Materials Sciences and Applicatio ns, 2011, 2, 1180-1187
doi:10.4236/msa.2011.29159 Published Online September 2011 (http://www.SciRP.org/journal/msa)
Copyright © 2011 SciRes. MSA
On the Electrical and Thermal Conductivities of
Cast A356/Al2O3 Metal Matrix Nanocomposites
El-Sayed Youssef El-Kady, Tamer Samir Mahmoud, Ali Abdel-Aziz Ali
Mechanical Engineering Department, Faculty of Engineering, King Khalid University (KKU), Abha, Kingdom of Saudi Arabia.
Email: Eyelkady@yahoo.com
Received May 13th, 2011; revised June 2nd, 2011; accepted June 7th, 2011.
ABSTRACT
To assess the effect of the dispersion of Al2O3 nanoparticles into A356 Al alloy on both the electrical and thermal con-
ductivities, A356/Al2O3 metal matrix nanocomposites (MMNCs) were fabricated using a combination of rheocasting
and squeeze casting techniqu es. Two different sizes of Al2O3 nanoparticles were dispersed into the A356 Al alloy, typi-
cally, 60 and 200 nm with volume fractions up to 5 vol%. The effect of the nanoparticles size and volume fra ction on the
electrical and thermal conductivities was evaluated. The results revealed that the A356 monolithic alloy exhibited better
electrical and thermal conductivities than the MMNCs. Increasing the nanoparticles size and/or the volume fraction
reduces both the thermal and electrical conductivities of the MMNCs. The maximum reduction percent in the thermal
and electrical conductivities, according to the A356 monolithic alloy, were about 47% and 38%, respectively. Such
percentages were exhib ited by A356/Al2O3 MMNCs containing 5 vol% of nanoparticles having 60 and 200 nm, respec-
tively.
Keywords: Nanocomposites, Thermal Conductivity, Electrical Conductivity, Aluminum Alloys
1. Introduction
Metal matrix nanocomposites (MMNCs) reinforced by
nanoscale particulates is one type of nanocomposite.
MMNCs reinforced with nanoparticles are widely used in
industry. Several applications of Al and Mg MMNCs are
found in automotive and aerospace industry [1,2].
Conventional particulate reinforced metal matrix com-
posites (MMCs) usually have a metallic phase (usually
Al or Mg) and a ceramic reinforcement phase (most
commonly SiC, Al2O3 and graphite particulates) com-
posed of micron scale particles. Ingot metallurgy (I/M)
(or casting techniques) and powder metallurgy (P/M) are
common methods to produce conventional MMCs [2].
MMCs have higher yield strength, stiffness and lower
coefficient of thermal expansion than those of the mono-
lithic matrix alloy. Currently, there are several fabrica-
tion methods of MMNCs, including mechanical alloying,
powder metallurgy, casting techniques, electrochemical
deposition, friction stir processing, laser and sol-gel
technology [1]. Each of these methods has its own ad-
vantages and disadvantages. Mechanical alloying, pow-
ered metallurgy and casting technique are the most
commonly used techniques for producing bulk MMNCs
[3]. With electrochemical deposition, laser technology
and sol-gel fabrication methods, only thin films or layer
of nanostructured MMNCs are formed.
The rheocasting (compocasting), as a semi-solid phase
process, can produce good quality MMCs [4,5]. The
technique has several advantages: It can be performed at
temperatures lower than those conventionally employed
in foundry practice during pouring; resulting in reduced
thermochemical degradation of the reinforced surface,
the material exhibits thixotropic behaviour typical of stir-
cast alloys, and production can be carried out using con-
ventional foundry methods. The preparation procedure
for rheocast composites consists of the incorporation of
the ceramic particles within very vigorously agitated
semi-solid alloy slurry which can achieve more homo-
genous particles distribution as compared with a fully
molten alloy. This is because of the presence of the solid
phase in the viscous slurry that prevents the ceramic par-
ticles from settling and agglomerating. However, the
composites produced by rheocasting suffer from the high
porosity content, which has a deleterious effect on the
mechanical properties. It has been reported that the po-
rosity can be reduced by means of mechanical working
such as extrusion or rolling on the solidied composites
On the Electrical and Thermal Conductivities of Cast A356/AlO Metal Matrix Nanocomposites1181
2 3
or by applying a pressure to the composite slurry during
solidication [6,7].
However the mechanical and tribological characteris-
tics of MMNCs were extensively studied [8,9], only limi-
ted data on the thermal and electrical behavior of such
nanocomposites are available [10,11]. Until now the ther-
mal and electrical behaviour of MMNCs were not suffi-
ciently determined and published. Accordingly, the aim
of the present work is to study the effect of the Al2O3
nanoparticles additives on both the electrical and the
thermal conductivities of A356 Al alloy. The effect of
the Al2O3 nanoparticles size and volume fraction of the
aforementioned conductivities was evaluated. The A356/
Al2O3 MMNCs were fabricated a combination of rheo-
casting and squeeze casting techniques.
2. Experimental Procedures
The A356 Al-Si-Mg cast alloy was used as a matrix. The
chemical composition of the A356 Al alloy is listed in
Table 1. Nano-Al2O3 particulates were used as reinforc-
ing agents. The Al2O3 nanoparticles have two different
average sizes, typically, 200 and 60 nm. Several A356/
Al2O3 MMNCs were fabricated with different volume
factions of nanoparticles up to 5 vol%.
The A356/Al2O3 MMNCs were prepared using a com-
bination of rheocasting and squeeze casting techniques.
Preparation of the composite alloy was carried out ac-
cording to the following procedures: About 1 kg of the
A356 Al alloy was melted at 680˚C in a graphite crucible
in an electrical resistance furnace. After complete melt-
ing and degassing by argon gas of the alloy, the alloy
was allowed to cool to the semisolid temperature of
602˚C. At such temperature the liquid/solid fraction was
about 0.7. The liquid/solid ratio was determined using
primary differential scanning calorimeter (DSC) experi-
ments performed on the A356 alloy. A simple mechani-
cal stirrer with three blades made from stainless steel
coated with bentonite clay was introduced into the melt
and stirring was started at approximately 1000 rpm. Be-
fore stirring the nano-particles reinforcements after heat-
ing to 400˚C for two hours were added inside the vortex
formed due to stirring. After that, preheated Al2O3
nanoparticles were introduced into the matrix during the
agitation. After completing the addition of Al2O3 nano-
particles, the agitation was stopped and the molten mix-
ture was poured into preheated tool steel mould and im-
mediately squeezed during solidication using a hydrau-
lic press of 50 ton capacity for 5 minutes.
Table 1. The chemical composition (wt%) of the A356 alloy.
Si Fe Cu Mn Mg Zn Al
6.6 0.25 0.11 0.002 0.14 0.026Bal.
The MMNCs were heat treated at T6 before testing.
The MMNCs were solution treated at 540˚C ± 1˚C for
three hours and then quenched in cold water. After cool-
ing specimens were artificially aged at 160˚C ± 1˚C for
12 hours.
Samples from the fabricated composites were cut from
the cast ingot for microstructural examinations. Speci-
mens were ground under water on a rotating disc using
SiC abrasive discs of increasing finesse up to 1200 grit.
Then they were polished using 10 µm alumina paste and
3 µm diamond paste. Microstructural examinations were
conducted using both optical and scanning electron mi-
croscopes (SEM). Microstructural examination was per-
formed in the unetched condition. The porosity of the
MMNCs was measured using the typical Archimedes
(water displacement) method.
The electrical measurements of the A356 monolithic
alloy and the A356/Al2O3 MMNCs were measured using
SIGMASCOPE SMP10 apparatus made by Helmut
Fischer GmbH + Co.KG, Germany. The apparatus can
measure the electrical conductivity of all non-magnetic
metals and even stainless steel, etc. Samples from the
MMNCs and the monolithic alloy having a cylindrical
shape of 20 mm diameter and 10 mm length were cut
from the cast and polished from the two end faces before
electrical conductivity measurements. The electrical con-
ductivity measurements were performed according to
ASTM E-1004. The apparatus measures the electrical
conductivity using the eddy current method.
The thermal conductivity of the MMNCs was measu-
red using the apparatus shown in Figure 1. The appara-
tus has been specially designed for the determination of
thermal conductivity for both good conductors and thin
specimens of insulants. The apparatus consists of a self-
clamping specimen stack assembly with electrically
heated source, calorimeter base, Dewar vessel enclosure
to ensure negligible loss of heat, and constant head cool-
ing water supply tank. A multipoint thermocouple switch
is mounted on the MMNCs specimen and two mercury
and glass thermometers are provided for water inlet and
outlet temperature readings as shown in Figure 2.
Three NiCr/NiAl thermocouples are fitted and connec-
tions are provided for a suitable potentiometer instrument
to give accurate temperature readings. The specimens
have cylindrical shape of about 10 mm diameter and 20
mm length. The end faces of these specimens are very
carefully prepared by grinding and lapping operations.
Two small holes were drilled in each specimen for inser-
tion of the thermocouples. Heat was applied to the top of
the stack of specimens by contact with the thermostati-
cally protected heater block, and is collected at the base
of the stack by contact with a water cooled calorimeter.
The heat transmitted through the specimens can thus be
Copyright © 2011 SciRes. MSA
On the Electrical and Thermal Conductivities of Cast A356/AlO Metal Matrix Nanocomposites
1182 2 3
Figure 1. A photograph of the apparatus used to measure
the thermal conductivity of the nanocomposites.
Figure 2. A schematic drawing shows the specimen, the
positions of the thermocouples and the heating and cooling
methods of the specimen.
calculated from a measurement of water flow and tem-
perature rise of the water. Heat losses by radiation and
conduction from the heater block other than from contact
with the specimen were neglected, since measurement is
made of heat collected from the samples rather than heat
delivered to them. A steady flow of cooling water is
maintained at the end away from the heating block and it
leaves at the end nearer to it. Thermometers Twi and Two
are provided to measure the temperatures of the inlet and
outlet water, respectively. Two holes of 1.5 mm diameter
are drilled in the specimen rod and thermocouples are
inserted in these holes to measure the temperatures T1
and T2 of the rod at these places. The temperatures of the
two thermometers and the two thermocouples rise ini-
tially and ultimately become constant when the steady
state is reached. The water coming out of the apparatus is
collected in a beaker for a fixed time measured by a
stopwatch and the flow rate of water is determined by
timing the collection of 100 cm3 per second sample of
water. The cross-section area of the rod is calculated by
measuring its diameter and the distance between the
holes in the rod was measured.
It is assumed that the cross-sectional area normal to
the flow of heat transfer is constant and the periphery is
insulated due to the use of Dewar vessel enclosure to
ensure negligible loss of heat. The heat transfer through
the specimen rod is governed by Fourier’s Law:
d
d
T
QkA
x
 (1)
where Q is the rate of heat transfer in the x-direction, k is
the thermal conductivity of the specimen material, A is
the cross-sectional area of the specimen normal to the
x-direction, and dT/dx is the temperature gradient in the
x-direction. At steady state condition the temperature of
the specimen does not change with time at any point and
the heat conducted along the specimen must go into the
flowing water. The heat taken by the water was calcu-
lated using the following equation:
p
wo wi
QmcT T (2)
where m is the mass flow rate of cooling water, Cp is the
specific heat of water, and Two, Twi are the cooling water
outlet and inlet temperatures, respectively. By applying a
thermal balancing hence, the same rate of heat transfer Q
is used in Equation (1) and (2). Thus,

21
pwowi
mc TTx
kAT T
(3)
The thermal conductivity measurement depends on the
measurement of the heat flux Q and temperature differ-
ence (T2 T1). The axial flow method has been long es-
tablished and has produced some of the most consistent
and highest accuracy results. The present measurement
focused mainly on reduction of radial heat losses in the
axial heat flow developed through the specimen from the
electrical heater mounted at one end.
3. Results and Discussion
3.1. Microstructural Observations
Figure 3 shows typical micrographs for the microstruc-
ture of the monolithic A356 alloy as well as the fabric-
cated A356/Al2O3 MMNCs after heat treatment. It is
clear from Figure 3(a) that the structure of the mono-
lithic A356 Al alloy consists of primary α phase (white
regions) and Al-Si eutectic structure (darker regions).
Fine needle-like primary Si particulates were distributed
along the boundaries of the α-Al dendrites. Figures 3(b)
and 3(c) show micrographs of A356/Al2O3 MMNCs con-
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On the Electrical and Thermal Conductivities of Cast A356/AlO Metal Matrix Nanocomposites1183
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40 μm
(a)
40 μm
(b)
40 μm
(c)
40 μm
(d)
10 μm
(e)
Figure 3. Optical micrographs for (a) A356 monolithic alu-
minum alloy; (b) A356/3 vol% Al2O3 (60 nm) nanocompo-
sites; (c) A356/3 vol% Al2O3 (200 nm) nanocomposites; (d)
A356/5 vol% Al2O3 (200 nm); (e) A356/5 vol% Al2O3 (200
nm) nanocomposites.
taining 3 vol% of Al2O3 nanoparticles having 60 and 200
nm, respectively. Clusters of Al2O3 nanoparticles in the
microstructure of the A356/Al2O3 MMNCs were ob-
served. Figure 3(d) shows high magnification optical
micrograph of Al2O3/5 vol% (200 nm) MMNCs. It is
clear that clusters of nanoparticles clusters are located
inside the -grains as well as near the eutectic structure.
Figure 4(a) shows high magnification SEM micrograph
of the 5 vol% Al2O3 nanoparticles (200 nm) showing that
nanoparticles are agglomerating near the Si particles of
the eutectic structure. The XRD analysis for the nanopar-
ticles is shown in Figure 4(b). Increasing the volume
fraction of the Al2O3 nanoparticles dispersed inside the
A356 alloy increases the agglomeration percent. The
(a)
(b)
Figure 4. (a) High magnification SEM micrograph of the 5
vol% Al2O3 nanoparticulates (200 nm) showing that na-
noparticulates are agglomerating near the Si particles of the
eutectic structure; (b) XRD analysis for the particles shown
in (a).
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On the Electrical and Thermal Conductivities of Cast A356/AlO Metal Matrix Nanocomposites
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MMNCs containing 5 vol% of Al2O3 nanoparticles ex-
hibited the highest agglomeration percent when com-
pared with those containing 1 and 3 vol%.
Porosity measurements indicted that the MMNCs have
porosity content lower than 2 vol%. Such low porosity
content is attributed to the squeezing process that was
carried out during the solidification of the MMNCs. In
cast MMCs, there are several sources of gases. The oc-
currence of porosity can be attributed variously to the
amount of hydrogen gas present in the melt, the oxide
film on the surface of the melt that can be drawn into it at
any stage of stirring and the gas being draw into the melt
by certain stirring methods. Vigorously stirred melt or
vortex tends to entrap gas and draw it into the melt. In-
creasing the stirring time allows more gases to be entered
into the melt and hence reduce the mechanical properties.
The amount of liquid inside the semi-solid slurry in-
creases with increasing the temperature which on the
other hand reduces the viscosity of the solid/ liquid slurry.
Nanoparticle distribution in the A356 Al matrix alloy
during the squeezing process depends greatly on the vis-
cosity of the slurry and also on the characteristics of the
reinforcement particles themselves, which inuence the
effectiveness of squeezing in to break up agglomerates
and distribute particles. When the amount of liquid inside
the slurry is large enough, the particles can be rolled or
slid over each other and thus breaking up agglomerations
and helping the redistribution of nanoparticles and im-
proving the microstructure.
3.2. The Thermal Conductivity of MMNCs
Figure 5 shows the variation of the thermal conductivity
against the time for different MMNCs specimens having
different volume fractions and sizes of Al2O3 nanoparti-
cles. The figure shows that there is a significant variation
of the thermal conductivity during the first few minutes
and it approaches to a relatively constant value after 120
minutes or two hours (Steady state condition). The A356/5
vol% contains Al2O3 nanoparticles of 200 nm and 60 nm
exhibited the maximum and the minimum values of
thermal conductivity of 213 and 105 W/m.K, respectively.
The present experimental measurements show a signify-
cant difference in the value of the average thermal con-
ductivity for different A356/Al2O3 MMNCs. The steady
state technique and the axial flow method are satisfactory
methods for measuring the thermal conductivity of
MMNCs.
The variation of the thermal conductivity of the A356/
Al2O3 MMNCs with the volume fraction of the nanopar-
ticles at several nanoparticles sizes is shown in Figure 6.
The thermal conductivities of the A356/Al2O3 MMNCs
showed disturbing results. It is suggested that the clus-
tering of the Al2O3 nanoparticles is the reason of such
(a)
(b)
(c)
Figure 5. Variation of average thermal conductivity with
the time for (a) the A356 monolithic alloys; (b) A356/Al2O3
60 nm) MMNCs and (c) A356/Al2O3 (200 nm) MMNCs. (
Copyright © 2011 SciRes. MSA
On the Electrical and Thermal Conductivities of Cast A356/Al2O3 Metal Matrix Nanocomposites
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Figure 6. The average thermal conductivity of the A356/Al2O3 MMNCs.
results. Generally, the Al2O3 nanoparticles addition de-
creased the thermal conductivity of the MMNCs when
compared with the A356 monolithic alloy. The MMNCs
containing up to 3% of 60 nm Al2O3 nanoparticles showed
better thermal conductivities than those containing 200
nm nanoparticles. Only MMNCs containing 3%-60 nm
and 5%-200 nm of Al2O3 nanoparticles showed practi-
cally the same thermal conductivity of A356. The reduce-
tion of the thermal conductivity of the MMNCs due to the
addition of the Al2O3 nanoparticles may attribute to the
insulating nature of the Al2O3 nanoparticles itself and the
increase of the porosity content with increasing the vo-
lume fraction of the nanoparticles. Figure 6 shows also
the percent of reduction/increase of the thermal conduc-
tivity of A356 due to the dispersion of Al2O3 nanoparti-
cles having several sizes and volume fractions. The ma-
ximum percentage of reduction of thermal conductivity
was about 47% for MMNCs containing 5 vol% of 60 nm
particulates.
The reduction of the thermal conductivity due to the
addition of ceramic nanoparticles was also reported by
Ke Chu et al. [11]. They reported that the addition of
carbon nanotubes (CNTs) showed no enhancement in
overall thermal conductivity of the Cu composites due to
the interface thermal resistance associated with the low
phase contrast of CNT to copper and the random tube
orientation. Besides, the composite containing 15 vol%
CNTs led to a rather low thermal conductivity due possi-
bly to the combined effect of unfavourable factors in-
duced by the presence of CNT clusters, i.e. large porosity
and lower effective conductivity of CNT clusters them-
selves.
The thermal conductivity of the conventional alumi-
nium based MMCs was investigated by many workers
[12,13]. It has been found that the thermal conductivity
of MMCs is mainly governed by the conductivity of the
individual phases, their volume fraction and shape, and
also by the size of the inclusion phase due to a nite
metal/ceramic interface thermal resistance. Beside these
factors, the results obtained from the current work show-
ed that the agglomeration % of the particulates has a sig-
nificant influence on the thermal conductivity of the
MMCs. Increasing the volume fraction of the particulates
increases the amount of agglomeration % of the particles
which can lead to disturbing results of the average ther-
mal conductivity of the MMCs. Particles free zones have
higher thermal conductivity than the particles clustered
zones. Another important factor that plays an important
role on the thermal conductivity of the MMCs is the fab-
rication technique and its processing conditions. The
casting techniques have several advantages than other
fabrication techniques such as powder metallurgy. These
advantages include lower cost and the capability of pro-
duction of large components. However, MMCs cast com-
ponents suffer from several defects such as particles seg-
regation and high porosity content. The choice of the
On the Electrical and Thermal Conductivities of Cast A356/AlO Metal Matrix Nanocomposites
1186 2 3
proper process parameters of the casting technique has a
significant effect in reduction, but not elimination, of the
particles segregation and porosity content. Squeezing the
MMNCs during solidification helped in reduction of the
porosity content of the MMNCs, however, it did not re-
duce the agglomeration % of the particles. The reduction
of the agglomeration % of the particles can be achieved
by the proper selection of the rheocasting process pa-
rameters which was the previous stage before the squee-
zeing process. The rheocasting process parameters in-
clude; the stirring speed; the shape, position and size of
the stirrer; the stirring temperature; the stirring duration
and pouring technique.
3.3. Electrical Conductivity of MMNCs
Figure 7 shows the variation of the electrical conductivity
of the A356/Al2O3 MMNCs, in MS/m, with the volume
fraction of the Al2O3 nanoparticles. The results revealed
that the MMNCs showed lower electrical conductivities
when compared with the A356 base alloy. Increasing the
volume fraction reduces the electrical conductivity of the
MMNCs. Moreover, the increasing the size of the nano-
particles from 60 to 200 nm reduced significantly the
electrical conductivity of the MMNCs. The maximum
reduction percent in the conductivity, with respect to the
A356 monolithic alloy, was about 38% and exhibited by
A356/Al2O3 MMNCs containing 5 vol% of nanoparticles
having 200 nm.
The reduction of the electrical conductivity of the
A356 due to the addition of the Al2O3 nanoparticles may
be attributed to the clustering of the nanoparticles which
are in fact insulations sites that reduce the electrical
conductivity. Moreover, it has been observed that the
nanoparticles are positioned and clustered at the grain
boundaries of Al grains and decrease the electrical con-
Figure 7. Variation of the electrical conductivity of the
A356/Al2O3 MMNCs with the volume fraction of the Al2O3
nanoparticles.
ductivity. The reduction of the electrical conductivity due
to the addition of ceramic nanoparticles noticed in the
current work has been observed also by Sheikh et al. [10].
In their work, carbon nanotubes were (CNT) added to
pure copper and MMNCs were fabricated using powder
metallurgy. The results revealed that increasing CNT
contents in the Cu composites decreases the electrical
conductivity. In contrast, El-Mahalla wi et al. [14] re-
ported that introducing Al2O3 nanoparticles to A356 al-
loy within the range 1 vol% tends to yield the alloy of
highest electrical conductivity of the MMNCs. The mor-
phology, and size of the eutectic silicon is believed to
have an effect on the electrical conductivity also and any
improvement on the eutectic morphology is believed to
induce a significant effect on the electrical conductivity
of the A356 alloy [14]. The significant effect induced on
cast alloys by adding Al2O3 particulates has been dis-
cussed and reported recently by some researchers. Ge-
nerally, the electrical conductivity of the MMCs is
mainly decided by the conductivity of the matrix alloy as
well as the shape, size and volume fraction of hard ce-
ramic alumina particles [15].
4. Conclusions
Based on the results obtained from the current work, the
following conclusions are drawn:
1) The A356 monolithic alloy exhibited better thermal
conductivity than the MMNCs. The A356/Al2O3 MMNCs
containing up to 3% of 60 nm Al2O3 nanoparticles
showed better thermal conductivities than the MMNCs
containing 200 nm nanoparticles.
2) The A356/Al2O3 MMNCs showed lower electrical
conductivities when compared with the A356 base alloy.
Increasing the volume fraction of the Al2O3 nanoparticles
reduces the electrical conductivity of the MMNCs.
Moreover, increasing the size of the Al2O3 nanoparticles
from 60 to 200 nm reduced significantly the electrical
conductivity of the MMNCs.
3) The maximum reduction percent in the thermal and
electrical conductivities, according to the A356 monoli-
thic alloy, were about 47% and 38%, respectively. These
percentages were exhibited by A356/Al2O3 MMNCs
containing 5 vol% of nanoparticles having 60 and 200
nm, respectively.
5. Acknowledgements
This work is supported by the King Abdel-Aziz City of
Science and Technology (KACST) through the Science
and Technology Center at King Khalid University
(KKU), Fund (NAN 08-172-7). The authors thank both
KACST and KKU for their financial support. Special
Thanks to Prof. Dr. Saeed Saber, Vice President of KKU,
Dr. Ahmed Taher, Dean of the Scientific Research at
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On the Electrical and Thermal Conductivities of Cast A356/Al2O3 Metal Matrix Nanocomposites
Copyright © 2011 SciRes. MSA
1187
KKU, and Dr. Khaled Al-Zailaie, Dean of the faculty of
engineering at KKU, for their support.
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