NiAl/Ni3Al - Al2O3composite formation by reactive ball milling

Article abstract of DOI:10.1007/s10973-005-0955-4

It has been presented that the bulk mechanical alloying occurring in the Ni-hydroxycarbonate-aluminium mixture can produce homogeneously refined composite of nickel aluminides with aluminium oxide. The control of mechanochemical phenomenon was carried out by phase identification of milling products as well as their structural and morphological characterization. As a result the thermoanalytical and X-ray powder diffraction measurements as well as scanning electron microscopy with backscattered electron images (BSE) and X-ray energy dispersive spectroscopy (EDS) were applied. It was found that the kinetics of mechanochemical reactions depends on the chemical nature of initial salt..

Full text of DOI:10.1007/s10973-005-0955-4

Journal of Thermal Analysis and Calorimetry, Vol. 82 (2005) 719–724
NiAl/Ni3Al – Al2O3 COMPOSITE FORMATION BY REACTIVE BALL
MILLING
K. Wieczorek-Ciurowa* and K. Gamrat
Institute of Inorganic Chemistry and Technology, Cracow University of Technology,
ul. Warszawska 24, 31-155 Cracow, Poland
It has been presented that the bulk mechanical alloying occurring in the Ni-hydroxycarbonate–aluminium mixture can produce ho-
mogeneously refined composite of nickel aluminides with aluminium oxide. The control of mechanochemical phenomenon was
carried out by phase identification of milling products as well as their structural and morphological characterization. As a result the
thermoanalytical and X-ray powder diffraction measurements as well as scanning electron microscopy with backscattered electron
images (BSE) and X-ray energy dispersive spectroscopy (EDS) were applied. It was found that the kinetics of mechanochemical re-
actions depends on the chemical nature of initial salt.
Keywords: aluminides, aluminium, high-energy ball milling, in situ reactions, mechanical alloying, nanocomposite, Ni-Al
intermetallics, Ni-hydroxycarbonate
Introduction
ment operating in carburizing furnace [6]. However,
NiAl has emerged as a possible high-temperature struc-
tural material due to its low density, high melting tem-
perature, good thermal conductivity and excellent oxi-
dation resistance. In factories they are used as transfer
rolls, heat-treating trays, die blocks, nuts and bolts and
corrosion resistance tool bits. These special features of
nickel aluminides result from their structures [10–12].
The literature data shows several examples of
mechanically synthesized nickel aluminides as well
Research over the last two decades has led to the
emergence of intermetallics, a new class of engineer-
ing materials with numerous attractive properties, that
include high oxidation and corrosion resistance as
well as relatively low densities combined with their
ability to retain strength and stiffness at high tempera-
tures. Intermetallics are one of the most important
group of nanostructured compounds [1–3]. Among
the processing methods to synthesize 'in situ'
nanophase materials, room temperature high-energy
ball milling appeared to be attractive for potential
large-scale economical production. Thus, the me-
chanical treatment allows formation of alloys, which
is difficult to obtain by conventional solidification
methods due to phase equilibrium constraints, as in
the case of Ti–Al alloys formation [4]. Furthermore,
this technique is universal for obtaining alloys with
metastable structures and compositions [5].
Nickel aluminides are examples of intermetallics
which typically work at elevated temperatures [6–9].
Intermetallics of the nickel–aluminium system exist-
ing in the composition region 40–75 at.% (see: phase
diagram in Fig. 1) are of interest due to their proper-
ties [10], in particular Ni3Al and NiAl phases. Thus,
Ni3Al phase is a structural alloy used as a strengthen-
ing phase in most superalloys. The Ni3Al – based al-
loys were the first intermetallic alloys fully commer-
cialized. Such alloys have a remarkable resistance to
carburizing making them an ideal choice for equip-
Fig. 1 Phase diagram of Ni–Al binary system [10]
*
Author for correspondence: kwc@usk.pk.edu.pl
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2005 Akadémiai Kiadó, Budapest

WIECZOREK-CIUROWA, GAMRAT
as composites of Ni–Al intermetallics with different
oxide powders using a Fritsch mill. This metal matrix
composite (MMCs) revealed a lamellar structure.
The purpose of this study was to determine the
possibility of nickel aluminides formation during me-
chanical treatment of Ni-hydroxycarbonate – Al sys-
tem. The choice of initial components was based on
our previous results of CuAl2–Cu9Al4/Al2O3
nanocomposite formation during several minutes in
Cu-hydroxycarbonate–Al system [25–27].
oxides [13–16]. Portnoy et al. [17] described mechan-
ical alloying of nickel aluminides from pure metals
performed in a vibratory ball mill in argon. It was
shown that the product of mechanical alloying de-
pends on the initial composition of milled mixture.
Thus, milling of the mixture containing 40–61 at.% Ni
brings about the formation of NiAl phase through a di-
rect exothermic reaction without formation of any inter-
mediate solid solution. However, alloying provided in
the system with 65–85 at.% of Ni involved solid solu-
tion Ni(Al) formation characterized by nanocrystalline
structure. Furthermore, it was shown that the product is
always a single phase with the minimum Gibbs energy.
In comparison to the mechanical alloying of
Ni–Al phase, self-propagating high-temperature syn-
thesis (SHS) realized classically by thermal ignition
was tested by Biswas et al. [18]. In general, occur-
rence of the SHS reaction depends on the ignition
temperature, which is typical for the reactant mixture.
Iltin et al. [19] showed that the combustion tempera-
ture of a mixture of nickel and aluminium powder is
1600–1700 K and a substantial decrease of such igni-
tion temperature can be achieved by applying the ini-
tial mechanical activation of alloyed powders. Then
the SHS reaction in the system Ni–Al starts at the
temperature 800–900 K [20, 21].
Experimental
Materials and apparatus
Ni-hydroxycarbonate in a hydrated form, Ni2(OH)2CO3
·3H2O, and aluminium powder (99.9% purity) as com-
mercial reagents were used. The two-component
(salt-metal) system, Ni2(OH)2CO3·3H2O–Al°, was pre-
pared as a physical mixture at a molar ratio of
NiO:Al=1:1.
A laboratory planetary mill with balls and mill-
ing container made of hardened steel was used. The
mass proportion of balls to sample was of 14:1. The
two milling vials were rotated at 1130 rpm. Alloying
was carried out in air for several minutes at room tem-
perature and atmospheric pressure in order to allow
comparison of results of the Cu2(OH)2CO3 –Al sys-
tem under the same conditions [25–27].
Formation of nickel aluminides – aluminium ox-
ide composite was observed recently [22] during re-
active ball milling in the system of nickel oxide and
aluminium. Activation process was performed in a
shaker/mill apparatus with a ball/material mass ratio
of 4:1. The formation of Ni3Al phase occurs accord-
ing to the following reaction (1).
Equipment and methods of phase identification
X-ray powder diffraction patterns were obtained us-
ing a Philips X’Pert Diffractometer (CuKa) in the 2q
range of 10–60°.
Thermoanalytical measurement was made in ar-
gon up to 1000°C using a SDT 2960 TA Instrument
with a heating rate of 24 K min–1.
3Ni2O3 + 8Al ® 2Ni3Al + 3Al2O3
(1)
‘In situ’ formation of composite with both
phases is a consequence of an exothermic reaction
process that occurs after induction time.
A Hitachi S-4700 instrument (SEM) equipped
with energy dispersive X-ray spectrometer was used
for microstructural examination and elemental
microanalysis. Atomic number contrast was observed
from the polished cross-sectioned samples. Speci-
mens were carbon-coated to obtain electrical conduc-
tivity. The BSE imaging and EDX elemental analyses
were performed at an electron beam voltage of 20 kV.
Another example of NiAl phase strengthened by
Y2O3 and applied as a composite ODS (oxide disper-
sion strengthened) was performed by Grahle et al.
[23]. In contrast to the above cases two-step prepara-
tion route was required. Initial powders of Ni and Al
were blended in two proportions: Ni–13.3 mass% Al
forming Ni3Al and Ni–50 mass% Al forming Ni2Al3
and NiAl3 phases. This was followed by reinforce-
ment –Y2O3 addition. The syntheses were performed
using a centrifugal vibratory ball mill under vacuum.
ODS–NiAl in powder form of a submicrometer and
nanocrystalline sized was obtained.
Results and discussion
Formation of nickel aluminides with Al2O3
Formation of nickel aluminides in the system of
Ni-hydroxycarbonate – aluminium is possible if two
metallic phases are present in the milled system. Alu-
minium, the initial reagent component of the treated
mixture should be present in sufficient amounts [28].
Olszówka-Myalska [24] obtained a composite
consisting of aluminium matrix/nickel aluminide by me-
chanical treatment of aluminium, nickel and aluminium
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NiAl/Ni3Al – Al2O3 COMPOSITE FORMATION BY REACTIVE BALL MILLING
Table 1 The phases which can be formed during mechanical alloying of Ni2(OH)2CO3·3H2O-Al system
Compound
Elemental concentration/ % (m/m) – calculated
ANF value
BSE grey tone levels
v. bright
Ni
100
86.7
78.4
78.6
68.6
59.3
42.1
55.5
44.2
0
Al
0
O
0
C
0
H
0
Ni
28.0
26.6
24.8
23.7
23.3
21.9
19.3
18.9
16.5
13.0
10.6
Ni3Al
13.3
21.6
0
0
0
0
Ni3Al5
0
0
0
NiO
21.4
0
0
0
NiAl
31.4
40.7
57.9
0
0
0
Ni2Al3
0
0
0
¯-
NiAl3
0
0
0
Ni2(OH)2CO3
Ni2(OH)2CO3×3H2O
Al
37.8
48.2
0
5.4
4.5
0
0.9
3.0
0
0
100
52.9
Al2O3
0
47.1
0
0
v.dark
Nickel forms as a result of two reactions: mechanical
decomposition of hydroxysalts–Eq. (1), and
aluminothermic reduction of nickel oxide –Eq.(2).
Ni2(OH)2CO3®2NiO+CO2+H2O
3NiO+2Al® 3Ni+Al2O3
(1)
(2)
(3)
xNi+Al®NixAl (where x=1 or 3).
These two metals may then combine to
intermetallic phases of NiAl and Ni3Al –Eq.(3) form-
ing nickel aluminides with alumina composite in the
treated system.
Reaction (2) is a typical self-propagating high
temperature synthesis (SHS) with ignition occurring
after the critical time of mechanical activation. Dur-
ing this time treated materials reach 'an active state'
through creation of structural defects and formation
of ‘fresh surfaces’. Then, the temperature in the mill-
ing vials increases abruptly indicating the occurrence
of the SHS reaction.
Fig. 2 X-ray diffraction patterns of Ni2(OH)2CO3×3H2O–Al
mixture after mechanical alloying for several minutes
Thermal analysis
Thermoanalytical measurement appeared to be versa-
tile for phase characteristics of milling products. The
course of TG–DTA curves in Fig. 3 confirmed the
presence of Ni-hydroxycarbonate in milling products
because of the thermal decomposition of mechanically
undecomposed part of Ni2(OH)2CO3 up to 350°C.
The decomposition degree of salt in mixture of
Ni2(OH)2CO3–Al after 15 min of milling was equal
80%. It is important to note that at the time of milling
XRD analysis
The X-ray powder diffraction patterns shown in Fig. 2
indicate formation of the qualitative phase composi-
tion of the mechanically activated mixtures of
Ni-hydroxycarbonate with aluminium. The presence
of intermetallic phases was confirmed. It may suggest
that few minutes of milling are sufficient for the initi-
ation of alloy formation in the studied system. NiAl
and Ni3Al phases are detectable in a broad peak in the
range of 2q (42–45°). The low intensity of this peak
indicates the weak crystallinity of phases undergoing
mechanical action. The lack of the peaks for
Ni-hydroxycarbonate indicates full decomposition of
the salt during milling or its amorphous form. It can be
assumed that Al2O3 appeared in low crystalline form.
Cu-hydroxycarbonate
decomposes
completely
[25–27]. However, the milling of Ni2(OH)2CO3 with-
out addition of Al results in only 8% of decomposi-
tion [28], in contrast to Cu2(OH)2CO3, which decom-
poses in about 65%. The possible difficulty in activa-
tion of Ni-salt is the presence of crystalline water
since water can stabilize the structure of compound.
Active reagent such as aluminium facilitates the de-
hydration as well as its decomposition to oxide.
J. Therm. Anal. Cal., 82, 2005
721

WIECZOREK-CIUROWA, GAMRAT
Table 2 Results of the EDX microanalysis for area (Fig. 4) of mechanically alloyed Ni2(OH)2CO3×3H2O–Al mixture
Elemental concentration/ % (m/m) - measured
Fig. 4, point
ANF value
Ni
Al
O
(Fe)
a
73.90
62.78
21.27
33.08
4.03
2.91
(0.79)
(1.23)
24.0
22.4
b
Table 3 Results of the EDX microanalysis for area (Fig.6) of mechanically alloyed Ni2(OH)2CO3×3H2O-Al mixture
Elemental concentration/ % (m/m) - measured
ANF value
Fig. 6, point
Ni
Al
O
(Fe)
a
31.46
50.93
38.80
29.60
24.86
16.47
(4.87)
(2.99)
17.1
20.2
b
Furthermore, one can observe the strong exo-
thermic effect at the temperature of about 620°C cor-
responding to the high-heat emission that results from
aluminothermic reaction between NiO and Al initi-
ated thermally during the experiment.
highest ANF values as seen in nickel and Ni3Al
phases while the darker contain phases with lower
ANF as indicated by Al and Al2O3. Therefore, the si-
multaneous information obtained from the BSE sig-
nals and EDS elemental microanalysis can be used for
estimation and localization of the phases observed on
the scanning electron microscopy.
The typical microstructure of the mechanically
alloyed of Ni2(OH)2CO3·3H2O–Al system for 15 min
is shown in Fig. 4. The variation of grey tone levels
indicates the strong differentiation in phases present
in the activated mixture. The correlation between
BSE image and the results of EDS analysis of point
a – indicates possible presence of nickel in this area
while in the point b – NiAl intermetallics. The results
of EDS analysis are shown in Table 2.
In contrast to the multiphase character of area in
Fig. 4, the microphotograph in Fig. 5 shows only one
phase, Ni-hydroxycarbonate. The large grain of such
phases reveals the presence of large fractures, proba-
bly formed during the collisions with milling balls.
Fig. 3 TG – DTA curves of Ni2(OH)2CO3×3H2O-Al mixture
(in inert atmosphere) after mechanical alloying for sev-
eral minutes
SEM analysis
Scanning electron microscopy (SEM) with the back-
scattered electron (BSE) imaging and quantitative en-
ergy dispersive X-ray microanalysis (EDS) of Ni, Al,
O and C were used for analysis of the composition and
microstructure of composite formed during mechanical
alloying of the Ni2(OH)2CO3·3H2O–Al system.
The BSE images being sensitive to changes in
the mean atomic number (ANF) of the sample indi-
cate variations in grey levels associaed with different
phases. The probable phases present in the composite
with their ANF values are shown in Table 1. The
brightest areas include phases characterised by the
Fig. 4 SEM microphotographs of mechanically alloyed
Ni2(OH)2CO3×3H2O-Al mixture shows a – Ni and
b – NiAl
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J. Therm. Anal. Cal., 82, 2005

NiAl/Ni3Al – Al2O3 COMPOSITE FORMATION BY REACTIVE BALL MILLING
aluminium oxide. Such phases are the result of mechani-
cal decomposition of Ni-salt, aluminothermic reduction
of Ni-oxide and mechanical alloying of Al with Ni.
Therefore, the mechanism of the intermetallics forma-
tion is the same as described earlier in the case of the
copper hydroxycarbonate–aluminium system [25–27].
However, the kinetics of mechanochemical reactions
described above under the same milling conditions dif-
fers in comparison to the Cu2(OH)2CO3–Al system. Af-
ter few minutes of mechanical treatment the Ni–Al
intermetallics formation is only initiated because the
mechanical decomposition of salt is not complete and
both Ni and Al are not fully synthesized.
Fig. 5 SEM microphotographs of mechanically alloyed
Ni2(OH)2CO3×3H2O–Al mixture shows the grain of
Ni2(OH)2CO3×3H2O
• Summing up, the kinetics limitations are influ-
enced by :
• difficulties with Ni2(OH)2CO3 decomposition due
to the hydrated form of this salt;
• aluminothermic reduction of NiO is more difficult
to achieve than that of CuO due to its higher
enthalpy formation [29] as well as the dilution ef-
fect of the reactants, NiO and Al, because of the
low decomposition degree of salt.
Acknowledgements
The authors would like to thank to Prof. Yu. G. Shirokov from
the Ivanovo State University of Chemistry and Technology
(Russia) for valuable discussions.
SEM and EDS microanalyses were carried out at the
Laboratory of Field Emission Scanning Electron Microscopy
and Microanalysis of the Institute of Geological Sciences at
the Jagiellonian University (Poland).
Fig. 6 SEM microphotographs of mechanically alloyed
Ni2(OH)2CO3×3H2O–Al mixture shows the beginning
of lamellar structure of NiAl/Ni3Al – Al2O3 composite
formation. The darker network corresponds to higher
amount of Al2O3, the brighter one – to higher content
of intermetallics
This work was supported by the Polish State Committee
for Scientific Research (KBN), grant No. 4T09B 070 24.
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The microstructure of the composite shown in
Fig.6 is typical for only few regions. As we can see
materials reveal higher value of homogeneity than
those in Fig. 5. From the EDS analysis and BSE im-
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The darker grey tone level of point a) is the result of
higher content of Al2O3. Typical for this microphoto-
graph is a very bright grey tone level of many places,
which are surrounded by darker network. This indi-
cates the lamellar morphology of composite of
intermetallic phase with aluminium oxide.
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