Preparation of Nb-25Si, Nb-37.5Si, Nb-66.6Si powders by high-energy ball milling and subsequent heat treatment

Article abstract of DOI:10.1016/j.jallcom.2006.08.135

The present paper reports on the syntheses of the Nb₃Si, α-Nb₅Si₃ and NbSi₂ compounds by mechanical alloying and subsequent heat treatment. The milling process was carried out in a planetary Fritsch P-5 ball mill under argon atmosphere using stainless steel balls and vials, rotary speed of 200 rpm and a ball-to-powder weight ratio of 10:1. Following, the milled powders were heat-treated at 1600 °C for 1 h in order to obtain the equilibrium microstructures. The milled and heat-treated powders were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), microanalysis via energy dispersive spectrometry (EDS). Results indicated that the silicon atoms were dissolved into the Nb lattice during milling to form supersaturated solid solutions. The Nb₃Si compound was formed after heat treatment at 1600 °C for 1 h only, while the Nb₅Si₃ and NbSi₂ compounds were successfully formed during ball milling. Results have indicated that a significant iron contamination close to 5 at% was found in mechanically alloyed Nb-25Si, Nb-37.5Si and Nb-66Si powders. Consequently, the nominal compositions of Nb-Si powders were altered and other phases were also formed after heat treatment at 1600 °C for 1 h.

Full text of DOI:10.1016/j.jallcom.2006.08.135

Journal of Alloys and Compounds 434–435 (2007) 509–513
Preparation of Nb–25Si, Nb–37.5Si, Nb–66.6Si powders by
high-energy ball milling and subsequent heat treatment
a
b
Bruno Bacci Fernandes , Erika Coaglia Trindade Ramos ,
a
a,∗
Gilbert Silva , Alfeu Saraiva Ramos
Universidade do Vale do Para ´ı ba, Av. Shishima Hifumi, 2911, 12244-000,
Urbanova, S a˜ o Jos e´ dos Campos, Brazil
a
b
Faculdade de Engenharia Qu ´ı mica de Lorena, Polo Urbo Industrial Gleba AI-6, s/n,
12600-000, Mondesir, Lorena, Brazil
Available online 6 October 2006
Abstract
The present paper reports on the syntheses of the Nb
3
5 3 2
Si, ␣-Nb Si and NbSi compounds by mechanical alloying and subsequent heat treatment.
The milling process was carried out in a planetary Fritsch P-5 ball mill under argon atmosphere using stainless steel balls and vials, rotary speed
◦
of 200 rpm and a ball-to-powder weight ratio of 10:1. Following, the milled powders were heat-treated at 1600 C for 1 h in order to obtain the
equilibrium microstructures. The milled and heat-treated powders were characterized by means of X-ray diffraction (XRD), scanning electron
microscopy (SEM), microanalysis via energy dispersive spectrometry (EDS). Results indicated that the silicon atoms were dissolved into the Nb
◦
lattice during milling to form supersaturated solid solutions. The Nb
Nb Si and NbSi
3
Si compound was formed after heat treatment at 1600 C for 1 h only, while the
5
3
2
compounds were successfully formed during ball milling. Results have indicated that a significant iron contamination close to
5
at% was found in mechanically alloyed Nb–25Si, Nb–37.5Si and Nb–66Si powders. Consequently, the nominal compositions of Nb–Si powders
◦
were altered and other phases were also formed after heat treatment at 1600 C for 1 h.
©
2006 Elsevier B.V. All rights reserved.
Keywords: Intermetallics; Mechanical alloying; X-ray diffraction
1
. Introduction
structures, which can be successfully obtained depending on
the starting powders, type of mill and milling parameters.
Various intermetallic compounds have been prepared by
different high-energy ball milling processes [7–10]. Previous
works reported on the phase transformations in Nb–16 at%
Si powders (eutectic composition) during ball milling using
a shaker mill, and the in situ explosive NbSi2 formation
in Nb–66 at% Si powders occurred during high-energy ball
milling [11,12]. The present work reports on the synthesis
Nb-based alloys have been considered for high-temperature
structural and microelectronic applications due to their physical
and chemical properties, such as high melting point and oxida-
tion resistance [1–5].
The following stable solid phases are reported in the accessed
Nb–Si phase diagram: NbSS (ss-solid solution), Nb3Si, ␣-
Nb Si3 (low temperature), ␤-Nb Si3 (high temperature), NbSi2
5
5
andSiSS. TheNb3Siphaseisformedfromthemeltbytheperitec-
by mechanical alloying of Nb3Si, ␣-Nb Si3 and NbSi2
5
tic reaction L + Nb Si3 ⇔ Nb3Si, while the Nb Si3 and NbSi2
compounds.
5
5
phases are formed by congruent transformations [6]. Eutectic
microstructure formed by the NbSS and Nb3Si phases can be
produced in as-cast alloys containing higher Nb amount.
Non-equilibrium powder processing techniques are
employed to produce nanocrystalline materials and metastable
2
. Experimental procedure
The Nb–25Si, Nb–37.5Si and Nb–66Si elemental powder mixtures (at%)
were used to produce by mechanical alloying the Nb Si, ␣-Nb Si , NbSi com-
3
5
3
2
pounds, respectively. The following high-purity starting powders were used in
thiswork:Nb(min99.0 wt%, angular, <200 mesh)andSi(99.999 wt%, irregular,
<
200 mesh).
∗
Corresponding author. Tel.: +55 12 3947 1103; fax: +55 12 3947 1149.
The milling process was carried out in a planetary Fritsch P-5 under argon
E-mail address: alfeu@univap.br (A.S. Ramos).
atmosphere using rotary speed of 200 rpm, stainless steel balls (19 mm diameter)
0
925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.08.135

5
10
B.B. Fernandes et al. / Journal of Alloys and Compounds 434–435 (2007) 509–513
and vials (225 ml), and a ball-to-powder weight ratio of 10:1. After different
milling times (1, 10, 30, 60 and 100 h) the milling process was interrupted and
small amounts of the milled materials were taken out for analysis. To minimize
the atmospheric contamination and a spontaneous ignition Ar-filled glove box
was used to manipulate the reactive powders. Following, the milled powders
The iron amount picked up in mechanically alloyed Nb–Si powders was also
determined.
3. Results and discussion
◦
were heat-treated at 1600 C for 1 h under argon atmosphere in order to obtain
equilibrium microstructures.
XRD patterns of Nb–25Si, Nb–37.5Si and Nb–66Si pow-
The milled and heat-treated powders were characterized by means of X-
ray diffraction (XRD), scanning electron microscopy (SEM) and microanalysis
via energy dispersive spectrometry (EDS). XRD experiments were performed
at room temperature in a Seifert equipment using Ni-filtered Cu K␣ radiation.
The computer program Powdercell [13] was used to index the phases present in
milled and heat-treated Nb–Si powders, and the phases present were indexed
according to the JCPDS database files [14]. SEM images were obtained in
the backscattered electron mode, and the Nb and Si contents of the phases
formed in heat-treated Nb–Si alloys were measured by microanalysis via EDS.
ders at different milling times are shown in Fig. 1. Initially, it
was noted a similar ball milling behavior between the Nb–25Si,
Nb–37.5Si and Nb–66Si powders, i.e., the silicon atoms were
dissolved into the Nb lattice to form supersaturated solid solu-
tions after milling for 10 h. At this moment, it occurred the
broadening and the reduction on the intensity of Nb peaks, and
the Si peaks disappeared in Nb–25Si and Nb–37.5Si powders
◦
Fig. 1. XRD patterns of (a) Nb–25Si, (b) Nb–37.5Si and (c) Nb–66Si powders at different milling times and after heat treatment at 1600 C for 1 h.

B.B. Fernandes et al. / Journal of Alloys and Compounds 434–435 (2007) 509–513
511
milled for 10 h. In addition, it was also noted that the Nb peaks
were slightly moved toward the direction of higher diffraction
angles, suggesting that metastable structures were formed. In
Nb–66Si powders milled for 10 h, it was noticed the presence
of Si peaks, indicating that its non-equilibrium solid solubility
was reached. The reduction on the intensity of Nb peaks was
more pronounced in powder mixtures containing higher silicon
amount. Following the ball milling, XRD results suggested that
Fig. 2. SEM images of (a) Nb–25Si, (b) Nb–37.5Si and (c) Nb–66Si powders at different milling times.

5
12
B.B. Fernandes et al. / Journal of Alloys and Compounds 434–435 (2007) 509–513
an amorphous phase was formed in Nb–25Si powders milled for
00 h. Previous works have reported that amorphous structures
can be formed in Nb–Si powders during the mechanical alloying
15]. In Nb–37.5Si and Nb–66Si powders, the Nb Si3 and NbSi2
100 h, and their particle sizes were then increased. Finer powder
particles with rounded morphology and a homogeneous distri-
bution were observed in mechanically alloyed Nb–37.5Si (60 h)
1
[
and Nb–66Si (30 h) powders, indicating that the ␣-Nb Si3 and
5
5
phases were formed after milling for 60 and 30 h, respectively.
However, the lower intensity values of characteristic peaks have
suggested that the atomic disordering of these intermetallic com-
pounds was reached in Nb–37.5Si and Nb–66Si powders milled
for 100 and 60 h, respectively.
NbSi2 phases were formed during ball milling. Finer powder
particles and clusters were found in Nb–37.5Si and Nb–66Si
powders milled for 100 and 60 h, respectively.
XRD patterns of mechanically alloyed Nb–25Si, Nb–37.5Si
◦
and Nb–66Si powders after heat treatment at 1600 C for 1 h
Fig. 2 shows the morphology of Nb–25Si, Nb–37.5Si and
Nb–66Si powders at different milling times. Initially, it was
noted an inhomogeneous distribution and irregular morphology
of starting powder particles. Large and rounded particles were
found in Nb–25Si powders milled for 10 h due to an intense cold-
welding mechanism occurred during ball milling with powder
mixtures containing a large amount of ductile component. In
Nb–25Si powders milled for 30 h, the particle sizes were reduced
probably owing the severe plastic deformation and/or formation
of an intermetallic phase. However, the cold-welding mecha-
nisms were more pronounced in Nb–25Si powders milled for
are shown in Fig. 1. Results indicated the major presence of the
Nb3Si, ␣-Nb Si3 and NbSi2 phases in Nb–25Si, Nb–37.5Si and
5
Nb–66Si, respectively. It was still noted that the peaks of the
Nb3Si phase were moved in the direction of lower diffraction
angles in heat-treated Nb–25Si powders. Peaks of the ␤-Nb Si3
5
(high temperature) phase were also indexed in XRD pattern of
the heat-treated Nb–37.5Si alloy.
SEM images of the heat-treated Nb–25Si, Nb–37.5Si,
Nb–66Si alloys are shown in Fig. 3. The microstructure of
the heat-treated Nb–25Si alloy indicated the presence of the
Nb3Si phase as matrix, which dissolved close to 9.0 at% Fe (see
◦
Fig. 3. Micrographs (SEM) of the (a) Nb–25Si, (b) Nb–37.5Si and (c) Nb–66Si alloys after heat treatment at 1600 C for 1 h.

B.B. Fernandes et al. / Journal of Alloys and Compounds 434–435 (2007) 509–513
513
Fig. 3a). Thus, the displacement of Nb3Si peaks in heat-treated
Nb–25Si powders previously cited occurred preferentially due
to an excessive iron contamination. In addition, it was also
observed the presence of another phase with Nb, Fe and Si con-
tents (at%) in the range 58.6–60.6, 20.1–26.2 and 13.3–15.6,
respectively. The microstructure of the heat-treated Nb–37.5Si
results suggested that an amorphous phase was formed in
Nb–25Si powders milled for 100 h, and no intermetallic peak
was identified in powder mixtures containing smaller Si amount.
The Nb3Si phase was successfully formed in Nb–25Si pow-
◦
ders after heat treatment at 1600 C for 1 h only, which presented
iron contents close to 9.0 at%. Other phases containing high
iron and chromium contents were also formed in heat-treated
Nb–25Si, Nb–37.5Si and Nb–66Si alloys due to an excessive
contamination from the milling medium.
alloy indicated the presence of the ␣-Nb Si3 phase as matrix,
5
which it presented low iron solubility (see Fig. 3b). Moreover,
it was also identified the presence of another phase containing
Nb, Fe, Cr and Si contents close to 35.5, 35.5, 8.0 and 21.0 at%.
In the heat-treated Nb–66Si alloy, the silicon contents of the
NbSi2 phase (matrix) were varied between 64.9 and 65.9 at%,
and another unknown phase with Nb, Fe, Cr and Si contents
close to 40.4, 16.5, 1.9 and 41.2 at% was also determined by
EDS analyses. Results have indicated that a significant iron con-
tamination lower than 5 at% was found in heat-treated Nb–25Si,
Nb–37.5SiandNb–66Sialloys. Consequently, thenominalcom-
positions of starting powders were altered and other phases were
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␣
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[
(
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