Synthesis of Fine Ceramic Particles in Molten Aluminum by

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Synthesis of Fine Ceramic Particles in Molten Aluminum by
Materials Transactions, Vol. 48, No. 9 (2007) pp. 2374 to 2377
#2007 The Japan Institute of Metals
Synthesis of Fine Ceramic Particles in Molten Aluminum by Combustion Reaction
Wataru Yoshida1 , Makoto Kobashi2 and Naoyuki Kanetake2
1
2
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
Ceramic particles (TiB2 and TiC) dispersed aluminum alloy was synthesized by a combustion reaction. Starting materials were aluminum,
titanium, boron and boron carbide. The heat of reaction between Ti/B and Ti/B4 C was too high to maintain the original configuration of the
blended powder compact. Aluminum was added to the blended powder mixture to control the adiabatic temperature of the reaction. Aluminum
could successfully control the adiabatic temperature and prevented the collapse of the precursor. The average size of the TiB2 and TiC particles
strongly depended on the amount of aluminum added as the diluents of the heat of reaction. By increasing the aluminum addition to the powder
phase, the average size of TiB2 and TiC particles synthesized by the combustion reaction decreased. The TiB2 and TiC particles were extracted
from aluminum matrix, and confirmed to have submicron size under the suitable conditions. [doi:10.2320/matertrans.MAW200783]
(Received April 25, 2007; Accepted June 14, 2007; Published August 10, 2007)
Keywords: combustion reaction, TiB2 , aluminum, titanium, boron, boron carbide, adiabatic temperature.
1.
Table 1 Compositions and sizes of starting materials.
Introduction
Molar ratio of Al-Ti-B system
In general, a chemical reaction that synthesizes intermetallics or ceramics generates a large amount of heat of
reaction.1,2) Combustion reaction is a process that makes use
of the strong exothermic heat to induce the sequential
chemical reaction at the neighboring part of the reacted zone.
The following reactions synthesize titanium diboride (TiB2 )
and/or titanium carbide (TiC) particles and generate large
quantity of heat of reactions.
Vol% of Al
Ti þ 2B ! TiB2
3Ti þ B4 C ! 2TiB2 þ TiC
Vol% of Al
ð1Þ
ð2Þ
When these reactions take place together with aluminum,
aluminum matrix composite with fine ceramic particles
dispersion can be fabricated. Since the combustion reaction
process uses heat of reaction, only a small amount of energy
is required to synthesize ceramics or intermetallics.3–5)
Hence, combustion reaction is an attractive process in terms
of productivity and cost-effectiveness.
By dispersing hard particles of submicron order, deformation resistance of material is strengthened by the effect of
pinning dislocations. Therefore, composites dispersing a fine
particle have superiority of high-temperature strength and
creep resistance.
The objective of this research is to synthesize fine ceramic
particle dispersed in aluminum by the combustion reaction
process. In this paper;
(1) Controlling adiabatic temperatures by changing powder
blending ratio
(2) Controlling the size of ceramic particles
are investigated and also discussed thermodynamically
2.
Experimental Procedure
Titanium powder (45 mm, 99.9%), aluminum powder
(45 mm, 99.99%), boron powder (45 mm, 99%) and boron
carbide powder (10 mm, 99%) were used as starting materials.
Molar blending ratio of starting materials and theoretical
volume fractions of aluminum matrix are shown in Table 1.
The mole blending ratio of titanium and boron powder was
Ti(45 mm)
1
B(45 mm)
2
Al(45 mm)
1.34
2.04
3.02
4.71
40
50
60
70
Molar ratio of Al-Ti-B4 C system
Ti(45 mm)
3
B4 C(10 mm)
1
Al(45 mm)
3.63
5.46
8.19
12.7
40
50
60
70
fixed to B/Ti = 2, and the mole blending ratio of titanium
and boron carbide powder was fixed to Ti/B4 C = 3. The
volume fraction of the aluminum powder was varied from 40
to 70 vol% in both cases. Cylindrical powder compacts
(10 mm in diameter, 8 mm in height) were made by a
compacting pressure of 150 MPa. The experimental apparatus is illustrated in Fig. 1. The powder compact was heated in
an induction furnace under an argon atmosphere to induce the
combustion reaction. After the reaction, the cross section of
the specimen was observed by a scanning electron microscope (SEM) and analyzed by an X-ray diffraction (XRD)
method. Detailed observations of ceramic particles were
carried out after extracting particles by immersing the
composite in hydrochloric acid solution for 20 s.
3.
Results and Discussion
3.1 Microstructure of Al-Ti-B system
Figure 2 shows the microstructure of specimens after the
reaction of Al-Ti-B system. TiB2 particles are dispersed
uniformly in all specimens. When the specimen had a high
aluminum volume fraction, Al3 Ti was remaining as an
intermediate product. Figure 2 shows the microstructure of
40 vol%Al, 50 vol%Al, 60 vol%Al, and 70 vol%Al specimens, and the average diameters of TiB2 particles are
1.25 mm, 1.12 mm, 0.57 mm and 0.31 mm, respectively. By
increasing the fraction of aluminum in specimen, the average
Synthesis of Fine Ceramic Particles in Molten Aluminum by Combustion Reaction
2375
Thermocouple
Al2O3 crucible
Induction coil
Carbon crucible
Pure Al
Mixed powder
Fig. 1
Schematic illustration of the experimental apparatus.
1µm
Fig. 3 SEM micrographs of TiB2 particles (a) 40 vol%Al (b) 50 vol%Al
(c) 60 vol%Al and (d) 70 vol%Al.
1
Cumulative frequency
TiB2
Al3Ti
0.9
0.8
0.7
0.6
0.5
0.4
40vol%
50vol%
60vol%
70vol%
0.3
0.2
0.1
0
TiB2
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
TiB2 particle size, D/µm
Fig. 4
Distribution of TiB2 particle size.
5µm
Fig. 2 SEM micrographs of cross section of specimens (Al-Ti-B) (a)
40 vol%Al (b) 50 vol%Al (c) 60 vol%Al and (d) 70 vol%Al.
diameter of TiB2 particles decreased. The formation of Al3 Ti
together with TiB2 was observed in 70 vol%Al specimen. As
for the Al-Ti-B reaction system, the first-stage reaction
between aluminum and titanium took place immediately after
the melting point of Al.6) This reaction raised the temperature
of the specimen and triggers the second-stage reaction
between titanium and boron. Therefore, when the amount of
aluminum was high (70 vol%Al), the temperature of the
specimen did not increase high enough by the first reaction,
and the TiB2 formation did not occur completely. High
magnification photographs of extracted TiB2 particles of
each specimen are shown in Fig. 3. A frequency distribution
of TiB2 particle size of each specimen was calculated as
shown in Fig. 4. As increasing the aluminum volume
fraction, irregularity of TiB2 particle size became smaller.
3.2 Microstructure of Al-Ti-B4 C system
Figure 5 shows the microstructure of specimens after the
reaction of Al-Ti-B4 C system. TiB2 and TiC particles are
dispersed uniformly in all specimens. Figure 5 shows the
microstructures of 40 vol%Al, 50 vol%Al, 60 vol%Al, and
70 vol%Al specimens, and the average diameters of TiB2
particles are 0.68 mm, 0.49 mm, 0.44 mm and 0.21 mm,
respectively. High magnification photographs of extracted
particles from each specimen are shown in Fig. 6. Similar
to the Al-Ti-B system, the size of particles generated in
aluminum matrix decreased by increasing aluminum volume
fraction. The ceramic particles synthesized from B4 C powder
became smaller than that of Al-Ti-B system. This is probably
because B4 C needed to decompose in prior to the reaction
and created finer nucleation sites of TiB2 and TiC.
3.3 Calculations of adiabatic temperature
The adiabatic combustion temperatures of blending powders containing boron and boron carbide were calculated by
2376
W. Yoshida, M. Kobashi and N. Kanetake
(Z
933
Cp ðAlÞdT þ Hm ðAlÞ
þx
T0
Z
)
Tad
Cp ðAlÞdT þ Hv ðAlÞ ¼ 0
þ
933
xAl þ 3Ti þ B4 C ! xAl þ 2TiB2
þ TiC þ 2Hf ðTiB2 Þ þ Hf ðTiCÞ
2Hf ðTiB2 Þ þ Hf ðTiCÞ
Z Tad
Z
þ2
Cp ðTiB2 ÞdT þ
T0
(Z
ð4Þ
Tad
Cp ðTiCÞdT
T0
933
þx
Cp ðAlÞdT þ Hm ðAlÞ
T0
Al3Ti
Z
Tad
)
Cp ðAlÞdT þ Hv ðAlÞ ¼ 0
þ
933
5µm
Fig. 5 SEM micrographs of specimens (Al-Ti-B4 C) containing (a)
40 vol%Al (b) 50 vol%Al (c) 60 vol%Al and (d) 70 vol%Al.
1µm
Fig. 6 SEM micrographs of ceramic particles (a) 40 vol%Al (b) 50 vol%Al
(c) 60 vol%Al and (d) 70 vol%Al.
the equations (3) and (4), respectively.7) The initial temperature was assumed as 300 K.
xAl þ Ti þ 2B ! xAl þ TiB2 þ Hf ðTiB2 Þ
Z Tad
Hf ðTiB2 Þ þ
Cp ðTiB2 ÞdT
T0
ð3Þ
x: mole fraction of aluminum melted at 933 K,
T0 : initial temperature (K),
Tad : adiabatic temperature (K),
Cp : heat capacity (kJ/molK),
Hf (TiB2 ): heat of formation of TiB2 (kJ/mol),
Hf (TiC): heat of formation of TiC (kJ/mol),
Hm (Al): latent heat of fusion of aluminum (kJ/mol),
Hv (Al): latent heat of vaporization of aluminum (kJ/mol)
In the Al-Ti-B system, the adiabatic temperatures of the
40 and 50 vol%Al specimens can be calculated as 2723 K,
which is the boiling point of aluminum. Hence, aluminum
vapor raised the internal pressure of powder compacts and
made the specimen broken into small pieces. The adiabatic
temperatures of 60 vol%Al and 70 vol%Al specimen are
2461 K and 2158 K, respectively. In these specimens, the
adiabatic temperatures are below the boiling point of
aluminum, and the original configuration of powder compact
was maintained. As for the Al-Ti-B4 C system, the adiabatic
temperature of 40, 50, 60 and 70 vol%Al specimens were
2723 K, 2573 K, 2243 K and 2008 K, respectively. Figure 7
shows the theoretically calculated adiabatic temperature
and the practically measured combustion temperature as a
function of aluminum addition. The theoretical data is higher
than the practically measured data at all range of aluminum
addition. This is because some fraction of reaction heat
was emitted to the ambient atmosphere in practice. The
temperature was effectively controlled in the wide range of
about 800 K both in theory and in practice. When ceramic
particle was synthesized by the combustion reaction, the size
of the ceramic particle was affected by the combustion
temperature because of larger growth of ceramic particle at
higher temperatures. Thus, the addition of aluminum could
be one of the effective methods to control the size of TiB2
and TiC particles. Relation between a ceramic particle size
and measured combustion temperatures is summarized in
Fig. 8. The particle size was effectively reduced to submicron levels by decreasing the combustion temperature to
around 1800 K or lower. It was also confirmed that the finer
ceramic particles were synthesized from the Al-Ti-B4 C
system.
Synthesis of Fine Ceramic Particles in Molten Aluminum by Combustion Reaction
1.4
TiB2 particle size, D/µm
Temperature, T/K
3000
2500
2000
1500
Calculated adiabatic temperature(Al-Ti-B)
Practical combustion temperature(Al-Ti-B)
Calculated adiabatic temperature(Al-Ti-B4C)
Practical combustion temperature(Al-Ti-B4C)
1000
500
0
30
40
50
60
70
80
Volume fraction of aluminum (%)
Fig. 7
Adiabatic temperature and the measured sample temperature.
This is probably because B4 C needed to decompose in
prior to the reaction and created finer nucleation sites of TiB2
and TiC.
4.
2377
Conclusion
Ceramic particle dispersed aluminum alloy was synthesized by combustion reaction and the effect of aluminum
addition as diluents on the particle size was also investigated.
The following results were obtained through the experiment.
1) The combustion temperature of samples becomes lower
by increasing aluminum volume fraction.
2) Particle size of the ceramic generated by combustion
reaction is smaller as the combustion temperature
becomes lower.
3) The smallest size of the ceramic particles is about
0.3 mm in 70 vol%Al sample.
1.2
1
0.8
0.6
0.4
Al-Ti-B
Al-Ti-B4C
0.2
0
1000
1500
2000
2500
Calculated adiabatic temperature, T/K
Fig. 8 Relationship between TiB2 particle size and sample temperature.
4) In comparison with the Al-Ti-B system, the size of
particles synthesized in Al-Ti-B4 C system is smaller.
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5) H. J. Feng, J. J. Moore and G. With: Metall. Trans. 23A (1992) 2373–
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