Reaction pathway of combustion synthesis of Ti5Si3 in Cu-Ti-Si system

Article abstract of DOI:10.1111/jace.12079

The reaction pathway of combustion synthesis (CS) of Ti₅Si ₃ in Cu-Ti-Si system was explored through a delicate microstructure and phase analysis on the resultant products during differential thermal analysis (DTA). The formation of Cu-Si eutectic liquids plays a key role in the reaction pathway, which provides easy route for reactant transfer and accelerates the occurrence of complete reaction. Cu initially reacted with Si to form Cu₃Si by a solid-state diffusion reaction, which further reacted with Cu to form Cu-Si liquids at the eutectic point of ~802°C; then Ti was dissolved into the surrounding Cu-Si liquids and led to the formation of Cu-Ti-Si ternary liquids; finally, Ti₅Si₃ was precipitated out of the saturated liquids by a solution-reaction-precipitation mechanism. The reaction pathway in CS of titanium silicide (Ti₅Si₃) could be described briefly as: Cu_(s) + Ti_(s) + Si _(s)→Cu₃Si_(s) + Ti_(s) + Si _(s)→(Cu-Si)_(l) + Ti_(s)→(Cu-Ti-Si) _(l)→Cu_(l) + Ti₅Si_3(s).

Full text of DOI:10.1111/jace.12079

J. Am. Ceram. Soc., 96 [3] 950–956 (2013)
DOI: 10.1111/jace.12079
© 2012 The American Ceramic Society
ournal
J
Reaction Pathway of Combustion Synthesis of Ti5Si3 in Cu–Ti–Si System
Hui–Yuan Wang, Si–Jie Lu, Wei Xiao, Guo–Jun Liu, Jin–Guo Wang,† and Qi–Chuan Jiang
¨
Key Laboratory of Automobile Materials of Ministry of Education and School of Materials Science and Engineering,
Nanling Campus, Jilin University, No. 5988 Renmin Street, Changchun 130025, China
The reaction pathway of combustion synthesis (CS) of Ti5Si3
in Cu–Ti–Si system was explored through a delicate micro-
structure and phase analysis on the resultant products during
differential thermal analysis (DTA). The formation of Cu–Si
eutectic liquids plays a key role in the reaction pathway, which
provides easy route for reactant transfer and accelerates the
occurrence of complete reaction. Cu initially reacted with Si to
form Cu3Si by a solid-state diffusion reaction, which further
reacted with Cu to form Cu–Si liquids at the eutectic point of
~802°C; then Ti was dissolved into the surrounding Cu–Si liq-
uids and led to the formation of Cu–Ti–Si ternary liquids;
finally, Ti5Si3 was precipitated out of the saturated liquids by
dominated by the solid–liquid interactions involving the dis-
solution of solid reactants into the liquid and the precipita-
tion of the final product.
An alloying addition to Ti–Si system was reportedly a pos-
sible way to improve the microstructures and mechanical
properties (e.g., fracture toughness) of Ti5Si3.23–28 Among
various alloying elements, K. J. Park et al.26 and H. C. Park
et al.27 stated that the addition of Cu acted as a sintering aid
and facilitated greatly the densification of Ti5Si3 during PM
process. Moreover, Kang et al.28 reported that a thorough
reaction among Ti, Si, and Cu by an explosive synthesis of
elemental powders resulted in the formation of a single-phase
Ti5Si3. In a previous study,29 we investigated the influence of
Cu addition on the CS of Ti5Si3 in Cu–Ti–Si system, and
found that the morphologies of yielded products transformed
from an obvious sintered–shaped coarse appearance to a
cobblestone–like smooth surface. Interestingly, Cu serves not
only as a diluent but also as a reactant that participates in
the CS process. However, the exact reaction mechanism has
not been clarified so far.
a solution–reaction–precipitation mechanism. The reaction
pathway in CS of titanium silicide (Ti5Si3) could be described
briefly as: Cu(s) + Ti(s) + Si(s)?Cu3Si(s)
+
Ti(s) + Si(s)?
.
(Cu–Si)(l) + Ti(s)?(Cu–Ti–Si)(l)?Cu(l) + Ti5Si3(s)
I. Introduction
HE synthesis of structural intermetallic compounds in
conventional methods is based on the classical processes
In this study, therefore, the principal objective is to
explore the reaction pathway of CS of Ti5Si3 in Ti–Si systems
with Cu addition. It is expected that the preliminary results
could shed some light on the understanding of the reaction
mechanism of Ti–Si system with a third metal addition.
T
of powder metallurgy (PM), hot isostatic pressed and reac-
tion hot pressed as well as arc melting,1–3 all of which share
the common feature of using strong external heating to accel-
erate processing. Compared with such conventional tech-
niques, however, combustion synthesis (CS), also termed as
self-propagating high-temperature synthesis, has attracted
more attention due to its advantages of lower energy con-
sumption, higher time efficiency and higher product purity,
which is preferred in preparing various refractory materials,
such as TiC, Al2O3, ZrB2, Ti3SiC2, MoSi2, Nb5Si3, and
Ti5Si3, etc.1,3–8 Recently, titanium silicide (Ti5Si3) has
attracted considerable attention due to its outstanding prop-
erties including high melting temperature (2130°C), low
density (4.32 g/cm3) and high hardness, as well as excellent
strength at elevated temperature and high oxidation resis-
tance,9–14 which consequently promote Ti5Si3 as a promising
material for high–temperature structural applications.9–18
To effectively control the CS process and eventually to
obtain desired reaction products, the reaction pathway of
Ti–Si system to form Ti5Si3 was investigated intensively.
Trambukis and Munir19 proposed that the combustion
reaction of Ti and Si proceeded via a series of intermediate
interactions in accordance with Ti–Si equilibrium phase rela-
tions, i.e., TiSi2?TiSi?Ti5Si4?Ti5Si3. Moreover, Riley
et al.20,21 reported that the CS of Ti5Si3 was triggered by the
phase transformation of a–Ti?b–Ti and followed by a direct
solid-state reaction of b–Ti and Si. In addition, Yeh et al.22
suggested that the formation mechanism of Ti5Si3 was
II. Experimental Procedure
The starting materials in the present study are commercial
powders of Cu (99.7% purity, ~45 lm), Ti (99.5% purity,
~15 lm), and Si (99.5% purity, ~15 lm). In Cu–Ti–Si
system, the Ti and Si powders in a ratio corresponding to
that of stoichiometric Ti5Si3 mixed with 0–50 wt% Cu were
used as blended powders. Moreover, to study the interactions
between each component in reactant mixtures, the Cu–Ti
and Cu–Si systems were also prepared. The compositions of
investigated systems are shown in Table I. All blended pow-
ders were mixed sufficiently by a ball-milling for 6 h. Differ-
ential thermal analysis (DTA) experiments were conducted
using the thermal analysis apparatus (SDT–Q600; TA Ltd,
New Castle, DE) with Al2O3 powders as the reference mate-
rials. A small amount of reactants weighing ~45 1 mg was
manually pressed into Al2O3 crucibles under flowing ultra
high purity argon (100 ml/min) and heated at a rate of 30°C/
min up to ~1200°C, except for some occasional interrupted
runs to obtain the intermediate reaction products. Repeated
experiments were conducted and the results with good
repeatability were presented.
For Cu–Ti and Cu–Si systems, the temperature-time pro-
files were obtained from the samples heated in an electric
resistance furnace (1.5 kW) under a protective atmosphere of
high purity argon (5 L/min). Powder blends were uniaxially
pressed into cylindrical compacts (6 mm in diameter and
Y. Zhao—contributing editor
6
1 mm in length) under pressures ranging from 70 to
75 MPa to obtain green densities of 67 2% theoretical
density. Subsequently, the compacts were placed on the
graphite-flat and heated at ~40°C/min to about 960°C. A
thermocouple pair of W-5% Re vs W-26% Re (0.5 mm in
Manuscript No. 31344. Received April 16, 2012; approved October 9, 2012.
†Author to whom correspondence should be addressed. e-mail: jgwang@jlu.edu.cn
950

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Reaction Pathway of Combustion
951
onset temperature of ~795°C was much lower than that
(~871°C) of the Ti–Si system, which suggested that the reac-
tion could be greatly facilitated by the Cu addition. With fur-
ther increase in temperature, a second exothermic hump with
the peak appearing at ~894°C was observed, which was very
close to that of the Ti–Si system. It is indicated that the igni-
tion reaction in Ti–Si system was changed by 10 wt% Cu
addition, whereas the combustion reaction was still domi-
nated by the reaction between b-Ti and Si, as the Cu content
is too meager to affect the whole exothermic reaction.
Table I. Reactant Compositions of the Investigated Systems
Designed composition
Cu (g)
Ti (g)
Si (g)
Ti5Si3
–
10
30
50
30
30
74.0
66.6
51.8
37.0
51.8
–
26.0
23.4
18.2
13.0
–
10 wt% Cu–Ti5Si3
30 wt% Cu–Ti5Si3
50 wt% Cu–Ti5Si3
Cu–Ti
Cu–Si
18.2
With 30 wt% Cu addition, surprisingly, a crossing feature
appeared on the DTA trace with the onset of ~810°C
[Fig. 1(a)]. According to our previous study,29 the crossing fea-
ture was mainly attributed to the high exothermic reaction,
leading to a rapid rise in the sample temperature, which, there-
fore, can be regarded as the CS reaction with a simultaneous
combustion mode. Moreover, with 50 wt% Cu addition, the
onset temperature of ~796°C was similar to those of 10 and
30 wt% Cu addition, and the exothermic peak appeared at the
temperature near 813°C. In addition, when Cu content was
increased over the range of 30–50 wt%, an endothermic peak
appeared at ~1048°C–1052°C on the heating stage (solid lines)
while an exothermic peak emerged at ~1034°C–1045°C on the
cooling stage (dashed lines), corresponding to the melting and
solidification of Cu, respectively.
diameter) inserted into the hole (2 mm in diameter and half
of the compact in length) drilled at the top of the compact
was linked up with a temperature acquisition recorder to
record the temperature-time curves.
The products retrieved from the DTA were analyzed for
their phase compositions by X–ray diffraction (XRD, D/Max
2500PC; Rigaku Ltd, Tokyo, Japan) using CuKa radiation.
Microstructures of the fracture surface of DTA products
were investigated by using field-emission scanning electron
microscope (FESEM, FEIXL–30; FEI Ltd, Hillsboro, OR)
equipped with an EDS analyzer (EDAX, SEDX–Genesis
2000; EDAX Inc, Mahwah, NJ).
According to XRD patterns [Fig. 1(b)], besides Ti5Si3,
some intermediate phases (Ti5Si4, TiSi2, and TiSi) and unre-
acted Ti are also detected in Ti–Si system, which suggests
that the reaction in DTA apparatus tends to be incomplete
without Cu addition. On the contrary, when reactions take
place in 10–50 wt% Cu–Ti–Si systems, the products only
consist of Ti5Si3 and Cu phases, without any transient phase,
indicating that the reactions are complete.
Therefore, on the basis of the way of decreasing onset
temperatures, we can see that the addition of Cu plays an
important role in the CS of Cu–Ti–Si system, which could
change the reaction pathway of Ti–Si system.
III. Results and Discussion
(1) DTA Results of Cu–Ti–Si Systems
In the previous study,29 we reported that addition of Cu to
Ti–Si system significantly decreases the onset temperature of
the reaction during DTA process. However, as the exact
reaction mechanism of the Cu–Ti–Si system is still not clear
so far, further research is continued in this study to reveal
the exact reaction pathway.
Figure 1(a) and (b) show typical DTA curves and XRD
patterns of DTA products of 0, 10, 30, and 50 wt% Cu–
Ti–Si systems, respectively. For Ti–Si system, only one exo-
thermic peak with an onset at ~871°C (near the transition
temperature of a-Ti?b-Ti of 882°C) and a maximum at
~913°C emerges on the curve, indicating the reaction between
Ti and Si is mainly dominated by the reaction of b-Ti and Si.
As for 10 wt% Cu–Ti–Si mixtures, an exothermic peak
appeared at the temperature close to 804°C. Note that the
(2) Interrupted Experiments of Cu–Ti, Cu–Si, Ti–Si, and
Cu–Ti–Si Systems
To determine phase formations and transitions in Cu–Ti–Si
system, the binary Cu–Ti and Cu–Si systems with compositions
(a)
(b)
Fig. 1. (a) DTA traces for elemental powder mixtures corresponding to Ti–Si with 0, 10, 30, and 50 wt% Cu additions, respectively, at a
constant heating rate of 30°C/min and up to ~1200°C in flowing high purity argon (100 ml/min) and (b) XRD patterns of the DTA products.

952
Journal of the American Ceramic Society—Wang et al.
Vol. 96, No. 3
(a)
(b)
Fig. 2. (a) DTA traces of Cu–Ti and Cu–Si systems at a constant heating rate of 30°C/min and up to ~1200°C in flowing high purity argon
(100 ml/min), (b) the temperature-time profiles for compacts corresponding to Cu–Ti and Cu–Si systems.
corresponding to 30 wt% Cu–Ti–Si system (Table I) were
heated to ~1200°C in DTA apparatus, as shown in Fig. 2(a).
For comparative purpose, the Cu–Ti and Cu–Si systems were
also heated to ~960°C in an electric resistance furnace under
flowing purity argon to obtain temperature-time profiles, as
indicated in Fig. 2(b). Furthermore, based on onset tempera-
tures of ~804°C–810°C in 10–50 wt% Cu–Ti–Si systems, a
series of interrupted experiments were conducted for Cu–Ti,
Cu–Si, Ti–Si and 30 wt% Cu–Ti–Si systems. The XRD pat-
terns and FESEM micrographs of resultant products are
shown in Figs. 3, 4, 5 and 6, respectively.
In Cu–Ti mixtures [Fig. 2(a)], an exothermic peak emerges
at ~887°C, followed by two endothermic peaks at ~978°C
and ~1015°C, respectively. Interestingly, the onset tempera-
ture of compacts in TE furnace was ~889 °C [Fig. 2(b)],
which was close to the first reaction temperature (887°C) in
the DTA [Fig. 2(a)], indicating that the reaction in TE fur-
nace and DTA apparatus share some similarity. According
to results of Liang et al.30,31 complex reactions had occurred
between Cu and Ti particles, as follows: after Cu–Ti mixtures
were heated to ~887°C, the Ti2Cu and TiCu phases were pro-
duced by solid–state diffusion reactions; subsequently, the
Cu–Ti eutectic liquids were formed by reactions of Ti2Cu
and TiCu corresponding to the endothermic peak of ~978°C.
Moreover, continuous heating could lead to the melting of
Ti2Cu into Cu–Ti liquids, corresponding to an endothermic
peak of ~1015°C. Therefore, it is logical to conclude that the
Cu–Ti and Cu–Ti–Si systems differ largely in reaction mecha-
nisms.
From DTA traces of Cu–Ti system [Fig. 2(a)], no obvious
reaction occurs below ~800°C, which is consistent with XRD
pattern [Fig. 3(a)] and FESEM micrograph [Figs. 4(a) and
(b)]. Compared with the raw powder mixture of Cu–Ti sys-
tem [Fig. 4(a)], although the mixture is heated to 800°C, it is
easy to distinguish between the cotton-like brightness parti-
cles of Cu and bulk-like gray particles of Ti [Fig. 4(b)].
Hence, we can deduce that the ignited reaction in Cu–Ti–Si
system is independent of the Cu–Ti reaction.
In Cu–Si mixtures [Fig. 2(a)], there is one sharp endother-
mic peak with the onset at ~801°C and the minimum at
~825°C. From the XRD pattern of resultant products heated
to ~800°C [Fig. 3(b)], we can observe that the Cu3Si phase is
detected besides unreacted Cu and Si phases. A small quan-
tity of Cu2O might be caused by the poor pressurization
during DTA process. Furthermore, it can be observed clearly
that some globular–like grains consisting of flake–white
matrix and gray–striped phase were formed in the FESEM
micrograph for the product heated to ~800°C [Fig. 4(c)]. The
matrix of the grains mainly corresponds to Cu3Si phases, as
identified by EDS (e.g. point +1:74 at.% Cu and 26 at.% Si;
point +2: 68 at.% Cu and 32 at.% Si; point +3:79 at.% Cu
and 21 at.% Si), while the gray–striped phase located in the
grains might be the Si phase (e.g., point +4:14 at.% Cu and
86 at.% Si; point +5:19 at.% Cu and 81 at.% Si; point
+6:20 at.% Cu and 80 at.% Si). Moreover, the globular–like
grain with smooth surface is a typical character of crystals
formed by solidification from liquids. Note that the phase
compositions in globular–like grain are in agreement with
the Cu–Si phase diagram,32 in which the Cu–Si eutectic liq-
uids can be formed at 802°C (eutectic temperature) by the
reaction of Cu3Si and Si. Through diffusion couples of Cu–Si
system annealed at 470°C for 8 h, Levin et al.33 observed
that the diffusion layer comprises the Cu3Si phase solely.
Consequently, it is rational to infer that during heating pro-
cess, the Cu3Si phase is produced initially by the solid-state
diffusion reactions of Cu and Si, which further reacts with
the remnant Si to form Cu–Si eutectic liquids at the onset of
~801°C. From Cu–Si profile [Fig. 2(b)], one can see that
there are two temperature platforms on heating and cooling
stages at ~807°C–809°C, respectively, which further confirm
the formation of Cu–Si liquids.
(a)
(b)
(c)
(d)
(e)
For Ti–Si system [Fig. 1(a) 0 wt%], only one exothermic
peak with an onset at ~871°C emerges on the DTA curve
[Fig. 1(a) 0 wt%]. After heating reactants to 800°C, the
Fig. 3. XRD patterns of (a) Cu–Ti, (b) Cu–Si, (c) Ti–Si, and (d)
30 wt% Cu–Ti–Si mixtures in DTA interrupted at ~800°C as well as
(e) 30 wt% Cu–Ti–Si mixtures in DTA interrupted at ~840°C.

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953
(a)
(b)
(c)
(d)
Fig. 4. Typical FESEM micrographs of the binary mixture in DTA (a) raw powder mixture of Cu–Ti mixture, (b) after heating to 800°C of
Cu–Ti mixture, (c) after heating to 800°C of Cu–Si mixture, and (d) after heating to 800°C of Ti–Si mixture.
(a)
(b)
Fig. 5. (a) Typical FESEM micrographs of 30 wt% Cu–Ti–Si system after heating to 800°C in DTA, and (b) is the local magnification of the
selected area in (a).
major phases detected are Ti and Si, together with a trace
amount of Ti5Si3 phase [Fig. 3(c)]. According to the
experiments of Ti–Si diffusion couples, Quenisset et al.34
reported that slight Ti5Si3 phase could be detected in Ti–Si
couples after being annealed at 600°C for 3 h. From micro-
structures of the mixture on heating to 800°C [Fig. 4(d)],
it can be observed that the elemental Ti and Si are readily
distinguished from each other, indicating that only slight
reaction occurred between Ti and Si when the temperature
reached to 800°C.
For 30 wt% Cu–Ti–Si system, when the heating run was
stopped at ~800°C, the major products identified were rem-
nant Cu, Ti, and Si phases, together with some amounts of
Cu3Si [Fig. 3(d)]. When the heating run was stopped at
~840°C, however, the dominant products became Ti5Si3 and
Cu phases [Fig. 3(e)]. Microstructures for DTA products of
Cu–Ti–Si system heated to 800°C and 840°C are shown in
Figs. 5 and 6, respectively. After interruption at 800°C, the
unreacted Cu, Ti, and Si can be easily identified [Figs. 5(a)
and (b)], corresponding to the XRD analysis. After
undergoing the exothermic event (840°C), one can see that
the Ti5Si3 grains synthesized exhibit a cobblestone-like mor-
phology with relatively smooth surface [Figs. 6(a) and (b)].
The morphology of Ti5Si3 grain agrees well with that of
Ti5Si3 particulate synthesized by CS reaction.29 Figs. 6(c)
and (d) show the FESEM and back scattering electron (BSE)
images, respectively, for the same viewing field, which clearly
indicates the bonding agents of Cu phase distribute in
boundaries of Ti5Si3 grains.
To estimate the reaction temperature of Cu–Ti–Si system
during CS process, we calculate the adiabatic combustion tem-
perature (Tad). It is supposed that the possible reactions in
Cu–Ti–Si system can be summarized as an overall reaction:
5
3
1
xCuþ ð1ꢀxÞTiþ ð1ꢀxÞSi!xCuþ ð1ꢀxÞTi5Si3 (1)
8
8
8
According to Eq. (1), the Tad of the Cu–Ti–Si system can be
calculated based on the relationship given by Moore and

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Journal of the American Ceramic Society—Wang et al.
Vol. 96, No. 3
(a)
(b)
(c)
(d)
Fig. 6. (a) Typical FESEM micrographs of 30 wt% Cu–Ti–Si system after heating to 840°C in DTA, (b) and (c) are the local magnifications of
the selected area in (a), respectively, and (d) BSE image of (c).
Feng,35 which assumes that the propagating mode is initiated
at a certain temperature of T0, as shown in Eq. (2).
correspond to melting points of Ti5Si3 (2403 K) and Cu
(1357 K), respectively. During the melting process (phase
transition), the Tad value remains constant despite of the
increase in Cu content. It can be seen that the 30 wt%
Cu–Ti–Si system corresponds to the Tad of 2403 K (2130°C),
which is equal to the melting point of Ti5Si3, but significantly
higher than those of Ti (1933 K), Si (1685 K), and Cu
(1357 K). Therefore, it is rational to believe that as the reac-
tion zone propagated as a wave front through the sample,
the reactants would convert to liquids driven by the exother-
micity of the reaction, which favors the precipitation of
Ti5Si3 particulates. Moreover, the cobblestone-like morphol-
ogy with relatively smooth surface of Ti5Si3 grains (Fig. 6) is
also a typical feature of crystal precipitated out of oversatu-
ration of liquids.29
Z
Tad
X
X
X
n DH
þ
n
C ðP Þ ꢁ dT þ
n LðP Þ ¼ 0 (2)
P
k
j
j
j
j
f;T0
T0
k
j
j
where DH
is the formation enthalpy at T0; CP(Pj) and
f;T0
LP(Pj) are the heat capacity and phase transformation
enthalpy (if the products go through a phase change, e.g.,
solid to liquid or liquid to gas) of the products, respectively;
nk is the stoichiometric coefficient of Eq. (1), while nj is the
stoichiometric coefficient of the products (Pj); and Tad refers
to the adiabatic combustion temperature.
According to Fig. 1(a), the preheat temperature is selected
as T0 = 1073 K (800°C), and the calculated result of Eq. (2)
is shown in Fig. 7. Except for two plateaus (horizontal lines),
one can see that the Tad value decreases with the increase in
Cu content (Fig. 7). Note that the two plateaus with Cu con-
tents ranging from 11.0 to 43.5, and 82.2 to 92.3 wt%
As reported by He and Stangle,36 the CS reaction begins
with an ignition process, in which at least a portion of the
sample’s surface is exposed to an external heat source for a
short time, to generate a self–propagating combustion wave
front that passes through the sample. Nevertheless, based on
the above analysis, it can be observed that neither in the
Ti–Si [Fig. 1(a)] nor in the Cu–Ti [Fig. 2(a)] system did exo-
thermic reactions occur evidently although the samples were
heated to ~800°C. For Cu–Si system [Fig. 2(b)], instead of
development of heat, the endothermic reaction absorbed a
large quantity of heat from the surroundings. So the ignition
reaction of Cu–Ti–Si system is not that of the binary mix-
tures of Ti–Si, Cu–Ti, and Cu–Si systems.
Combining the results of Figs. 1, 2 and 3, we suggest that
the reaction between Cu–Si eutectic liquids and Ti triggers
the combustion reaction in Cu–Ti–Si system. In other words,
adding Cu to the Ti–Si system would result in the formation
of Cu–Si eutectic liquids prior to the a-Ti?b-Ti transition,
which changes the ignition reaction mechanism and, there-
fore, is much indispensable for the CS of Cu–Ti–Si system.
(3) Reaction Pathway in Cu–Ti–Si System
On the basis of aforementioned results, a model illustrating
the reaction pathway is proposed in the 30 wt% Cu–Ti–Si
Fig. 7. Effect of Cu content on the combustion temperature of
Cu–Ti–Si system with a preheat temperature of 1073 K (800°C).

March 2013
Reaction Pathway of Combustion
955
(a)
(b)
(c)
(f)
(e)
(d)
Fig. 8. A model illustrating the reaction pathway of combustion synthesis (CS) in Cu–Ti–Si system.
system during CS, as shown in Fig. 8(a)–(f). With the
increase in temperature, the compound Cu3Si was formed
initially via a sluggish solid-state diffusion reaction between
Cu and Si particles [Fig. 8(b)], and then the continuous heat-
ing further promoted Cu3Si and Si to form Cu–Si eutectic
liquids at ~802°C [Fig. 8(c)]; simultaneously Ti was dissolved
into the surrounding Cu–Si binary liquids and led to the
formation of Cu–Ti–Si ternary liquids [Fig. 8(d)]. Once
the liquids reached a saturation stage with [Ti] and [Si] in
the course of continuous dissolution, the Ti5Si3 precursors
were precipitated out of the Cu–Ti–Si liquids, leading to an
abrupt increase in temperature and thus initiating the com-
bustion reaction [Fig. 8(e)]. The heat generated by the reac-
tion significantly promoted more Ti and Si dissolved into the
liquids, making the Cu–Ti–Si liquids sufficiently supersatu-
rated quickly, and then plenty of Ti5Si3 could be developed
from the liquids by a solution–reaction–precipitation mecha-
nism with the continual consumption of [Ti] and [Si] [Fig. 8(f)].
In short, the reaction pathway could be described briefly as:
Cu(s) + Ti(s) + Si(s)?Cu3Si(s) + Ti(s) + Si(s)?(Cu–Si)(l) + Ti(s)?
(Cu–Ti–Si)(l)?Cu(l) + Ti5Si3(s). Finally, unlike the Ti–Si sys-
tem, the reaction was complete with the addition of Cu and
the final products only contained Ti5Si3 and Cu stable
phases without any intermediate phases.
When 10 wt% Cu was added to Ti–Si system, the yielded
Cu3Si was insufficient to form enough Cu–Si and Cu–Ti–Si
liquids, resulting in the precipitation of a small amount of
Ti5Si3 out of the liquids. Consequently, the released heat,
corresponding to the exotherm of ~804°C [Fig. 1(a)], could
not ignite the whole combustion reaction. As a result, the CS
was still dominated by the reaction between b-Ti and Si, cor-
responding to the exotherm of ~894°C. As the Cu content
was further increased to 50 wt%, however, the heat of
absorption was also increased by excessive Cu, leading to the
disappearance of crossing feature in DTA curves, corre-
sponding to the exotherm of ~813°C [Fig. 1(a)].
Therefore, the addition of Cu to Ti–Si system thoroughly
changed the reaction pathway by the prior formation of bin-
ary Cu–Si and ternary Cu–Ti–Si liquids during CS, which
provides an easier route for the reactant transfer and acceler-
ates the occurrence of complete reaction.
IV. Conclusions
The addition of Cu to Cu–Ti–Si system significantly pro-
motes the reaction between Ti and Si by the prior formation
of Cu–Si and Cu–Ti–Si liquids, which provides an easy route
for reactant transfer and accelerates the occurrence of com-
plete reaction. The reaction pathway of 30 wt% Cu–Ti–Si
system during CS could be described as: the Cu3Si was
formed initially via solid-state diffusion reaction between Cu
and Si particles; with the temperature increasing, Cu–Si
eutectic liquids were formed at ~802°C between Cu3Si and
Si; then Ti was dissolved into the surrounding Cu–Si liquids
and led to the formation of Cu–Ti–Si ternary liquids; mean-
while, once the liquids reached a saturation stage with [Ti]
and [Si] in the course of continuous dissolution, the Ti5Si3
precursors were precipitated out of the Cu–Ti–Si liquids,
leading to an abrupt increase in temperature and thus initiat-
ing the combustion reaction. The heat generated by the reac-
tion significantly promoted more Ti and Si to dissolve into
the liquids, making the Cu–Ti–Si liquids sufficiently supersat-
urated quickly, and finally plenty of Ti5Si3 could be devel-
oped from the liquids by a solution–reaction–precipitation
mechanism with the continual consumption of [Ti] and [Si].
Acknowledgment
This work is supported by The National Science Foundation of China (no.
50671044), The National Basic Research Program of China (973 Program)
(no. 2012CB619600) and Foundation of Jilin University for Distinguished
Young Scholars.
Furthermore, it is interesting to note that there is a large
difference between Cu–Ti–Si system and Cu–Ti–C system.
According to the results reported by Liang et al.30,31, the
solid-state reaction occurred firstly between metallic Cu and
Ti phases, followed by the formation of Cu–Ti eutectic liq-
uids, and then the nonmetallic C dissolved into binary liquids
forming Cu–Ti–C liquids. However, in this study, the solid-
state reaction took place initially between metallic Cu and
nonmetallic Si, followed by the production of Cu–Si eutectic
liquids; subsequently the metallic Ti was dissolved into
binary liquids forming Cu–Ti–Si liquids.
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