In situ synthesis of FeSi-Al2O3 nanocomposite powder by mechanical alloying

Article abstract of DOI:10.1016/j.jallcom.2009.12.020

FeSi-Al₂O₃ nanocomposite powder was successfully synthesized via mechanical alloying of SiO₂ and Al powders in stainless steel cup and balls. Effects of the Al morphology, milling time and annealing temperature were invest

Full text of DOI:10.1016/j.jallcom.2009.12.020

Journal of Alloys and Compounds 492 (2010) 226–230
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
In situ synthesis of FeSi–Al O nanocomposite powder by mechanical alloying
2
3
M. Zakeri , M.R. Rahimipour , S.Kh. Sadrnezhadb
a,∗
b
a
Ceramic Department, Islamic Azad University (Saveh Branch), PO Box 39187/366, Saveh, Iran
Ceramic Department, Materials and Energy Research Center, Karaj, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 October 2009
Received in revised form
FeSi–Al O nanocomposite powder was successfully synthesized via mechanical alloying of SiO2 and Al
powders in stainless steel cup and balls. Effects of the Al morphology, milling time and annealing temper-
ature were investigated. Structural and morphological evolutions were monitored by X-ray diffraction
2
3
2
9 November 2009
(XRD) and scanning electron microscopy (SEM), respectively. Results show that the reduction of TiO2 by
Accepted 2 December 2009
Available online 29 December 2009
spherical Al initiates after 30 h and completes after 45 h of milling. However, there is no reaction with
flaky Al up to 45 h of milling. The mean grain size of 9 nm was obtained for FeSi at the end of milling.
FeSi–Al2O3 nanocomposite powder was stable and maintained its nanocrystalline nature after annealing
Keywords:
Nanostructured materials
Mechanical alloying
◦
at 1000 C. Combination of mechanical activation and heat treatment led to the formation of Fe0.42Si2.67
and mullite phases in the 10 h milled sample.
©
2009 Elsevier B.V. All rights reserved.
1
. Introduction
Alternatively, nano-metric FeSi–Al O3 powders can be produced
2
through high energy reactive milling of the mixtures of SiO , Al
2
Nanocrystalline materials are very attractive since the reduc-
and Fe powders. Mechanical alloying (MA) is basically a dry and
high energy ball milling process which has been used to syn-
thesize alloys, oxide-dispersion-strengthened alloys, amorphous
alloys and various intermetallics compounds [10,11].
tion of the grain size at the nanometric scale can improve their
physical and mechanical properties. Due to their attractive high
temperature properties, silicides of various metals have been the
subject of numerous investigations .In particular, the nanocrys-
talline FeSi alloys have been the subject of various structural and
magnetic studies from many researchers [1–3]. This silicide is not
poisonous, highly resistant against oxidation and can be used in
air without special precaution. Mechanical properties of this mate-
rial can be improved by addition of a hard secondary phase such
The aim of this work is to synthesize FeSi–Al O nanocomposite
2
3
powder by ball milling of low cost starting materials. The effect of
the milling time and annealing temperature were also investigated.
2
.
Experimental details
The MA experiments were performed in a planetary ball mill at nominal room
as Al O . Preparation in nanostructure is another mechanism to
2
3
temperature with a vial rotation speed (cup speed) of 500 rpm. Two kinds of Al,
Flaky (MERK Co., 99.9 wt.%, <100 ␮m) and spherical (Fluka Co., 99.9 wt.%, <200 ␮m)
were used (Fig. 1). Al and SiO2 (Hamedan Pro., 99.9 wt.%, <200 ␮m) powders were
mixed on the basis of following reaction:
improve mechanical properties [4].
Two different methods have been developed in industry. The
first method, based on the classical melting process, does not yield
a homogeneous bulk product directly [5]. A very long period of
annealing is required after the solidification. The second procedure
uses powder metallurgical techniques to obtain the nanostructured
powder of this material. There are several reports on the prepa-
ration of FeSi or FeSi2 by powder metallurgical techniques [5–9],
3SiO2 + 4Al → 3Si + 2Al2O3
(1)
The reduced Si will be reacted with the introduced Fe from the stainless steel
cup and balls on the basis of following reaction:
Fe + Si → FeSi
(2)
but there is no attempt to produce the FeSi–Al O3 nanocomposite
powder by this method.
On the other hand the starting materials and iron impurity were mixed to give
the FeSi–Al2O3 composite as follow [12]:
2
FeSi–Al O nanocomposite powder can be obtained easily by
◦
= −628.9 kJ, ꢀG◦ = −598.7 kJ) (3)
2
3
3SiO2 + 4Al + 3Fe → 3FeSi + 2Al O (ꢀH
2
3
298
298
direct mixing of nano-alumina and iron silicide powders. But
the resulting heterogeneous microstructure and high cost of the
starting materials are two important set backs of this method.
The ball to powder weight ratio (BPR) was 10:1. Seven balls with 20 mm and
five balls with 10 mm diameter were used in the MA experiments. The mixture of
the powders with the stainless steel balls was charged into a stainless steel cup
(
250 ml) under argon atmosphere. For preventing of excess agglomeration some
process controlling agent (PCA) was used (1 wt.% stearic acid). Samples for analysis
were removed by interrupting the mill at various intervals. Heat treatment of the
∗ Corresponding author at: Material Science Department, Islamic Azad University
−1
milled powders was performed in a tube furnace in argon atmosphere (2 l min ).
◦
(
Saveh Branch), Saveh, Iran. Tel.: +98 255 2241552; fax: +98 255 2240111.
The heating rate was 10 C/min and the holding time at the maximum temperature
◦
E-mail address: M zakeri@iau-saveh.ac.ir (M. Zakeri).
(600, 700 and 800 C) was 2 h.
0
925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.12.020

M. Zakeri et al. / Journal of Alloys and Compounds 492 (2010) 226–230
227
Fig. 1. SEM images of as received Al powders: (A) flaky and (B) spherical morphology.
X-ray diffraction patterns were recorded on a Philips diffractometer (30 kV
and 25 mA) with Cu k˛1 radiation (ꢁ = 1.5404 Å). The recorded XRD patterns were
used for the calculation of crystallite size and strain. Prior to calculations from
the XRD peaks, the background was automatically removed and the k˛2 radi-
ation was stripped from the scans using the computer software X-pert High
Score developed by PANalytical B.V. Company (Netherlands). The morphology and
the particle size of the MAed powders were examined by the SEM operating at
reducing of SiO2 during milling. SiO2 reduction may be performed
by some different mechanisms [12]:
◦
SiO2 = Si + O2 ꢀG = 907, 100 − 175T, T = 298–1685 K
(4)
◦
SiO2 + C = Si + CO2 ꢀG = 513, 000 − 175.84T,
2
5 kV.
T = 298–1685 K
(5)
3
. Results and discussion
The oxygen pressure must approximately be zero for SiO reduc-
tion at room temperature during milling on the basis of reaction
2
Fig. 2 shows the XRD patterns of the milled powders with flaky
Al. SiO2 and Al reflections can be seen after 6 h of milling. With
increasing milling time to 45 h, there is no change except the peak
broadening and intensity decreasing. Reaction (3) can be performed
during milling at room temperature because of its negative Gibbs
free energy, but kinetically parameters of this reaction do not per-
mit to takes place. Al morphology may be one of these parameters,
so new MA experiments were performed with spherical Al mor-
phology. As seen in Fig. 3, there is no difference in the obtained
results during milling up to 30 h. All of the starting material reflec-
tions disappeared at longer milling time (45 h). On the other hand,
the FeSi reflections appeared in the pattern of the 45 h milled sam-
ple. FeSi formation indicates the wearing of iron impurity and
Fig. 2. XRD patterns of the milled powders with flaky Al.
Fig. 3. XRD patterns of the milled powders with spherical Al.

2
28
M. Zakeri et al. / Journal of Alloys and Compounds 492 (2010) 226–230
(
4). The oxygen pressure in the cup can be approximately esti-
mated 0.03 atm on the basis of argon purity (99.9%) and its pressure
3 atm). The vial temperature must be exceeding from 6220 K at this
(
oxygen pressure for SiO2 reduction by reaction (4) that it is impos-
sible during milling [12]. Reaction (5) cannot take place because
of its positive Gibbs free energy at room temperature (460 kJ) at
standard condition. And also CO2 pressure must approximately be
zero for SiO2 reduction at room temperature that cannot be sat-
isfied during this condition of milling. Thus, it can be concluded
that the SiO2 reduction was performed by Al (reaction (1)). This
reaction has negative Gibbs free energy at room temperature and
can easily take place during milling. The reaction of reduced Si as a
product of reaction (1) with Fe as a product of cup and ball wear-
ing led to the formation of FeSi (reaction (2)). The large difference
2
between the mass absorption coefficients of FeSi (227.66 cm /g)
2
Fig. 4. XRD patterns of the 10 h milled powders after annealing at different temper-
atures.
and Al O (39.1 cm /g) leads to the absorption of the Al O inten-
2
3
2
3
sity by FeSi [13]. Further milling up to 70 h had no change except
the peak broadening that is due to the microstructure refinements.
Effect of the mechanical activation was studied by the heat treat-
ment of the 10 h milled powder at different temperatures. The as
milled (10 h) powder includes starting materials and only activated
◦
during milling. As seen in Fig. 4, the annealing at 600 C had no con-
siderable effect except the peak sharpening that is due to the grain
◦
growth and strain release [11]. At higher temperature at 800 C,
SiO was partially reduced by Al (reaction (1)) and led to the forma-
2
tion of Al O and Fe0.42Si2.67. The remained SiO2 and synthesized
2
3
◦
Al O form mullite (Al4.59O9.7Si1.41) at 1000 C. Thermal stability
2
3
of the synthesized FeSi–Al O3 composite was also investigated by
2
the annealing of the 45 h milled powder at different temperatures.
There is no change in the synthesized phase except the peak sharp-
ening (Fig. 5). On the other hand, the FeSi–Al O composite powder
2
3
◦
is thermally stable up to 1000 C. Alumina reflections appeared in
this pattern due to the considerable grain growth and strain release
that will be discussed in continue.
Microstructure refinement during milling and annealing were
investigated by the measurement of mean grain size and micros-
Fig. 5. XRD patterns of the 45 h milled powders after annealing at different temper-
atures.
Fig. 6. W–H calculation results of the milled powders: (A) flaky and (B) spherical Al.
Table 1
Mean grain size and microstrain of the milled and annealed powders.
Aluminum morphology
Phase
Flaky
SiO2
Spherical
Spherical
FeSi
SiO2
FeSi
Method
W–H
W–H
Scherrer
Scherrer
Milling time (h)
6
–
15
–
73
25
–
45
45
–
35
5
–
10
–
116
20
–
69
30
–
13
–
45
–
11
–
70
–
9
45
45
800
12
45
1000
34
◦
Annealing temperature ( C)
Mean grain size (nm)
Microstrain (%)
600
11
–
693
0.5
277
0.4
0.59
0.73
1.0
0.44
1.03
–
–
–

M. Zakeri et al. / Journal of Alloys and Compounds 492 (2010) 226–230
229
Fig. 7. SEM images of the milled powders: (A) 5 h, (B) 5 h at higher magnification (HM), (C) 20 h, (D) 20 h at HM, (E) 30 h, (F) 30 h at HM, (G) 70 h and (H) 70 h at HM.
train. The well-known Williamson–Hall (W–H) [14] and Scherrer
13] methods were used. For measurement of SiO mean grain size
annealing at different temperatures. The maximum mean grain size
[
of 34 nm was obtained after annealing of 45 h milled powder at
2
◦
and strain, the W–H method was used due to the existence at least
three reflections in the patterns. The results of these measurements
are shown in Fig. 6. The slopes of these straight lines represent the
microstrain and their intercepts are a symbol of mean grain size.
The accuracy of the measurements can be confirmed by the good
1000 C.
Fig. 7 shows the morphological evolutions of the milled powder
with spherical Al. As seen in Fig. 7A, the 5 h milled powder includes
a distribution of large agglomerates and small particles. Existence
of ductile Al in this sample led to the more agglomeration during
milling. These agglomerates are clearly shown in Fig. 7B at higher
magnification. Agglomeration of the milled powders with ductile
Al continues up to 20 h of milling (Fig. 7C and D). With increas-
ing milling time to 30 h, work hardening of the powder led to the
fracturing and size decreasing of the agglomerates. Formation of
2
regression coefficients (R ) of these lines. The calculated mean grain
size and microstrain of the milled and annealed powders are shown
in Table 1. Longer milling time led to the smaller grain size as well as
larger strain for SiO in both Al morphologies. With a same behavior
2
for FeSi, a nanocomposite powder with the mean grain size of 9 nm
was obtained at the end of milling. In spite of considerable grain
growth, this composite maintains its nanocrystalline nature after
FeSi and Al O3 brittle phases also lead to the fracturing of these
2
agglomerates that are presented in Fig. 7E and F. With continuing

2
30
M. Zakeri et al. / Journal of Alloys and Compounds 492 (2010) 226–230
this behavior, a narrow size distribution of small agglomerate was
obtained at the end of milling (Fig. 7G and H).
[2] B. Zuo, T. Sritharan, Ordering and grain growth in nanocrystalline Fe75Si25 alloy,
Acta Mater. 53 (2005) 1233–1239.
[
[
[
3] C.T. Sims, Non metallic materials for gas turbine engines: are they real, Adv.
Mater. Process. 6 (1991) 32.
4] L. Takacs, Combustion phenomena induced by ball milling, Mater. Sci. Forum
4
. Conclusion
269-272 (1998) 513–522.
5] C. Gras, E. Gaffet, F. Bernard, J.C. Niepce, Enhancement of self-sustaining reac-
tion by mechanical activation: case of Fe–Si system, Mater. Sci. Eng. A 264
(1999) 94–107.
FeSi–Al O nanocomposite powder was successfully synthe-
2
3
sized via mechanical alloying of SiO2 and Al powders in stainless
steel cup and balls. FeSi formation initiated after 30 h and com-
pleted after 45 h of milling. However, there is no reaction with
flaky Al up to 45 h of milling. Longer milling time led to the smaller
grain size as well as larger strain. The mean grain size of 9 nm was
[
6] Ajay Gupta, Pratima Dhuri, Metastable phase formation in Fe–Si system using
non-equilibrium techniques, Mater. Sci. Eng. A 304–306 (2001) 394–398.
[7] R.R. Rodrıguez, G.A. PerezAlcazar, H. Sanchez, J.M. Greneche, Milling time
effects on the magnetic and structural properties of theFe70Si30 system, Micro-
electron. J. 39 (2008) 1311–1313.
[
8] Z. Benseba, B. Bouzabat, A. Otmani, Study of nanocrystalline FeSi alloys pre-
pared by mechanical alloying, J. Alloys Comp. 469 (2009) 24–27.
obtained for FeSi at the end of milling. FeSi–Al O3 nanocomposite
2
powder was stable and maintained its nanocrystalline nature after
annealing at 1000 C. However, combination of mechanical acti-
vation and heat treatment led to the formation of Fe0.42Si2.67 and
mullite phases in the 10 h milled sample.
[9] T.J. Zhou, Z. Yu, Y.W. Du, The microstructure and magnetic properties of
nanocrystalline Fe100−xSix alloys, Appl. Phys. A 70 (2000) 75–78.
10] C.C. Koch, Mater. Sci. Technol. 15 (2001) 193.
[11] J.S. Benjamin, T.E. Volin, Mechanism of mechanical alloying, Metall. Trans. B 5
(8) (1974) 1924.
12] O. Kubaschewski, C.B. Alcock, Materials Thermochemistry, 6th ed., Oxford Perg-
amon Press, 1993.
13] B.D. Cullity, Elements of X-ray Diffraction, 2nd ed., Addison–Wesley Publishing,
◦
[
[
References
[
1
977.
[
1] M. Li, R. Birringer, W.L. Johnson, R.D. Shull, Nanocrystalline Fe–Si phase by
mechanical attrition and its soft magnetic properties, Nanostruct. Mater. 3
[14] G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminum and
wolfram, Acta Metal. 1 (1953) 22–31.
(
1993) 407–412.