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Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.11, pp.1027-1035, 2010
jmmce.org Printed in the USA. All rights reserved
Mechanically Alloyed Carbon Nanotubes (CNT) Reinforced Nanocrystalline
AA 4032: Synthesis and Characterization
M.S. Senthil Saravanan, S.P. Kumaresh Babu*, K. Sivaprasad
Advanced Materials processing laboratory,
Department of Metallurgical and Materials Engineering,
National Institute of Technology, Tiruchirappalli- 620 015, India
*Corresponding author: firstname.lastname@example.org
Powder metallurgy has emerged as a promising technique to develop carbon nanotubes
reinforced metal matrix composites. In this work, high energy ball mill was used to disperse
multi-walled carbon nanotubes in AA 4032 alloy. The nanocrystalline nature of AA 4032 was
obtained by milling the elemental powders for 30 hours. Different mass fractions of carbon
nanotubes were added in the matrix at end of 29th hour. The phase evolution and changes in
crystallite size and lattice strain were analyzed using X- ray diffractometer (XRD). SEM analysis
reveals the morphological changes during the milling process. Transmission electron
microscopy (TEM) studies reveal the presence of CNTs in the matrix and evidenced the uniform
distribution of CNTs inside the matrix.
Keywords: Multi walled carbon nanotubes, Nanocrystalline, AA 4032, XRD, SEM and TEM
In recent years considerable scientific research has been carried out to develop ultra high strength
light weight, stiffer and wear-resistant engineering materials for extraordinary performance.
Metal matrix composites (MMC) are regarded as excellent materials to obtain superior properties
to those of the constituent phases and also to satisfy the above requirements .
Aluminum alloys have a great diversity of industrial application because of their low density and
good workability but the use of these alloys is limited due to their relatively low yield stress. The
interest in developing new aluminium based alloys has grown largely in the past years as part of
the ongoing research for new advanced materials to meet the challenges and demands for high
performance alloys. These alloys are used in automotive, aerospace and many other structural
1028 M.S. Senthil Saravanan, S.P. Kumaresh Babu, K. Sivaprasad Vol.9, No.11
applications because of their high specific strength and good corrosion resistance [2-6]. The
interest in finding super reinforcements for metallic matrices has been growing considerably over
the past few years, largely focusing on investigating their contribution to the enhancement of the
mechanical properties of metal matrix composites. Carbon nanotubes (CNTs) have received
widespread attention in the past decade owing to their unique physical, chemical, thermal and
mechanical properties [7-9]. CNTs are the most promising of all nanomaterials and have been
studied for their potential application in storage systems, composite materials, field emission
displays, nanoscale sensors, nanoscale programmable logic circuits and other electronic
applications . The CNTs possess relatively low density varying from 0.8 to 1.8 g/cc for
SWNTs and 1.4 to 1.8 g/cc for MWNTs . The estimated high Young’s modulus and tensile
strength of the nanotubes reinforcements make it possible use in composite materials for
improved mechanical properties . Powder metallurgy (PM) technique offer production of
near net shaped aluminum composite and fabrication of CNT reinforced MMC has been
attempted by many researchers through traditional powder metallurgy. However these attempts
have not been fully successful because of the agglomeration of CNTs caused by the Van der
Waals forces between CNTs [13-17].
Further mechanical alloying by high energy ball milling is one of the promising methods to
nanocrystalline composites [18, 19]. In this present work, synthesis of novel aluminum based
nanocomposites by combining AA 4032 alloy powders and multi walled carbon nanotubes
(MWNTs) through mechanical milling and powder metallurgy
Elemental powders of AA 4032 were used as starting material to prepare alloys of composition
given in Table 1. Multi walled carbon nanotubes (MWNTs) synthesized by electric arc discharge
method  were used as reinforcement. Raman spectroscopy and transmission electron
microscopy were used to characterize the synthesized MWNTs.
TABLE 1 Chemical composition of AA 4032 alloy:
Name of the elements Concentration (wt %) Mesh size Purity (%)
The milling process was carried out at room temperature using tungsten carbide balls and Vials
in Fritsch Pulverisette P5 high energy ball mill. The ball to powder mass ratio and rotational
speed were maintained as 10:1 and 300 rpm. In addition to the AA 4032 powders, a small
amount of toluene was added as process control agent. Ball milling experiment were stopped at
every 30 minutes to prevent overheating. Powder samples were taken at regular time intervals of
5, 10, 15, 20, 25 and 30 h for phase and structural analyses. Different weight fractions (1.0, 1.5.
2.0 wt %) of MWNTs were added to the AA 4032 powder mixture at end of 29th h of total
Vol.9, No.11 Mechanically Alloyed Carbon Nanotubes 1029
milling time to avoid any structural damage of CNTs caused by the ball milling and to prevent
The X-ray powder diffraction was studied on Rigaku, Japan system with monochromatic Cu Kα
operated at 45 kV with a current beam of 10 mA. The intensity was measured over a diffraction
angles range from 0 to 80° with scanning step of 0.2°. The microstructure was examined using
HITACHI S3000H scanning electron microscope. A Philips CM 200 FEG Transmission
electron microscope (TEM) was used to investigate grain size and dispersion of nano tubes
inside the matrix.
3. RESULTS AND DISCUSSION
Raman analysis of synthesized MWNTs is shown in Figure 1. The Raman spectra show the
presence of disorder in CNT samples with two strong peaks at 1338 and 1571 cm-1. The peak at
1571cm-1 is due to the vibration of carbon atoms of the graphite called first order G-band, and
the one at 1338 cm-1 called D-band, which is due to the vibrations of carbon atoms with dangling
bonds in the disordered plane structure . Fig. 2 shows the multi walled nature with inner and
outer diameter as 3 and 30 nm respectively
Fig. 1: Raman Spectra of MWNTs Fig. 2: HRTEM image of MWNTs
3.2 X-ray Diffraction Analysis
Fig. 3 shows the XRD pattern of powders milled for up to 30 hours. It was observed that some of
the peaks correspond to the starting materials tend to broaden or disappear with the milling time.
The above characteristics may be due to high structural defect during mechanical alloying,
dissolution of elemental powders into the matrix and grain size reduction. The intensity of major
element Al peak decreases and broadened with milling time. Similarly the major alloying
1030 M.S. Senthil Saravanan, S.P. Kumaresh Babu, K. Sivaprasad Vol.9, No.11
element Si starts to decrease during milling time which may be due to the partial dissolution of
Si in Al lattice. The formation of solid solution can be achieved by ball milling.
Fig. 3: XRD pattern of ball milled powders
The lattice parameter of aluminum decreases with milling time. In early stages of milling, lattice
parameter decrease with limited variation and it increases during 20 to 25 h of milling. The
reason for Al peaks shifted to higher angles during the initial stages of milling where all the
elemental powders except magnesium start to dissolve in the Al matrix. The increase in the
lattice parameter may be attributed that the Mg starts to dissolve or due to the prolonged milling,
Si atoms are rejected from Al lattice which evidenced the nanocrystalline structure of the powder
The peak shift may be due to the reduction in crystallite size of aluminum and increase in the
lattice strain induced during mechanical alloying process. Al –Aqeeli et al  reported the same
kind of trend in their work, which may be attributed variation in lattice parameter due to
dissolved solute element.
The crystallite size and lattice strain were estimated from the broadening of XRD peaks using
Williamson – Hall formula 
cos += D (1)
Here β is the full width half maximum, λ is the X-ray wavelength (1.5406), θ is the Bragg angle
and D is the effective crystallite size normal to the reflecting planes and ε is the lattice strain. The
instrumental broadening corrected line profile breadth β were calculated by computer software
(XRD-analyzer) based on each reflection of 2θ. The first four Al reflections (1 1 1), (2 0 0), (3 1
1) and (2 2 2) were used to construct a linear plot of βcosθ against 4sinθ. Then, crystallite size
(D) was obtained from the intercept c and the strain (ε) from the slope.
Vol.9, No.11 Mechanically Alloyed Carbon Nanotubes 1031
During mechanical alloying, powder particles were subjected to repeated deformation, cold
welding and fragmentation, structural changes like reduction in grain size and development of
lattice strain induced during milling. Therefore XRD analysis was used to characterize crystallite
size and lattice strain which is shown in Fig. 4. It is clear that crystallite size of Al decreased
significantly from 140 nm to 17 nm from 1 to 30 h of milling. The crystallite size decrease
rapidly at the early stages of milling and then it is fixed at about 17nm. The lattice strain
increases with milling time due to the distortion effect caused by dislocation in the lattice. In
early stages of milling, severe plastic deformation of particles causes a deformed lattice with
density dislocations. However, long milling time gives nanocrystalline structure .
Fig. 4: Grain size and strain variation with milling time
It was observed that lattice strain keeps on increasing but the crystallite size remains in steady
state in the final stage of milling. The reason for the increase in the lattice strain at the final
stages may be due to dissolution of elemental powders .
3.2 Morphological Changes
Figure 5 shows the SEM images of AA 4032 alloy powder taken at the regular intervals (0h, 5h,
10h, 15h, 20h, 25h, and 30h) of milling process. The AA 4032 particles are equiaxed and
irregular in shape at the initial stage. After 5 h milling, the particles deform into flake like shape
due to the ductile nature of aluminum. The powder had a broad distribution of irregular particles.
The alloying elements give a homogeneous mixture after 5h milling. The particle shape became
plate like and flattened after 10 h milling. The plate like particles are work hardened after 15 h
milling, hence continuation of milling, cold welding and fracture mechanism is still activated.
The flake like morphology still remains even after 15 h milling with decreased particle size.
After 20 h, a low aspect ratio of the powders was seen and then there is a narrow distribution in
the particle size. Further milling up to 30 h, shows a uniform change in the particle size.
1032 M.S. Senthil Saravanan, S.P. Kumaresh Babu, K. Sivaprasad Vol.9, No.11
Fig. 5: SEM micrographs of AA 4032 at different milling times
Figure 6 shows bright field images of 2 wt % CNT reinforced AA 4032 nanocomposites. The
MWNTs are dispersed quite homogeneously in the composite. The inset shows the
Vol.9, No.11 Mechanically Alloyed Carbon Nanotubes 1033
corresponding selected area diffraction (SAED) pattern of AA 4032 reinforced with 2 wt %
MWNTs. The obtained ring pattern in SAED evidenced the formation of nanostructured
composite phase. It was found that no additional spots were formed, which confirms absence of
other phases. In conventional Carbon/aluminium composites, Al4C3 was grown on the prismatic
planes of carbon fiber. In our work, no carbides were detected at the Al/CNT interfaces, which
show a better chemical stability of MWNT. In the present work, MWNTs were added to the
matrix at the end of 29th h of milling which may be the reason for the homogeneous dispersion.
When CNTs were added at the early hrs, agglomeration and clustering occur due to lengthy
milling hours. Another noticeable observation in TEM picture was less structural damage of
CNTs. The uniform distribution of MWNTs in the composite effectively inhibits matrix
deformation and produces strengthening effect .
Fig. 6: TEM micrograph of CNT reinforced AA 4032 alloy powders; inset shows the SAED
Multi-walled carbon nanotubes reinforced AA4032 alloy nanocomposites have been successfully
synthesized by high energy ball milling. TEM analysis shows that MWNTs posses high
mechanical and chemical stability. MWNTs were dispersed homogeneously in the AA4032
matrix composites. Effect of milling time on grain size and lattice strain was clearly studied and
there was decrease in grain size and increase in strain with increase in milling time.
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