fe titanium
alloy, Materials Science and Engineering A, 528 (2011) 3,
1138–1142, doi:10.1016/j.msea.2010.09.084
8 R. Yang, Y. S. Zhu, F. Y. Gao, Diffusion bonding for titanium alloy,
Development and Application of Materials, 28 (2013) 5, 109–115,
doi:10.3969/j.issn.1003-1545.2013.05.023
9 R. K. Shiue, S. K. Wu, C. H. Chan, The interfacial reactions of infra-
red brazing Cu and Ti with two silver-based braze alloys, Journal of
Alloys and Compounds, 372 (2004) 1–2, 148–157, doi:10.1016/
j.jallcom.2003.09.155
10 M. F. Wu, C. Yu, Z. S. Yu, K. Qi, R. F. Li, Dissolution behaviour of
Ti/Cu contact reaction, Materials Science and Technology, 21 (2005)
2, 250–254, doi:10.1179/174328405X18683
11 V. N. Eremenko, Y. I. Buyanov, S. B. Prima, Phase diagram of the
system titanium-copper, Powder Metallurgy and Metal Ceramics, 5
(1966) 6, 494–502, doi:10.1007/BF00775543
12 J. L. Murray, The Cu-Ti (Copper-Titanium) system, Journal of Phase
Equilibria, 4 (1983) 1, 81–95, doi:10.1007/BF02880329
L. FEI et al.: FORMATION MECHANISM OF DIFFUSION-REACTION LAYER FOR A Cu/Ti ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1025–1029 1029
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 4: Relationship between the thickness of the interfacial
reaction layer and the holding time under pulse-current heating