Formation of Ta-Al intermetallics by combustion synthesis involving Al-based thermite reactions

Article abstract of DOI:10.1016/j.jallcom.2009.10.203

Thermite-based combustion synthesis was conducted to investigate in situ formation of tantalum aluminides (Ta₂Al, TaAl, TaAl₂, and TaAl₃) and Al₂O₃ from the powder compacts of Al and Ta₂O₅ under a molar ratio ranging from Al/Ta₂O₅ = 13/3 to 28/3. Effects of the sample stoichiometry on the combustion behavior and degree of phase conversion were studied. Experimental evidence showed that upon ignition, the synthesis reaction proceeded in a self-sustaining manner featuring a planar combustion front or two localized reaction zones traveling spirally. The combustion front temperature varies between 1135 and 1460 °C. In agreement with composition dependence of the reaction exothermicity, the reaction front velocity in the axial direction increased from 5.8 to 7.2 mm/s with molar ratio of Al/Ta₂O₅ from 13/3 to 16/3, beyond which the flame velocity decreased substantially. The slowest reaction front with an axial velocity of 1.15 mm/s was observed in the sample of Al/Ta₂O₅ = 28/3. Based upon the XRD analysis, in situ formation of the composites composed of Al₂O₃ and tantalum aluminides was confirmed. For the samples with Al contents of Al/Ta₂O₅ = 13/3-18/3, an increase in the Al/Ta₂O₅ ratio improved the production of tantalum aluminides and caused the change in the dominant aluminide phase from Ta₂Al, TaAl, to TaAl₂. Due to lack of adequate reaction time, the phase conversion was not stoichiometrically achieved in the samples of Al/Ta₂O₅ = 13/3-18/3. However, it was found that the slower reaction front provided longer reaction time to complete the phase conversion almost stoichiometrically in the samples of Al/Ta₂O₅ = 22/3-28/3, which yielded the Al-rich phases TaAl₂ and/or TaAl₃ in the final products.

Full text of DOI:10.1016/j.jallcom.2009.10.203

Journal of Alloys and Compounds 491 (2010) 153–158
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Formation of Ta–Al intermetallics by combustion synthesis involving Al-based
thermite reactions
C.L. Yeh∗, H.J. Wang
Department of Aerospace and Systems Engineering, Feng Chia University, 100 Wenhwa Road, Seatwen, Taichung 40724, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Thermite-based combustion synthesis was conducted to investigate in situ formation of tantalum alu-
minides (Ta2Al, TaAl, TaAl2, and TaAl3) and Al2O3 from the powder compacts of Al and Ta2O5 under a
molar ratio ranging from Al/Ta2O5 = 13/3 to 28/3. Effects of the sample stoichiometry on the combustion
behavior and degree of phase conversion were studied. Experimental evidence showed that upon igni-
tion, the synthesis reaction proceeded in a self-sustaining manner featuring a planar combustion front or
two localized reaction zones traveling spirally. The combustion front temperature varies between 1135
and 1460 ◦C. In agreement with composition dependence of the reaction exothermicity, the reaction front
velocity in the axial direction increased from 5.8 to 7.2 mm/s with molar ratio of Al/Ta2O5 from 13/3 to
16/3, beyond which the flame velocity decreased substantially. The slowest reaction front with an axial
velocity of 1.15 mm/s was observed in the sample of Al/Ta2O5 = 28/3. Based upon the XRD analysis, in
situ formation of the composites composed of Al2O3 and tantalum aluminides was confirmed. For the
samples with Al contents of Al/Ta2O5 = 13/3–18/3, an increase in the Al/Ta2O5 ratio improved the pro-
duction of tantalum aluminides and caused the change in the dominant aluminide phase from Ta2Al,
TaAl, to TaAl2. Due to lack of adequate reaction time, the phase conversion was not stoichiometrically
achieved in the samples of Al/Ta2O5 = 13/3–18/3. However, it was found that the slower reaction front
provided longer reaction time to complete the phase conversion almost stoichiometrically in the samples
of Al/Ta2O5 = 22/3–28/3, which yielded the Al-rich phases TaAl2 and/or TaAl3 in the final products.
© 2009 Elsevier B.V. All rights reserved.
Received 24 September 2009
Received in revised form 23 October 2009
Accepted 23 October 2009
Available online 29 October 2009
Keywords:
Composite materials
Intermetallics
Solid state reactions
X-ray diffraction
Tantalum aluminides
1. Introduction
Because the reactions between metallic reactants are normally less
exothermic than those between metals and non-metal elements
(e.g., B, C, and N), the SHS technique applied to the preparation of
aluminides often requires additional treatments or modifications,
such as prior heating [13,18,19], electric-field activation [15,17],
microwave assistance [16], and high-energy mechanical milling
[21,22].
Intermetallic compounds based upon aluminides of several
transition metals such as iron, nickel, titanium, cobalt, niobium,
and tantalum have been recognized as potential candidates for
the high-temperature structural applications [1–5]. Many fabri-
cation routes for aluminides of the transition metals have been
developed; for instance, mechanical alloying, hot pressing, reac-
tion synthesis, melting, casting, etc. [6,7]. With the merits of time
and energy savings, combustion synthesis particularly in the mode
of self-propagating high-temperature synthesis (SHS) represents
a promising alternative to produce a variety of refractory mate-
rials, such as borides, carbides, nitrides, carbontrides, silicides,
aluminides and other intermetallics [8–10]. Most of the research
effort in the preparation of aluminide compounds by SHS from
metallic powders of their constituents has been devoted to the alu-
minides of nickel (NiAl and Ni3Al) [11–14], titanium (TiAl, Ti3Al, and
TiAl3) [15–18], cobalt (CoAl) [19,20], and niobium (NbAl3) [20–22].
On the other hand, when compared with the conventional SHS
process using elemental powder compacts, formation of transi-
tion metal aluminides by combustion synthesis of the thermite
type is generally more energetic and starts with less expensive
metal oxides as the source of transition metals [23]. For example,
by means of the thermite-based SHS process with Al and Nb2O5
as the reactants, Yeh and Wang [24] successfully fabricated two
Nb-rich aluminides Nb3Al and Nb2Al, both of which cannot be
formed through the reaction between Nb and Al particles due to
weak exothermicity. Moreover, the integration of combustion syn-
thesis with Al-based thermite reactions leads to in situ formation
of the aluminide matrix composite reinforced by Al2O3 [25–27].
Many composites comprising an aluminide matrix (such as FeAl,
TiAl, NiAl, and NbAl3) and a reinforcing phase of Al2O3 have been
shown to possess improved high-temperature strength and creep
resistance [28–31].
∗
Corresponding author. Tel.: +886 4 24517250x3963; fax: +886 4 24510862.
E-mail address: clyeh@fcu.edu.tw (C.L. Yeh).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.10.203

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C.L. Yeh, H.J. Wang / Journal of Alloys and Compounds 491 (2010) 153–158
Fig. 1. Phase diagram of Ta–Al system [34].
Among various aluminides of transition metals, the compounds
According to reactions (1)–(4), the starting powder mixtures were formulated with
a molar ratio of Al/Ta2O5 ranging from 4.33 (13/3) to 9.33 (28/3). Moreover, the
standard reaction enthalpy (ꢀH) of reactions (1)–(4) was calculated with ther-
modynamic data of Refs. [35,36] and presented in Table 1 under unit mole of the
product (including the aluminide phase and Al2O3). Due to an appreciable amount
of heat generated from the production of Al2O3, reactions (1)–(4) are about 10-fold
more exothermic than the formation of tantalum aluminides from their constituent
elements (i.e., their enthalpies of formation).
Under the prescribed stoichiometry, the reactant powders were dry mixed in
a ball mill and then cold-pressed into cylindrical test specimens with a diame-
ter of 7 mm, a height of 12 mm, and a compaction density of 50% relative to the
theoretical maximum density (TMD). The SHS experiment was conducted in a
stainless-steel windowed combustion chamber under an atmosphere of high-purity
argon (99.99%). The ignition was accomplished by a heated tungsten coil with a volt-
age of 60 V and a current of 1.5 A. Details of the experimental setup and methods of
measurement approach were previously reported [13,19].
in the Ta–Al system have been of great interest for their excellent
mechanical properties and high degree of structural complexity
[32,33]. The Ta–Al phase diagram depicted in Fig. 1 shows the exis-
tence of four aluminides, including Ta2Al, TaAl, TaAl2, and TaAl3
[34]. However, the enthalpy of formation (ꢀHf) associated with
each tantalum aluminide is relatively low and varies between −22
and −30 kJ/mol [35], which is responsible for the unfeasibility of
formation of the tantalum aluminide by SHS from the elemental
powder compact. Therefore, instead of using Ta and Al powders,
this study employs tantalum oxide (Ta2O5) and Al as the starting
materials to prepare tantalum aluminides through thermite-based
combustion synthesis. Such a process essentially combines two
consecutive stages: the first one produces elemental Ta and by-
product Al2O3 via the thermite reaction that concurrently liberates
substantial reaction heat to initiate the second stage, which yields
aluminide compounds from the interaction between the reduced
Ta and Al.
3. Results and discussion
3.1. Observation of combustion characteristics
In this study, the Al2O3-reinforced tantalum aluminides were
obtained through the SHS process of the thermite type and the alu-
minide phase formed in the product was investigated with respect
to the initial composition of the reactant compact (i.e., the molar
ratio of Al to Ta2O5). Additionally, the effect of sample stoichiom-
etry was studied on the combustion temperature as well as the
spreading mode and propagation rate of the combustion wave.
Two typical combustion sequences in association with the reac-
tant compacts of different starting compositions of Ta2O5:Al = 3:16
and 3:22 are respectively illustrated in Fig. 2(a) and (b). As shown in
Fig. 2(a), upon ignition a nearly planar combustion front forms and
traverses the entire sample self-sustainingly in about 1.6 s. How-
ever, Fig. 2(b) reveals that shortly after initiation the planar front
is transformed into two localized combustion zones moving along
a spiral trajectory on the sample surface. In the recorded image
2. Experimental methods of approach
The present study adopts tantalum oxide Ta2O5 (Strem Chemicals, 99.8% purity)
and elemental Al (Showa Chemical Co., <40 ␮m, 99% purity) powders as the raw
materials. Reactions (1) through (4) correspond to stoichiometric formation of four
different tantalum aluminides, Ta2Al, TaAl, TaAl2, and TaAl3, with Al2O3.
Table 1
Standard reaction enthalpy (ꢀH) per unit mole of product for reactions of Al with
Ta2O5 under different stoichiometries.
Thermite-based combustion synthesis reactions
ꢀH (kJ/mol)
13Al + 3Ta2O5 → 3Ta2Al + 5Al2O3
16Al + 3Ta2O5 → 6TaAl + 5Al2O3
22Al + 3Ta2O5 → 6TaAl2 + 5Al2O3
28Al + 3Ta2O5 → 6TaAl3 + 5Al2O3
(1)
(2)
(3)
(4)
13 Al + 3 Ta2O5
→
3 Ta2Al + 5 Al2O3
−288
−217
−218
−220
8
8
8
8
16
11
3
6
5
Al + Ta2O5
→
→
→
11 TaAl + Al2O3
11
11
22
11
3
6
5
Al + Ta2O5
11 TaAl2
+
+
11 Al2O3
11
28
11
3
6
5
Al + Ta2O5
11 TaAl3
11 Al2O3
11

C.L. Yeh, H.J. Wang / Journal of Alloys and Compounds 491 (2010) 153–158
155
Fig. 2. Recorded SHS sequences recorded from powder compacts with starting stoichiometric ratios of Ta2O5:Al = (a) 3:16 and (b) 3:22. (A scale with a unit of mm is optically
superimposed on the left-hand side of each image.).
Fig. 3. Photographs of synthesized products obtained from powder compacts with starting stoichiometric ratios of Ta2O5:Al = (a) 3:16 and (b) 3:22.

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C.L. Yeh, H.J. Wang / Journal of Alloys and Compounds 491 (2010) 153–158
by Al as its latent heat, which lowers the overall reaction exother-
micity and accounts for the decrease of the flame speed shown in
Fig. 4. In addition, the low axial velocity of the reaction front for the
powder compacts with Al/Ta2O5 ≥ 22/3 is also a consequence of the
spiral propagation of the reaction zone, under which the combus-
tion wave travels not only longitudinally but also transversely.
3.3. Measurement of combustion temperature
Six combustion temperature profiles recorded from the
Ta2O5–Al powder compacts with different initial stoichiometries
are depicted in Fig. 5, where the abrupt rise in temperature sig-
nifies rapid arrival of the combustion front and the peak value
represents the reaction front temperature. It is evident that the
temperatures curves of two different shapes are seen. One features
a single peak corresponding to the longitudinal progression of a
planar front (for Al/Ta2O5 = 13/3, 16/3, 18/3, and 20/3 in Fig. 5). The
other accounting for the spinning motion of two consecutive reac-
tion zones is characterized by double spikes (for Al/Ta2O5 = 22/3
and 26/3 in Fig. 5). The combustion front temperatures presented
in Fig. 5 range between 1135 and 1460 ◦C, which are lower than the
melting points of tantalum aluminides in Fig. 1 and Al2O3 (2054 ◦C).
This explains that the burned sample almost retains its original
shape after combustion. In agreement with the composition depen-
dence of combustion velocity, the combustion front temperature
increases with Al content, reaches a maximum at Al/Ta2O5 = 16/3,
and then decreases with further increase of Al.
Fig. 4. Effect of molar ratio of Al/Ta2O5 of reactant compacts on flame-front propa-
gation velocity of SHS processes.
at t = 1.60 s of Fig. 2(b), two combustion zones are circled. Obvi-
ously, it took longer time of approximately 2.9 s for the spinning
combustion wave to arrive at the bottom of the sample. Accord-
ing to Ivleva and Merzhanov [37], once the heat flux liberated from
self-sustaining combustion is no longer sufficient to maintain the
steady propagation of a planar front, the combustion front forms
one or several localized reaction zones. In view of the compara-
ble reaction enthalpies reported in Table 1 for the reactions with
powder compacts of Ta2O5:Al = 3:16 and 3:22, the appearance of
the spinning combustion wave in the sample of Ta2O5:Al = 3:22 is
essentially attributed to consumption of more reaction enthalpy
as the latent heat of Al (28.7 kJ/mol) by the sample with a higher
content of Al. As a result, the experimental observations of this
study indicated that for the powder compacts with a molar ratio
of Al/Ta2O5 ≥ 22/3, the combustion front was restricted typically
within two localized regions propagating in a spinning manner,
similar to that of Fig. 2(b).
Fig. 3(a) and (b) shows two photographs of the synthesized
products respectively obtained from the sample compacts with
starting stoichiometric ratios of Ta2O5:Al = 3:16 and 3:22. The prod-
uct in Fig. 3(a) subjected to the propagation of a planar combustion
front exhibits a porous appearance and a metallic shine. For the
synthesized product experiencing a spinning combustion wave,
Fig. 3(b) displays spiral marks on the rugged exterior of the product.
3.4. Phase constituent of SHS products
The XRD patterns of the products synthesized from the reac-
tant compacts of Ta2O5:Al = 3:13 and 3:14 are depicted respectively
in Fig. 6(a) and (b), which confirm in situ formation of tantalum
aluminides and Al2O3. Despite the production of two kinds of alu-
minides (Ta2Al and TaAl), Fig. 6(a) and (b) indicates a considerable
amount of elemental Ta left unreacted in the final products. The
remnant Ta was diminished by introducing more Al, which also
led to the appearance of another intermetallic TaAl2 when the Al
content reached the proportion of Al/Ta2O5 = 16/3, as shown in
Fig. 6(c). Furthermore, Fig. 6(d) reveals that for the powder com-
pact of Ta2O5:Al = 3:18, TaAl2 becomes the dominant aluminide. In
view of reactions (1) and (2), the presence of multiple aluminide
phases and free Ta in Fig. 6(a) and (c) signifies incomplete conver-
sion, which is presumably due to lack of sufficient reaction time
caused by rapid propagation of the reaction front.
3.2. Measurement of flame-front propagation velocity
The flame-front propagation velocity (Vf) in the axial direction
was determined from the recorded combustion images and plotted
in Fig. 4 as a function of the starting stoichiometry of the reactant
compact. The flame-front velocity was found to increase from 5.8 to
7.2 mm/s with increasing molar ratio of Al/Ta2O5 from 4.33 (13/3)
to 5.33 (16/3), beyond which the flame velocity decreased signifi-
cantly with Al content and arrived at a minimum of 1.15 mm/s in
the sample of Al/Ta2O5 = 28/3. The increase of combustion velocity
observed in Fig. 4 is mainly due to an improvement in the degree
of aluminide formation, which leads to a higher reaction temper-
ature and hence a faster combustion front. This is confirmed by
the fact that the amount of elemental Ta left unreacted in the final
product was reduced by increasing the Al content in the sample.
On the other hand, as mentioned above, the increase of the ratio of
Al/Ta2O5 in a powder compact means more reaction heat consumed
Fig. 5. Dependence of combustion temperature on Al content of Ta2O5–Al powder
compacts.

C.L. Yeh, H.J. Wang / Journal of Alloys and Compounds 491 (2010) 153–158
157
Table 2
Summary of phase composition of SHS products with respect to their initial
stoichiometries.
Stoichiometry
Ta2O5:Al
Phase composition of SHS products
Dominant
Secondary
Others
aluminide(s)
aluminide(s)
3:13
3:14
3:15
3:16
3:18
3:20
3:22
3:24
3:26
3:28
Ta2Al
Ta2Al
Ta2Al
TaAl
TaAl2
TaAl2
TaAl2
TaAl2
TaAl3
TaAl3
TaAl
TaAl
TaAl, TaAl2
Ta2Al, TaAl2
TaAl, Ta2Al
Ta2Al
Ta, Al2O3
Ta, Al2O3
Ta, Al2O3
Ta, Al2O3
Al2O3
Al2O3
Al2O3
Al2O3
Al2O3
TaAl3
TaAl2
TaAl2
Al2O3
pound TaAl3 is identified along with TaAl2. Because TaAl2 is trivial
in Fig. 7(d), the product obtained from the sample of Ta2O5:Al = 3:28
is practically a composite of TaAl3 and Al2O3, as described by reac-
tion (4). Fig. 7(a) and (d) means that in contrast to reactions (1)
and (2), the complete conversion of reactions (3) and (4) is verified.
Better phase transformation achieved in the powder compacts of
Al/Ta2O5 ≥ 22/3 is most likely attributed to the longer reaction time
provided by slower propagation of the reaction front.
A summary of the product composition with respect to the
starting stoichiometry of the Ta2O5–Al sample is given in Table 2.
It was found that an increase in the molar ratio of Al/Ta2O5
not only improved the aluminide formation, but also altered the
dominant aluminide phase. No elemental Ta was observed in
the final products until the molar ratio of the sample reached
Al/Ta2O5 = 18/3 and above. The Ta-rich phase Ta2Al was identi-
fied as the dominant aluminide for the initial compositions of
Al/Ta2O5 = 13/3–15/3, while the Al-rich phase TaAl2 was recognized
for Al/Ta2O5 = 18/3–24/3 and TaAl3 for Al/Ta2O5 = 26/3 and 28/3.
Besides, due to the complexity of the aluminide phases formed from
the sample of Al/Ta2O5 = 16/3, the monoaluminide TaAl is granted
to be the major aluminide according to reaction (2).
Fig. 6. XRD patterns of SHS products obtained from powder compacts with Ta2O5:Al
(a) 3:13, (b) 3:14, (c) 3:16, and (d) 3:18.
On the other hand, Fig. 7(a) shows in situ formation of Al2O3
and TaAl2 from the sample of Ta2O5:Al = 3:22, which represents
complete conversion according to reaction (3). With a larger molar
ratio of Al/Ta2O5, as shown in Fig. 7(b)–(d), the other Al-rich com-
4. Conclusions
This study presents a comprehensive investigation of the
thermite-based SHS process associated with in situ formation of
tantalum aluminides and Al2O3. The experiments were conducted
with Al–Ta2O5 powder compacts under a molar ratio of Al/Ta2O5
between 13/3 and 28/3. For the samples of Al/Ta2O5 = 13/3–21/3,
the SHS process featured a planar combustion front. However, due
to a decrease in the overall reaction exothermicity, the reaction
in the samples of Al/Ta2O5 = 22/3–28/3 was confined to two local-
ized areas propagating spirally. The increase of the Al content in
the sample contributed to an improvement in the formation of
tantalum aluminides, but it enlarged the fraction of the reaction
heat consumed as the latent heat of Al. Therefore, the combus-
tion temperature as well as the reaction front velocity exhibited
an increase with molar ratio of Al/Ta2O5, approached their max-
ima at Al/Ta2O5 = 16/3, and then decreased constantly with further
increase in Al.
For the samples with Al contents of Al/Ta2O5 = 13/3–16/3, the
final products contained free Ta and aluminide compounds like
Ta2Al, TaAl, and TaAl2. As the molar ratio of Al/Ta2O5 increased up
to 18/3 and above, the elemental Ta reduced from Ta2O5 was fully
converted into TaAl2 and/or TaAl3. Moreover, the phase conversion
was stoichiometrically achieved in the samples of Ta2O5:Al = 3:22
and 3:28, from which in situ composites of TaAl2–Al2O3 and
TaAl3–Al2O3 were produced, respectively.
Fig. 7. XRD patterns of SHS products obtained from powder compacts with
Ta2O5:Al = (a) 3:22, (b) 3:24, (c) 3:26, and (d) 3:28.

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