Materials Sciences and Applications, 2011, 2, 390-398
doi:10.4236/msa.2011.25050 Published Online May 2011 (http://www.SciRP.org/journal/msa)
Elevated Temperatures Tensile Characteristics
of Cast A356/Al2O3 Nanocomposites Fabricated
Using a Combination of Rheocasting and
Squeeze Casting Techniques
El-Sayed Youssef El-Kady, Tamer Samir Mahmoud, Mohamed Abdel-Aziz Sayed
Mechanical Engineering Department, Faculty of Engineering, King Khalid University (KKU), Abha, Kingdom of Saudi Arabia.
Received December 24th, 2010; revised March 21st, 2011; accepted April 6th, 2011.
ABSTRACT
In the present investigation, the tensile properties of A356/Al2O3 nanocomposites at both ambient and elevated tem-
peratures were studied. The A356/Al2O3 nanocomposites were fabricated using a combination between the rheocasting
and squeeze casting techniques. The A356 matrix alloy was reinforced with Al2O3 nanoparticulates having average
sizes of 60 nm and 200 nm with different volume fractions up to 5 vol%. The results revealed that the A356/Al2O3 na-
nocomposites exhibited better mechanical properties than the A356 monolithic alloy. Such improvement in the me-
chanical properties was observed at both room and elevated temperatures up to 300˚C. Increasing the volume fraction
and/or reducing the size of Al2O3 nanoparticulates increase both the tensile and yield strengths of the nanocomposites.
Keywords: Nanocomposites, Mechanical Properties, Rhecocasting, Squeeze Casting, Aluminum Alloys
1. Introduction
Metal matrix composites (MMCs) exhibit attractive phy-
sical and mechanical properties such as high tensile,
creep and fatigue strengths, superior wear resistance, and
improved thermal stability. Such properties allow these
materials to have numerous applications in the aerospace,
automobile and military industries [1,2]. Normally, mi-
cro-size ceramic particles are used to improve the yield
and ultimate strength of the metal. However, the ductility
of the MMCs deteriorates with high ceramic particle
concentration [3-5]. Recently, it is of interest to use nano-
size ceramic particles to strengthen the metal matrix,
while maintaining good ductility [6-8]. With nanoparti-
cles reinforcement, high temperature creep resistance and
better fatigue life could be achieved.
The production of MMCs is mainly achieved using
two routes; either casting techniques, such as stir casting,
or powder metallurgy techniques, such as mechanical
alloying. Each of these processes has certain advantages
and limitations. One main challenge in mechanical al-
loying is the difficulty to keep clean surfaces assuring
coherent contact between the powders used [9]. Casting,
as a liquid phase process, is capable of producing as-cast
lightweight bulk components of MMCs. However, liquid
phase casting processes usually results in poor properties
obviously due to the defects arising from high melting
temperatures at which the reinforcement is usually added.
It is difficult to obtain uniform dispersion of the ceramic
particles in liquid metals due to high viscosity, poor wet-
tability in the metal matrix, and a large surface-to-volume
ratio. These problems induce agglomeration and cluster-
ing [10].
The rheocasting (compocasting), as a semi-solid phase
process, can produce good quality MMCs [6,11]. 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
conventional foundry methods. The preparation proce-
dure for rheocast composites consists of the incorpora-
tion of the ceramic particles within very vigorously agi-
tated semi-solid alloy slurry which can achieve more
homogenous particles distribution as compared with a

Elevated Temperatures Tensile Characteristics of Cast A356/Al2O3 Nanocomposites Fabricated Using a Combination
of Rheocasting and Squeeze Casting Techniques
391
fully molten alloy. This is because of the presence of the
solid phase in the viscous slurry that prevents the ceramic
particles 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 [11]. It has been reported that the
porosity can be reduced by means of mechanical working
such as extrusion or rolling on the solidified composites
or by applying a pressure to the composite slurry during
solidification [12-14].
However the mechanical characteristics of Al-based
MMCs reinforced with microsize ceramic particulates at
elevated temperatures was extensively studied [15-20],
only limited data on the elevated temperatures bebaviour
of Al-based nanocomposites are available. Until now the
elevated temperatures mechanical properties for such
nanocomposites were not sufficiently determined and
published. This lack of data is more important when it is
considered that elevated temperatures are often found in
industrial applications, where the Al-based nanocompo-
sites are being considered as candidates to replace steel
or aluminum alloys. Typical examples are piston liners
and cylindrical heads of automobile engines, as well as
brake rotors.
The aim of the present investigation is to study the
tensile behavior of A356/Al2O3 nanocomposites at both
room and elevated temperatures up to 300˚C. The nano-
composites were fabricated using a combination of rheo-
casting and squeeze casting techniques. The effect of
nanoparticles size and volume fractions on the tensile
properties of the nanocomposites was investigated.
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 have two different ave-
rage sizes, typically, 200 and 60 nm were used as rein-
forcements. Several A356/Al2O3 nanocomposites con-
taining different volume factions of Al2O3 nano-particu-
lates up to 5 vol%.
The A356/Al2O3 nanocomposites were fabricated us-
ing a combination of rheocasting and squeeze casting
techniques. Fabrication of the composite alloy was car-
ried out according to the following procedures: About 1
kg of the A356 Al alloy was melted at 680˚C in a grap-
hite crucible in an electrical resistance furnace. After
complete melting and degassing by argon gas, the alloy
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 0.026 Bal.
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
primarily differential scanning calorimeter (DSC) ex-
periments performed on the A356 alloy. A simple me-
chanical stirrer with three blades made from stainless
steel coated with bentonite clay (see Figure 1) was in-
troduced into the melt and stirring was started at app-
roximately 1000 rpm. The Al2O3 nano-particles were
heated at 400˚C for two hours before dispersion inside
the vortex formed due to stirring. After that, preheated
Al2O3 nanoparticles were introduced into the matrix dur-
ing the agitation. After completing the addition of Al2O3
nanoparticles, the agitation was stopped and the molten
mixture was poured into preheated low carbon steel
mould and immediately squeezed during solidification. T
Figure 2 shows a photograph of the mould used for
squeezing the nanocomposites and the ingot after squee-
zing.
Samples from the cast nanocomposite ingots were cut
from the cast ingot for microstructural examinations.
Specimens were ground under water on a rotating disc
using silicon carbide abrasive discs of increasing finesse
Figure 1. The three blades stirrer.
(a) (b)
Figure 2. The mould used to squeeze the nanocomposites (a)
and the ingot after squeezing (b).

Elevated Temperatures Tensile Characteristics of Cast A356/Al2O3 Nanocomposites Fabricated Using a Combination
of Rheocasting and Squeeze Casting Techniques
392
Samples from the cast nanocomposite ingots were cut
from the cast ingot for microstructural examinations.
Specimens were ground under water on a rotating disc
using silicon carbide 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 microscopes (SEM). Microstructural
examination was performed in the unetched condition.
The porosity of the nanocomposites was measured using
the typical Archimedes (water displacement) method.
The nanocomposites were heat treated at T6 before
hardness and tensile testing. The nanocomposites were
solution treated at 540 ± 1˚C for three hours and then
quenched in cold water. After cooling specimens were
artificially aged at 160 ± 1˚C for 12 hours. Microhard-
ness was measured on polished samples using the
Zwick/Roll microhardness tester. The tests were carried
out by applying an indentation load of 25 g with a Vic-
kers indenter. Minimum of ten readings were conducted
for each specimen and the average value were considered.
Tensile tests were carried out on composites after heat
treatment at T6. Tensile tests were carried out at ambient,
100, 150, 200, 250 and 300˚C. The standard specimen
used is shown in Figure 3. The specimens were ma-
chined longitudinally from the nanocomposite cast ingots.
The tensile tests were carried out using 200 kN Shimadzu
universal testing machine and a gripping device for
non-threaded-end specimen according to ASTME 8-094.
For each condition, minimum three specimens were
tested and the average value was calculated.
The elevated temperatures tensile tests were conducted
using electrical furnace. To insure that the temperature
inside the furnace is stable, the specimen was heated to
the test temperature for 30 min before each experiment.
The temperature inside the furnace has an accuracy of
±5˚C. The cross head speed used in all tensile tests was 1
mm/min. The ultimate tensile strength (UTS), 0.2% yield
stress (YS) and ductility (%E) measured by the elonga-
tion % were calculated.
3. Results and Discussion
3.1. Microstructural Observations
Figure 4 shows example micrographs of the microstruc-
ture of the monolithic A356 alloy as well as the A356/
Figure 3. The tensile specimen (dimensions in mm).
Al2O3 nanocomposites after heat treatment. It is clear
from Figure 4(a) that the structure of the monolithic
A356 Al alloy consists of primary α phase (white regions)
and Al-Si eutectic structure (darker regions). Needle-like
primary Si particulates were distributed along the boun-
daries of the α-Al dendrites. Figures 4(b) and 4(c) show
micrographs of nanocomposites containing 3 vol% of
Al2O3 nanoparticulates having 60 and 200 nm, respect-
tively. Clusters of nanoparticles in the microstructure of
the A356/Al2O3 nanocomposite were observed. The na-
nocomposites containing 5 vol% exhibited the highest
agglomeration percent when compared with those con-
taining 1 and 3 vol% (see Figure 4(d)). Figure 4(e) shows
high magnification micrograph of Al2O3/5 vol% (200 nm)
nanocomposites. It is clear that clusters of nanoparticles
clusters are located inside the  -grains as well as near the
eutectic structure. Figure 5(a) shows high magnification
SEM micrograph of the 5 vol% Al2O3 nanoparticulates
(200 nm) showing that nanoparticulates are agglomerat-
ing near the Si particles of the eutectic structure. The XRD
analysis for the nanoparticles is shown in Figure 5(b)
indicates the presence of Al2O3 nanoparticulates near the
eutectic structure. Also, it has been observed that in-
creasing the volume fraction of the nanoparticlutes dis-
persed inside the A356 alloy increases the agglomeration
percent.
Porosity measurements indicted that the nanocompo-
sites have porosity content lower than 2 vol%. Such low
porosity content is attributed to the squeezing process
carried out during the solidification of the nanocompo-
sites. Generally in cast MMCs, there are several sources
of gases. The occurrence 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
drawn into the melt by certain stirring methods [6,11].
Vigorously stirred melt or vortex tends to entrap gas and
draw it into the melt. Increasing 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 influence 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-

Elevated Temperatures Tensile Characteristics of Cast A356/Al2O3 Nanocomposites Fabricated Using a Combination
of Rheocasting and Squeeze Casting Techniques
393
(a) (b)
(c) (d) (e)
Figure 4. Optical micrographs for (a) A356 monolithic aluminum alloy; (b) A356/3 vol% Al2O3 (60 nm) nanocomposites; (c)
A356/3 vol% Al2O3 (200 nm) nanocomposites; (d) A356/5 vol% Al2O3 (200 nm) (e) A356/5 vol% Al2O3 (200 nm) nanocompo-
sites.
(a) (b)
Figure 5. (a) High magnification SEM micrograph of the 5 vol% Al2O3 nanoparticulates (200 nm) show ing that nanoparticu-
lates are agglomerating near the Si particles of the eutectic structure; (b) XRD analysis for the particles shown in (a).
proving the microstructure.
3.2. Hardness of Nanocomposites
Figure 6 shows the variation of the microhardness of the
nanocomposites with the volume fraction of Al2O3 na-
noparticulates. It has been found that the nanocomposites
exhibited higher average microhardness than the A356
monolithic alloy. The average microhardness of the na-
nocomposites increases with increasing the volume frac-
tion of the Al2O3 nanoparticulates. The nanocomposites
containing 200 nm Al2O3 nanoparticulates exhibited
slightly higher average microhardness when compared
with the nanocomposites containing 60 nm Al2O3 nano-
particulates. The increase of the hardness of the A356
alloy due to the addition of Al2O3 nanoparticulates may
attribute to the increase of the resistance to localized
plastic deformation.
The increase of the hardness due to the addition of ce-
ramic nanoparticulates to aluminum and magnesium al-
loys were reported by many workers [7,8,21]. For exam-

Elevated Temperatures Tensile Characteristics of Cast A356/Al2O3 Nanocomposites Fabricated Using a Combination
of Rheocasting and Squeeze Casting Techniques
394
Figure 6. Variation of the microhardness of the nanocom-
posites with the volume fraction of Al2O3 nanoparticulates.
ple, Ansary et al. [21] studied the mechanical properties
of A356.1 Al alloy reinforced with nano-sized MgO (50
nm) up to 5 vol%. The A356.1/MgO nanocomposites were
fabricated via stir casting method. The results showed
that the hardness of all composites is higher than A356.1
monolithic alloy due to the presence of MgO nanopar-
ticulates with high hardness. The nanocomposites with 5
vol% content of MgO exhibited lower hardness than
samples with 2.5 vol% MgO due to the presence of more
porosity with the higher content of MgO.
3.3. Tensile Properties at Room Temperature
Figure 7 shows the variation of the tensile properties of
the nanocomposites as well as the A356 unreinfrced ma-
trix at room temperature. The tensile tests results reveled
that both ultimate tensile and yield strengths of the nano-
composites are greater than the A356 monolithic alloy.
For example, the A356 monolithic alloy exhibited aver-
age UTS and YS of about 221 and 181 MPa, respect-
tively. While the nanocomposites containing 5 vol% of
60 nm Al2O3 nanoparticles showed the maximum UTS
and YS of about 282 and 245 MPa, respectively. This
shows an increase of the UTS and YS by about 27% and
35%, respectively. The A356/Al2O3 nanocomposites ex-
hibited higher ductility when compared with the A356
monolithic alloy. For example, the A356 monolithic al-
loy exhibited average % elongation of about 4% while
the nanocomposites containing 5 vol% of 60 nm Al2O3
nanoparticles showed average % elongation of about 8%.
This shows an increase of the elongation percent by
about 50%. The nanocomposites reinforced with smaller
Al2O3 nanoparticles size (i.e. 60 nm) exhibited higher
UTS and YS when compared with the nanocomposites
having larger Al2O3 nanoparticles size (i.e. 200 nm). In-
creasing the volume fraction of Al2O3 nanoparticles from
(a)
(b)
(c)
Figure 7. Tensile properties of A356/Al2O3 nanocomposites
at room temperature; (a) ultimate tensile strength (UTS), (b)
0.2% yield strength (YS) and (c) elongation %.
1% to 5% increased considerably both the UTS and YS.
While, increasing the volume fraction of Al2O3 nanopar-

Elevated Temperatures Tensile Characteristics of Cast A356/Al2O3 Nanocomposites Fabricated Using a Combination
of Rheocasting and Squeeze Casting Techniques
395
ticles slightly increased the ductility of the nanocompo-
sites.
The improvement of the tensile properties of the cast
aluminum alloys due to the addition of nanoparticulates
was reported by many workers [6,22]. For instance,
El-Mahallawi et al. [6] studied the processing of A356
Al-Si alloy containing up to 5% vol% nanosize Al2O3
particles having size less than 500 nm. Nanocomposites
were prepared using rheocasting (semi-solid) casting
route. The results showed that introducing nano-particles
into semi-solid slurries promises to be a successful route
for producing a new generation of cast metal matrix na-
nocomposites. The nanocomposites showed high strength
values associated with superior ductility compared to the
alloy without particles addition under the same casting
conditions. Young et al. [22] studied the tensile behavior
of A356 Al alloy reinforced with nanosize Al2O3 par-
ticulates (30 nm). Nanocomposites containing up to 2
wt% of Al2O3 nanoparticulates were fabricated using ul-
trasonic processing at 760˚C. The results showed that
with only 2 wt% nanosize SiC, the yield strength of as-
cast Al alloy A356 was improved approximately 50%,
which is significantly better than what Al alloy with the
same percentage of micro-particle reinforcement can
offer. They also noted that there is little change in the
elongation and ultimate tensile strength.
The improved strength and ductility exhibited by the
nanocomposites fabricated by the combination of rheo-
casting and squeeze casting technique may be attributed
to: 1) The high effective viscosity of the molten slurry
that prevents particles from settling, floating, or agglo-
merating during the stirring process; 2) The application
of squeezing pressure during solidification which reduces
greatly the porosity that may developed from the rheo-
casting step. These reasons lead to better distribution of
the ceramic phase, reduction of porosity and hence better
mechanical properties.
The strengthening mechanism for the metal matrix
nanocomposites may attributed to the higher dislocation
density in nanocomposites due to the addition of nano-
ceramic phase [22,23]. The difference of the coefficients
of thermal expansion between the aluminum matrix and
the uniformly dispersed nano-Al2O3 could develop high
dislocation density, and the Al2O3 naoparticulates can
work as the barriers for dislocations movement. It has
been shown that the properties of nanocomposites would
be enhanced considerably even with a very low volume
fraction due to the high dislocation density of matrix
metal.
3.4. Tensile Properties at Elevated Temperatures
Figure 8 and Figure 9 show the variation of the tensile
(a)
(b)
(c)
Figure 8. Variation of the A356/Al2O3 (60 nm) nanocom-
posites with temperature (a) ultimate tensile strength (UTS),
(b) 0.2% yield strength (0.2% YS) and (c) the elongation %.
properties with temperature for the nanocomposites con-
taining 60 and 200 nm Al2O3 nanoparticulates, respec-

Elevated Temperatures Tensile Characteristics of Cast A356/Al2O3 Nanocomposites Fabricated Using a Combination
of Rheocasting and Squeeze Casting Techniques
396
(a)
(b)
(c)
Figure 9. Variation of the A356/Al2O3 (200 nm) nanocom-
posites with temperature (a) ultimate tensile strength (UTS),
(b) 0.2% yield strength (0.2% YS) and (c) the elongation %.
tively. The results showed that the nanocomposites exhi-
bited better elevated temperature mechanical properties
than the A356 unreinforced alloy. The UTS, 0.2% YS and
ductility of the nanocomposites up to 300˚C are still
higher than that of A356 monolithic alloy. For example, at
150˚C, the UTS of A356 monolithic alloy was 170 MPa
while the UTS of A356/5 vol% Al2O3 (60 nm) nano-
composites was 241 MPa. Again, it has been found that
reducing the size of the nanoparticulates from 200 nm to
60 nm and/or increasing the volume fraction of the
nanoparticles from 1% to 5%, increase the tensile proper-
ties of the nanocomposite at elevated temperatures.
A sharp decrease of the tensile properties of the A356
monolithic alloy was observed after 150˚C. The tem-
perature of about 150˚C was a critical temperature for the
tensile properties of the A356 monolithic alloy. The na-
nocomposites exhibited a critical temperature of about
200˚C. Such observation was noticed for both alloys
containing Al2O3 nanoparticulates of size of 60 and 200
nm. It can be concluded that at temperature higher than
200˚C, the matrix becomes soft, then the particles flow
with the matrix, left voids, and hence reduce the strength.
The manner by which the reinforcing Al2O3 nanopar-
ticulates affect the tensile strength of the aluminum alloy
composite can be described in terms of work hardening.
Beyond macroscopic yield a power law is representative
of the stress versus strain curve. It is expressed by the
Equation (1):

n
p
K   (1)
where: K is the monotonic strength coefficient (intercept
at plastic strain εp = 1) and ‘n’ is the work hardening or
strain hardening exponent. In the A356/Al2O3 nanocom-
posites, with a large coefficient of thermal expansion
(CTE) mismatch strain between the aluminum alloy metal
matrix and the reinforcing nano-Al2O3 particulates, the
plastic deformation of the ductile aluminum alloy metal
matrix, in the presence of the discontinuous nano-Al2O3
reinforcements, is essentially nonuniform, i.e. heteroge-
neous. This is because of the hard, brittle and elastically
deforming particles resisting plastic flow of the soft, duc-
tile and plastically deforming aluminum alloy metal ma-
trix. The plastic deformation induced dislocations, or slip
dislocations, become dominant when the plastic strain
exceeds the thermal mismatch strain and the two effects
eventually act in synergism so that they can be lumped
together.
The increased yield strength (∆σy) of the matrix of the
Al2O3 nanoparticles reinforced A356 aluminum nano-
composites, due to dislocation generation and accumula-
tion, and assuming the dislocations to be uniformly dis-
persed in the metal matrix, can be estimated using the
Equation (2) [24]:

Elevated Temperatures Tensile Characteristics of Cast A356/Al2O3 Nanocomposites Fabricated Using a Combination
of Rheocasting and Squeeze Casting Techniques
397

12
vGb



 (2)
where ∆σy is the increase in yield strength of the metal
matrix composite over that of the unreinforced matrix
alloy (aluminum alloy A356), G is the shear modulus
(GPa) of the metal matrix, b is the Burgers vector, ρ is
the increase in dislocation density of the composite ma-
trix over that of the unreinforced matrix density, and α is
a constant and is equal to 1.25 for aluminum.
The increase in flow stress of the discontinuously
reinforced aluminum metal matrix composites over the
unreinforced matrix alloy is proportional to the CTE
mismatch strain if the dislocations generated by CTE
mismatch strain are dominant. The mismatch strain (εα)
induced in the particle is [25]:

pM
T

 
(3)
where: αp and αM are CTE of the ceramic particle (Al2O3)
and the matrix (aluminum alloy A356) respectively, and
both the matrix and the particle are assumed to be iso-
tropic in stiffness and CTE. ∆T is the net temperature (To
– Tambient) change when the particulate-reinforced alumi-
num alloy metal matrix is quenched from an elevated
temperature (say To).
Accordingly, the contributions to strengthening of the
A356/Al2O3 nanocomposites can be arises from the
mutually competitive influences of the following mecha-
nisms: 1) Strengthening due to large differences in ther-
mal coefficients of expansion between constituents of the
composite, i.e. aluminum alloy and Al2O3, resulting in
misfit strains due to differential thermal contraction at the
Al/Al2O3 interfaces. The misfit strain and concomitant
misfit stresses generate dislocations. The increased dis-
location density generated to accommodate the misfit
strains provides a positive contribution to strengthening
the matrix of the nanocomposites; 2) Dispersion streng-
thening caused by the presence of reinforcing Al2O3
nanoparticles in the A356 aluminum alloy metal matrix
and the additional stress required for slip dislocations to
by-pass a reinforcing Al2O3 nanoparticles.
4. Conclusions
According the results obtained from the current investi-
gation, the following conclusions can be pointed out:
1) The A356/Al2O3 nanocomposites exhibited better
tensile properties than the A356 monolithic alloy. Such
improvement in the tensile properties was observed at
both room and elevated temperatures up to 300˚C. In-
creasing the volume fraction of the Al2O3 nanoparticu-
lates increases both the tensile and yield strengths of the
nanocomposites.
2) The nanocomposites having 60 nm Al2O3 nanopar-
ticulates showed better room and elevated tensile proper-
ties than those containing 200 nm Al2O3 nanoparticu-
lates.
3) The nanocomposites exhibited higher critical tem-
perature, at which the tensile properties decrease sharply,
than the A356 monolithic alloy. The nanocomposites and
the A356 unreinforced alloy showed critical temperatures
of 200˚C and 150˚C, 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
KKU, and Dr. Khaled Al-Zailaie, Dean of the faculty of
engineering at KKU, for their support.
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