J. Kor. Powd. Met. Inst., Vol. 20, No. 3, 2013
DOI: 10.4150/KPMI.2013.20.3.221
<PM리뷰>
Fabrication of Fe-TiB2 Composite Powder by High-Energy Milling
and Subsequent Reaction Synthesis
H. X. Khoa, N. Q. Tuan, Y. H. Lee, B. H. Lee, N. H. Vieta, and J. S. Kim*
School of Materials Science and Engineering, University of Ulsan, San-29, Mugeo-2 Dong,
Nam-Gu, Ulsan, 680-749, Korea
a
School of Materials Science and Engineering, Hanoi University of Science and Technology,
No 1, Dai Co Viet Street, Hai Ba Trung dist, Hanoi, Vietnam
(Received June 11, 2013; Accepted June 24, 2013)
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Abstract TiB2-reinforced iron matrix composite (Fe-TiB2) powder was in-situ fabricated from titanium hydride
(TiH2) and iron boride (FeB) powders by the mechanical activation and a subsequent reaction. Phase formation of the
composite powder was identified by X-ray diffraction (XRD). The morphology and phase composition were observed
and measured by field emission-scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy
(EDS), respectively. The results showed that TiB2 particles formed in nanoscale were uniformly distributed in Fe matrix.
Fe2B phase existed due to an incomplete reaction of Ti and FeB. Effect of milling process and synthesis temperature on
the formation of composite were discussed.
Keywords: Fe-TiB2 composite, Mechanical activation, Solid state reaction, Heat treatment
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nique has been developed to incorporate ceramic particulates into metal matrices. The major advantage of in
situ process is that the dispersed ceramic phase is created by the chemical reaction between elements of their
compounds so that the particles are formed within the
metal matrix and are clean, non-oxidized particle-matrix
interfaces with higher interfacial strength, improved wettability and better particle-size distribution [3]. The synthesis of TiB2 phase by various methods have been
given: laser cladding [4-5], plasma transferred arc (PTA)
[6], aluminothermic reduction [7], and self-propagating
high temperature synthesis (SHS) [2, 8]. They indicate
that TiB2 particles are able to form in situ via different
routes from various initial materials. Some of them used
pure elements as starting materials which is costly unfavourable. Most of them exhibited large TiB2 particle size.
The present work aimed at investigating the feasibility
of fabricating TiB2-reinforced Fe matrix composite powder by a combination of mechanical activation and heat
treatment. It is expected that the route is able to provide a
novel process for rapid, simple and cost-effective synthe-
1. Introduction
Fe-based metal matrix composites have found many
applications in tools, dies and wear-resistant parts for last
several decades. Adding refractory particles to the Fe
matrix as dispersoid improves mechanical properties of
the matrix and increases wear property [1-2]. Among
various ceramic particulates, TiB2 is considered to be one
of the best reinforcements for steel matrix due to its high
melting point (2980°C), high hardness (3400 kg/mm2,
only lower than diamond, BN and B4C), high elastic
modulus, good corrosion resistance and chemical inertness. In terms of thermal and electrical conductivities,
TiB2 is favoured in selection because of its high thermal
(60-120 W/mK) and electrical (~105 S/cm) conductivities.
One of the most common techniques of producing
these types of particulate-reinforced metal-matrix composites is powder metallurgy. An important factor influencing the structure and properties of the composites is
the ceramic metal interface [2]. Recently, in situ tech-
*Corresponding Author : Ji Soon Kim, TEL: +82-52-259-2244, FAX: +82-52-259-1688, E-mail: jskim@ulsan.ac.kr
221
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H. X. Khoa, N. Q. Tuan, Y. H. Lee, B. H. Lee, N. H. Viet, and J. S. Kim
sis method, because of its effective dispersion of the fine
reinforcing particles, for Fe-TiB2 composite using titanium hydride (TiH2) and ferro-boron (FeB) as initial
materials. Effect of milling process and heat treatment on
the phase formation of composite powder was included
in this study.
mation of powder was identified by X-ray diffractometer
using Cu Kα radiation. The microstructure and chemical
elements of the composite powders were observed and
analyzed by field emission scanning electron microscope
(FE-SEM) equipped with energy dispersive spectroscopy
(EDS).
2. Experimental
3. Results and Discussion
Commercial ferro-boron alloy, FeB, and titanium hydride,
TiH2, powders were used as starting materials. The TiH2
and FeB powder mixture was accurately weighed in the
ratio of 27.2 wt% TiH2 and 72.8 wt% FeB to give a Fe38.4wt% TiB2 composite powder corresponding to the
following reaction equation:
3.1. Powder characteristics
The morphology of the starting powders is shown in
Fig. 1. Table 1 shows a summary of the particle size
analysis. The mean particle size (D50) of TiH2 and FeB
are 12.5 and 7.0 µm, respectively.
Fig. 3 shows morphology of the FeB-TiH2 powder
mixture after high-energy milling (a-b) and its cross-section(c). TiH2 and FeB are brittle materials, so these powders get fractured easily during milling and their particle
size is reduced significantly. Particle size analysis in the
D10 and D50 terms are 1.2 and 3.0 µm respectively. These
values prove that the FeB and TiH2 powder became finer
after 2h of high-energy milling in comparison with the
starting powders. But the D90 value increased after milling process (D90 = 23.5 µm) by formation of agglomerates as shown in Fig. 3(a). The image taken on the surface of
agglomerate (Fig. 3(b)) shows fine and rounded particles of
2FeB(s) + TiH2(s) → 2Fe(s) + TiB2(s) + H2(g) ↑
(1)
FeB in a form of lump was manually crushed and
screened to remove the large fragment of FeB. The powder mixtures were prepared either by turbular mixing for
2h or by high-energy milling for 2 h in a high-energy ball
mill (AGO-2, [9]). Stainless steel vials and balls (5 mm
in diameter) were used. The balls-to-powder weight ratio
was 20:1. The vials were evacuated and filled with 0.3
MPa pure argon gas before each run to prevent oxidation during processing. The rotational velocity of mill
axis was 500 rpm.
To fabricate Fe-TiB2 composite powders, the reaction
of TiH2 and FeB was carried out in a tube furnace at
1200oC in flowing Ar gas with heating rate of 5oC/min
and holding time of 1 and 5 hours.
Particle size was measured using laser scattering particle size analyser, Mastersizer 2000 (Malvern). Phase for-
Table 1. Result of particle size analysis of starting powders
(volume distribution)
Powder
D10
(µm)
D50
(µm)
D90
(µm)
TiH2
FeB
3.3
3.1
12.5
7.0
38.6
16.8
Fig. 1. FE-SEM images of starting powders; (a) TiH2 and (b) FeB powder.
Journal of Korean Powder Metallurgy Institute (J. Kor. Powd. Met. Inst.)
Specific Area
(m2/g)
0.80
1.05
Fabrication of Fe-TiB2 Composite Powder by High-Energy Milling and Subsequent Reaction Synthesis
223
decreased and distributed in a broad range from nanometer to several micrometer.
EDS line scan (Fig. 3(d)) on the particle outlines
showed the distribution of Fe, Ti and B. The gradual
change of Fe and Ti indicates the close contact between
FeB and TiH2.
Fig. 2. XRD pattern of the FeB-TiH2 powder mixture prepared
by high-energy milling.
FeB and TiH2. To observe the agglomerates more clearly,
the cross-section of agglomerate was investigated (Fig.
3(c)). Agglomerates formed a dense structure; the TiH2
(dark phase) homogeneously distributed around the FeB
particles. TiH2 is brittle and can be milled to nanoscale
by high-energy milling for short periods of time [10].
The more brittle component TiH2 gets more easily fragmented and embedded around the less brittle component
FeB. After 2h high-energy milling, the FeB particle size
3.2. Thermal activation of powder mixtures
Fig. 4 shows DSC curve obtained under a constant
heating rate of 10 Kmin−1 for the FeB-TiH2 powder mixture prepared by turbular mixing and high-energy milling, respectively. DSC curve for tubular mixing shows
two thermal events represented by the first two peaks at
470°C and 520°C assigning for endothermic nature of the
dehydrogenation process of TiH2 compound [10], while
the only a peak appeared at 370°C for the high-energy
milling powder mixture. The shift to lower temperature
for dehydrogenation events can be explained by effect of
milling that decreases particle size [10]. At higher temperature, beginning at 1030°C for turbular mixing and
910°C for high-energy milling powder mixtures, respec-
Fig. 3. (a) Fe-SEM image of FeB-TiH2 powder mixture after high-energy milling for 2h at 500 rpm, (b) the selected area with
high magnification, (c) the polished cross-section, and (d) the result of EDS line scan across the particle in (c).
Vol. 20, No. 3, 2013
224
H. X. Khoa, N. Q. Tuan, Y. H. Lee, B. H. Lee, N. H. Viet, and J. S. Kim
Fig. 4. DSC curves of FeB-TiH2 powder mixture after (a) 2h
turbular mixing and (b) high-energy milling at 500 rpm for
2h (b).
tively, the DSC curves shifted up signaling for appearing
of thermal events. These events can be chemical solid
reaction between Ti formed from the dehydrogenation
process of TiH2 with boron in FeB to form TiB2 phase.
3.3. Phase composition and microstructure of FeBTiH2 powder mixture after heat treatment
Fig. 5 shows the XRD patterns of the Fe-TiB2 composite
powders by in situ synthesis from FeB-TiH2 powder mixture. The identified peaks are assigned to Fe, Fe2B and TiB2,
respectively. There is a clear split of the peak around 45
when increasing temperature of 1100°C to 1200°C.
The possible solid state reactions involved in synthesizing Fe-TiB2 composite powder has been considered. The
reactions leading to formation of Fe-TiB2 composite can
be suggested as follows [11-14]:
Fig. 5. XRD patterns of high-energy milled powder mixture
after synthesis reaction at given conditions of temperature
and time.
Ti + 2Fe2B → TiB2 + 4Fe
(4)
The overall reaction of these is reaction (1) presented
above. FeB phase was not found but Fe2B phase exists
within the resolution of the XRD pattern. This means the
reaction (4) was not completely finished, the remained
Fe2B phase was also reported in [4-7]. During the process, TiH2 and FeB might interact to form some possible
compounds such as TiB2, TiB or Fe2B, etc. The Gibbs
free energies of formation for these compounds are [2,
14]:
Ti + 2B → TiB2
∆G1300K = -275.5 kJ.mol−1
(6)
2Fe + B → Fe2B
∆G1300K = -67.2 kJ.mol−1
(7)
TiH2 → Ti + H2 ↑
(2)
Ti + B → TiB
∆G1300K = -155.3 kJ.mol−1
(8)
Ti + 4FeB → TiB2 + 2Fe2B
(3)
Fe + Ti → FeTi
∆G1450K = -31.0 kJ.mol−1
(9)
Fig. 6. FE-SEM image shows (a) morphology and (b) EDS result of Fe-TiB2 composite powder after reaction synthesis at
1200oC for 5 hour.
Journal of Korean Powder Metallurgy Institute (J. Kor. Powd. Met. Inst.)
Fabrication of Fe-TiB2 Composite Powder by High-Energy Milling and Subsequent Reaction Synthesis
These value indicates that TiB2 is thermodynamically
most favourable and the most stable phase. The reason
for the formation of Fe2B can be explained by reaction
(3), when B in FeB particles reacts with Ti on surface to
form TiB2, and B content in FeB particle was depleted
causing the transform of FeB to Fe2B by the newly pure
Fe on surface.
The morphology of Fe-TiB2 composite powder after
heat treatment at 1200oC for 5 hour is given in Fig. 6. It
is hardly to identify TiB2 particle on the surfaces but Fe,
Ti and B peaks appearing in EDS spectra implies that the
ultrafine TiB2 particles in the Fe matrix.
225
Alternatively, TiB2 particles can be identified using
etching step to remove Fe matrix and to leave TiB2 nanoparticles on the surface (Fig. 7(a)). The nanoscale TiB2
particles form networks surrounding on the surface as
shown in Fig. 7(b).
Fig. 8 shows cross-section images of powder particles
after heat treatment and describes the evolution of composites microstructure after thermal reaction. Microstructure is composed of round-bright phase embedded in the
dark phase (Fig. 8(a)). Comparing Fig. 8(a) with Fig.
3(c), it can be seen that the fine FeB particles in Fig.
3(c), particle size below 1mm, were disappeared in
Fig. 7. FE-SEM image of Fe-TiB2 composite powder after etching: (a) Low and (b) high magnification.
Fig. 8. FE-SEM image of microscopic cross-section of Fe-TiB2 composite powder after (a) polishing and (b) etching. (c) Result of
EDS line scan across the area marked in (a). In (d) the formation of microstructure is given schematically.
Vol. 20, No. 3, 2013
226
H. X. Khoa, N. Q. Tuan, Y. H. Lee, B. H. Lee, N. H. Viet, and J. S. Kim
Table 2. Result of EDS composition analysis on cross-section
surface marked in Fig. 8(a)
Points
Fe
Ti
Atomic (%)
Position
Phase
composition
1
2
96.07
95.62
3.93
4.38
Centre
Centre
Fe2B-Fe
3
4
83.36
83.79
16.64
16.21
Layer
Layer
Fe2B-Fe-TiB2
5
72.11
27.89
Boundary
Fe- TiB2
6
46.86
53.14
Dark phase
Fe- TiB2-Ti
microstructure of Fig. 8(a). It means that the fine FeB
particles completely reacted with Ti to form TiB2 particles in Fe matrix. Fig. 8(b) shows the dark phase surface
after etching, which is composed of Fe and TiB2. It is
noteworthy that the TiB2 particle is extremely fine, around
50 nm, in comparison with previous results reported by
others. The received TiB2 particle size was very coarse
from several to tens micrometer [2, 4-8]. With coarse FeB
particles, TiB2 was formed firstly on the surface, gradually
created network of TiB2 particles surrounding Fe2B core
(Fig. 8(d)). This formation might prevent the further diffusion of boron to the matrix boundary and remained a large
amount of Fe2B in the powder after heat treatment.
Fig. 8(c) is result from EDS line scanning of bright
phase on the Fig. 8(a), showing distribution of Fe, Ti and
B. Fig. 8(d) shows the schematic figure on mechanism of
phase formation after heat treatment. The concentration
of Ti and B are higher at boundary (Point ⑤ in Fig. 8(c))
due to TiB2 layer as mentioned above (Fig. 8(d)). The
bright phase has Fe concentration much higher than that
the dark phase, while Ti distribution is higher in the dark
phase, as presented in the Table 2.
Table 2 shows composition of selected points analysed
on microscopic cross-section surface of the composite.
Most of Ti was concentrated in the dark phase, it is
clearly that the dark phase is almost the Fe-TiB2 composite, in which a small amount of Ti can be included that is
hardly to detect within the resolution of X-ray diffraction. The bright phase was assigned for Fe2B-Fe due to
the highest Fe concentration in this phase.
4. Conclusions
Fe-TiB2 composite powder was in situ synthesized
from FeB and TiH2 precursors by using high-energy
mechanical milling and subsequent heat treatment process.
1) The FeB-TiH2 powder mixture after mechanical
milling is fine with agglomerates. The agglomerate was
composed of fine TiH2 embedded around FeB particles.
The process did not generate new phases, but lowered
temperature of TiH2 dehydrogenation events.
2) DSC analysis confirms that TiH2 dehydrogenation
occurs above 500ºC for starting powder and 370ºC for
milled powder mixtures, while TiB2 formation is at
higher temperature, above 910ºC.
3) A large amount of Fe2B phase exists in final product after heat treatment. The TiB2 particles create network surrounding the surface of coarse Fe-Fe2B particles.
Microstructure of the composite is characterized by the
Fe-TiB2 that was formed from fine FeB particles, embedded around the Fe-Fe2B. Nanoscale TiB2 particles were
uniformly distributed in the Fe matrix with particle size
around 50 nm.
Acknowledgements
This work was supported by the 2008 Research Fund
of the University of Ulsan (2008-0116).
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