Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.2, pp 93-106, 2009
jmmce.org Printed in the USA. All rights reserved
Comparison of the Me chanical Properties of AL6061/Albite and
AL6061/Graphite Metal Matrix Composites
A. Ramesh 1, J. N. Prakash 1*, A. S. Shiva Shankare Gowda2 and
Sonnappa Appaiah 3
1Department of Mechanical Engineering, Alpha College of Engineering
Bangalore-562149, INDIA
2 Dr.M.G.R Educational and Research Institute,
Chennai-600 095, INDIA
3Department of Industrial Engineering and Management, M.S.R.I.T,
Bangalore-560 075, INDIA
* Corresponding author’s e-mail address: prakash_jnp_ace@yahoo.com
ABSTRACT
The aim of this present investigation is to carry out a comparative study of the mechanical
properties of AL6061/Albite composites containing albite(NaAlSi3O8) particulates, which are
naturally occurring plagioclase feldspar and AL6061/graphite particulate composites
containing graphite particles. The reinforcing particulates in the MMC’s vary from 0% to 4%
by weight. The 'vortex method' of production was employed to fabricate the composites, in
which the reinforcements were poured into the vortex created by stirring the molten metal by
means of a mechanical agitator. The composites so produced were subjected to a series of
tests.
The results of this study revealed that as the Albite particle content was increased, there were
significant increases in the ultimate tensile strength, hardness and Young's modulus,
accompanied by a reduction in its ductility. There was, however, only a very marginal
increase in the compressive strength, where as in graphite reinforced composites as the
graphite content was increased, there were significant reduction in hardness and monotonic
increases in the ductility, ultimate tensile strength (UTS), compressive strength and Young's
modulus of the composite, An attempt is made in the paper to provide explanations for these
phenomena.
KEYWORDS: Aluminium alloys; Albite; Graphite; Composites; Mechanical properties
93
94 A. Ramesh, J. N. Prakash, A. S. Shiva Shankare Gowda and Sonnappa Appaiah Vol.8, No.2
1. INTRODUCTION
Metal matrix composites (MMC’s) are increasingly becoming attractive materials for
advanced aerospace applications because their properties can be tailored through the addition
of selected reinforcements [1-2]. In particular, particulate reinforced MMC’s have recently
found special interest because of their specific strength and specific stiffness at room or
elevated temperatures [3]. It is well known that the elastic properties of metal matrix
composites are strongly influenced by micro structural parameters of the reinforcement such
as shape, size, orientation, distribution and volume fraction [4].
Aluminium-based Metal Matrix Composites (MMCs) have received increasing attention in
recent decades as engineering materials. The introduction of a ceramic material into a metal
matrix produces a composite material that results in an attractive combination of physical and
mechanical properties which cannot be obtained with monolithic alloys. There is an
increasing need for knowledge about the processing techniques and mechanical behaviour of
particulate MMCs in view of their rising production volumes and their wider commercial
applications. Interest in particulate reinforced MMCs is mainly due to easy availability of
particles and economic processing technique adopted for producing the particulate-reinforced
MMCs. Aluminium alloy-based particulate-reinforced composites have a large potential for a
number of engineering applications. Interest in reinforcing Al alloy matrices with ceramic
particles is mainly due to the low density, low coefficient of thermal expansion and high
strength of the reinforcements and also due to their wide availability. Among the various
useful aluminium alloys, aluminium alloy 6061 is typically characterized by properties such
as fluidity, castability, corrosion resistance and high strength-weight ratio. This alloy has
been commonly used as a base metal for MMCs reinforced with a variety of fibres, particles
and whiskers [5-7].
In recent years, considerable work has been done on graphite reinforced metal matrix
composites which exhibit low friction, low wear rate and excellent antiseizing properties. The
graphite in these composites presumably imparts improved tribological properties to the
composites through the formation of a graphite-rich film on the tribo-surface which provides
solid lubrication. Journal bearings made of graphite particle dispersed composites perform
much better than conventional bearing alloys [8].
Graphite particles of size ranging from 50 to 200 μm yield the best results [9]. Manufacturing
automotive pistons out of graphite reinforced composites instead of other conventional
materials resulted in a saving of 5-7% on fuel and lubricating oil [10]. The presence of
graphite in the matrix improves its oil spreadability over the contact surface, thus reducing
the tendency to score or seize. Graphite, which consists of carbon atoms arranged in a layer-
like structure, displays a very low coefficient of friction while sliding on another clean
surface, thus suggesting that it can be used as solid lubricants [11]. Because of this solid
lubricative property, graphite in the form of particles has a wide range of applications in
composite materials which are used to make components requiring great wear resistance such
as engine bearings, pistons, piston rings and cylinder liners [12]. Although fibre
Vol.8, No.2 Comparison of Mechanical Properties 95
reinforcements lead to marked enhancement in its properties, composites using particulate or
discontinuous reinforcement for high-volume applications are being increasingly sought. The
most widely used reinforcements have been SiC, alumina, graphite, B4C and TiC. These have
been used to achieve improvements in selected properties. Most of the published data pertain
to the mechanical properties of particulate-reinforced MMCs deal with tensile properties
while only a relatively small amount of data has been obtained dealing with compression
properties, although it is generally known that the compressive strength of an MMC is
invariably higher than its UTS. Such MMCs are quite brittle compared with monolithic
materials and have values of percentage elongation typically less than 5 per cent. For
unreinforced aluminium alloy 6061, Awerbuch et al. [13] have shown that the deformation in
compression could be greater than 50 per cent for specimens having a length-diameter ratio
L/D equal to unity. Such effects could also be responsible for the apparently high value of the
compressive strength. Hence, in the present investigation, importance is also given to the
compressive properties of the MMCs, together with the tensile properties such as the UTS,
ductility, hardness and Young's modulus.
In the present investigation, aluminium alloy AL6061 was used as the matrix material.
AL6061 alloy has the highest strength and ductility of the aluminium alloys with excellent
machinability and good bearing and wear properties [14]. Most of the particulate reinforced
metal matrix composites are produced by liquid metallurgy, sometimes known as the 'vortex
method' [15], although many different processes for fabricating these cast composites are
also available which have been reported by various researchers. In the present work, the
'vortex method' of producing AMC’s , in which graphite and albite particulates have been
used as the candidate reinforcements of particulate sizes ranging from 100 to 150 μm and
added to the vortex formed in the AL6061 melt above its liquidus temperature. Since the
ductility, ultimate tensile strength (UTS), compressive strength, Young's modulus and
hardness of the composite material are all vital properties of a structural material, the present
investigation aims at studying these properties in the AL6061 alloy/ particulate composites.
2. EXPERIMENTAL
2.1 Preparation of Composites
In the present investigation, the materials tested were MMCs based on aluminium alloy 6061
(containing 0.4 per cent Mg and 0.75 per cent Si) and containing graphite particulates of
size 50 microns. The AL6061-graphite composites were prepared by the vortex method [9].
The graphite contents used for the preparation of the composites were 0%, 1%, 2% and 4%.
This is because graphite compositions of 7% and above would lead to rejection from the melt
[16]. Addition of graphite into the molten aluminium alloy melt above its liquidus
temperature of 5000C was carried out by creating a vortex in the melt using a mechanical
stainless steel stirrer coated with aluminite (to prevent migration of ferrous ions from the
stirrer material into the aluminium alloy melt). The melt was rotated at a speed of 500 rpm in
order to create the necessary vortex. The graphite particles were preheated to 400°C and
added to the melt through the vortex at the rate of 0.1 kg/min. A small amount of magnesium,
96 A. Ramesh, J. N. Prakash, A. S. Shiva Shankare Gowda and Sonnappa Appaiah Vol.8, No.2
which improves the wettability of the graphite particles, was added along with the graphite,
and the melt was thoroughly stirred and subsequently degassed by passing nitrogen through
at the rate of 2-3 l/min. The molten metal was then poured into permanent moulds for casting.
AL6061-Albite composites were produced using the same technique as used for graphite
reinforced composites. The reinforcement used was albite (NaAlSi3O8) particulates of
diameter 90-150 μm, which is naturally occurring plagioclase feldspar. Basically consisting
of silicates, it is abundantly available in the Earth's crust. Albite ranges from white to dark
grey in colour and is extremely wear resistant, having a Moh hardness of about 6.5, almost
rivaling that of SiC but exceeding that of alumina. Its specific gravity is about 2.6, which is
much lower than those of SiC (3.1) and alumina (4.0). It does not react with the matrix
material in this case, even at elevated temperatures, and has a low coefficient of thermal
expansion of 2.3 x 10-6 K-1. Such properties, coupled with its relative lightness, make albite a
superior substitute for alumina and SiC as reinforcement material in MMCs. In the present
work, the weight fraction of albite particulates used for making the MMCs was in the range
of 0-4 per cent. In this technique, the albite particulates were slowly added into the vortex
created in the molten aluminium alloy 6061 by stirring it with an impeller at 500 rpm. The
melt containing the reinforcement was then poured in to permanent moulds for casting. The
unreinforced matrix material was cast in the same manner, except that no albite particulates
were added. The aluminium-alloy based composites containing various albite and graphite
contents, namely 1, 2, 3 and 4 wt %, were fabricated and tested, and their properties were
compared with those of the unreinforced matrix. The tensile fracture specimens (Figures 3
and 4) were investigated by scanning electron microscopy (SEM) using a JEOL JSM 5410
instrument
2.2 Testing of Specimens
All tests were conducted in accordance with ASTM standards. Tensile tests were conducted
at room temperature using a universal testing machine (UTM) in accordance with ASTM
Standard E 8-82. The tensile specimens of diameter 8.0 mm and gauge length 75 mm were
machined from the cast composites with the gauge length of the specimens parallel to the
longitudinal axis of the castings. For each composite, four tensile test specimens were tested
and the average values of the UTS, Young's modulus and ductility (in terms of percentage
elongation) were measured. The hardness tests were conducted in accordance with ASTM
Standard E 10 using a Brinell hardness tester with a ball indenter of 2.5 mm diameter and a
load of 31.25 kg. The load was applied for 30 secs. Eight hardness readings were taken for
each specimen at different locations to circumvent the possible effects of particle segregation.
Compression tests were conducted on a UTM in accordance with ASTM Standard E 9 at
room temperature. In this test the compression loads were gradually increased and the
corresponding strain was measured until the specimen failed. Each result is an average of
four readings.
Vol.8, No.2 Comparison of Mechanical Properties 97
3. RESULTS AND DISCUSSIONS
Table 1 and 2 shows the results for ductility (in terms of percentage elongation), ultimate
tensile strength (UTS), compressive strength, hardness and Young's modulus obtained for the
composites containing various amounts of graphite and albite. Each value represented is an
average of six measurements. The results are repeatable in the sense that each individual
result did not vary more than 5% from the mean value.
All these results are represented graphically in Figures 4-14. The optical micrographs Figure
1 and Figure 2 shows the graphite reinforced AL6061/Graphite composite and Albite
reinforced AL6061/Albite composite at low and higher magnifications. The microstructures
of the composites shown differ only in the degree of reinforcing particulates clustering;
localized enrichment of particulates is visible in the matrix.
Figure 1. Micrograph showing graphite particle distribution in AL6061/ Graphite
composites
Figure 2. Micrograph showing Albite particle distribution in AL6061/ Albite
composites
98 A. Ramesh, J. N. Prakash, A. S. Shiva Shankare Gowda and Sonnappa Appaiah Vol.8, No.2
Figure 3. Tensile Fractured SEM image of
4% by weight Graphite reinforced AL6061
based MMC
Figure 4. Tensile Fractured SEM image of
4% by weight Albite reinforced AL6061
based MMC
3.1 Ultimate Tensile Strength
Figure 5 is a graph showing the effect of reinforcement content on the UTS of cast
AL6061/graphite and AL6061/Albite particulate composites. It can be seen that as the
graphite content increases, the UTS of the composite material increases monotonically by
significant amounts. In fact, as the graphite content is increased from 0% to 4%, the UTS
increases by about 25%. These results are in accordance with those obtained by Pillai [17]
who reported similar findings. This increase in UTS may be due to the graphite particulates
acting as barriers to dislocations in the microstructure [18]. One great advantage of this
dispersion-strengthening effect is that it is retained even at elevated temperatures and for
extended time periods because the particles are unreactive with the matrix phase [19].
Table 1: Mechanical Properties of AL6061-Graphite composites containing varying
amounts of Graphite
Graphite
particulate
content
(%)
UTSMpa Brinell
Hardness
(HB)
Young’s
modulus
Gpa
Ductility(%
elongation)
Ultimate
Compression
Strength
(Mpa)
0 153.90 51.41 71.47 5.90 743.03
1 173.80 44.58 73.65 7.31 847.90
2 181.20 40.64 76.34 8.47 946.10
3 182.10 39.98 77.10 8.60 1028.72
4 191.65 37.38 78.02 9.40 1067.21
Figure 5 also shows the effect of the albite particulate content on the UTS of the composites.
It can be seen that the UTS of the composite increases monotonically by over 30 per cent as
the albite particulate content is increased from 0 to 4 wt%. The increase in UTS can be
attributed to the presence of hard albite particulates that impart strength to the matrix alloy,
thereby providing enhanced resistance to tensile stresses. There is a reduction in the
interspatial distance between the hard albite particulates, which causes an increase in the
dislocation pile-up as the particulate content is increased. This leads to a restriction in the
Vol.8, No.2 Comparison of Mechanical Properties 99
plastic flow due to the random distribution of the particulates in the matrix, thereby providing
enhanced tensile strength to the composites. Vogelsang et al. [5], believed that the
improvement in the UTS may be due to the matrix strengthening following a reduction in
composite grain size, and the generation of a high dislocation density in the matrix as a result
of the difference between the thermal expansion coefficients of the metal matrix and the
albite particulate reinforcement.
Table 2: Mechanical Properties of AL6061-Albite composites containing varying
amounts of Albite
Albite
particulate
content
(%)
UTSMpa Brinell
Hardness
(HB)
Young’s
modulus
Gpa
Ductility(%
elongation)
Ultimate
Compression
Strength
(Mpa)
0 153.9 51.41 71.47 5.90 743.03
1 178.17 72.22 80.71 5.25 753.41
2 182.34 73.57 85.47 4.57 757.5
3 184.21 75.60 88.00 4.10 767.86
4 201.34 99.10 99.95 3.95 780.01
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0123
Percentage of Albite and Graphite
content
Percentage variation in UTS(M pa)
4
AL6061/GRAPHITE
AL6061/ALBITE
153.9
173.8 181.2 182.1
191.65
153.9
178.17 182.34 184.21
201.34
0
50
100
150
200
250
01234
Percentage of reinfor c m ents
UTS(Mpa)
AL6061/GRAPHITE
AL6061/ALBITE
Figure 5. Effect of the albite and graphite
content on UTS.
Figure 6. Bar graph of UTS values of
AL6061/Graphite and AL6061/Albite metal
matrix composites.
Similar observations were made by Ghosh and Ray [20], who fabricated Al203particulate-
reinforced aluminium alloy composites using the compo-casting method and by McCoy et al.
[21] who produced TiB2-particulate-reinforced aluminium alloy composites with particulate
contents ranging from 10 to 25 vol %. An increase in the UTS was observed as the TiB2
100 A. Ramesh, J. N. Prakash, A. S. Shiva Shankare Gowda and Sonnappa Appaiah Vol.8, No.2
content was increased from 10 to 18 vol %. One significant advantage of this dispersion-
strengthening effect is that it is retained even at elevated temperatures and for extended
periods of time because the albite particulates do not react with the matrix phase [19]. It is
noteworthy that the most spectacular increase in the UTS occurs when the albite particulate
content is increased from 0 to 1 wt %. Quantitatively, this increase in the UTS is over 15 per
cent. Subsequent equivalent additions of albite particulates, however, do not result in such
great increases in the UTS. The bar graph (Fig. 6) shows the comparative changes that the
albite and graphite reinforcements have on the tensile strength on the matrix alloy AL6061.
3.2 Hardness
Hardness, which is described as a measure of a material's resistance to surface indentation,
may be thought of as a function of the stress required to produce some specific types of
surface deformation. Figure 7 is a graph showing the effect of graphite and albite
reinforcements on the hardness of cast AL6061/graphite and AL6061/Albite particulate
composites. It can be seen that as the graphite content increases, the hardness of the
composite material decreases monotonically by significant amounts. In fact, as the graphite
content is increased from 0% to 4% the hardness decreases by about 27%. One would have
expected that as the UTS of a material increases, so would the hardness, as is very evident for
the common engineering metals like steel, and indeed for many composites too. Nevertheless,
the opposite is seen in this particular composite, whereby the hardness drops as the UTS
increases. There is a good reason for this phenomenon, though, since graphite, being a soft
dispersoid, does not contribute positively to the hardness of the composite. Instead, as
mentioned above in the case of ductility, the graphite added, being an effective solid lubricant
[22,8-11], eases the movement of grains along the slip planes, rendering the material more
easily deformable under the indenter of the hardness tester. K.H.W Seah et al., [23] have
reported a reduction in hardness from 107 BHN to 77 BHN (about 28% differences) on
addition of similar weight percentages of graphite to ZA-27 (Zinc Aluminium) alloy. Such a
monotonic decrease in the hardness of the composite as graphite content is increased poses a
limit to how much graphite may be added to enhance its other mechanical properties, since
hardness is directly related to wear resistance. Consequently, a compromise is necessary
when deciding how much graphite should be added to enhance the ductility, UTS,
compressive strength, and Young's modulus of the composite without sacrificing too much of
its hardness, especially in components like engine bearings, pistons, piston rings and cylinder
liners, in which wear resistance is of paramount importance.
Figure 7 is a graph showing the effect of percentage in hardness, with increase in albite
particulate content on the hardness of the composites. It is evident that, as the percentage of
albite particulates is increased from 0 to 4 wt %, the hardness of the composite increases
monotonically and significantly to almost twice its original value. Zhu and Liu [24] also
observed a similar increase in hardness when ZA alloy is reinforced with short alumina
fibres. Various other researchers have also reported that the addition of hard ceramic
particulates or short fibres to metal alloys could lead to improved strength, wear resistance
and hardness [6-7]. A similar effect was observed by Sood et al. [25] for TiC reinforced
aluminium alloy MMCs. They found that the hardness linearly increases with increasing
volume percentage of TiC. The increase in hardness is to be expected since it is observed that
the increased UTS of most engineering materials such as steel and many composites lead to
an increased hardness. Albite particulates, being hard (6 on the Moh scale), exhibit a greater
resistance to indentation by the hardness tester and hence enhanced hardness since the
hardness, after all, it is a measure of the resistance of a material to surface indentation and is a
Vol.8, No.2 Comparison of Mechanical Properties 101
function of the stress required to produce some specific type of surface deformation [26]. The
increased hardness is also attributed to the fact that the hard albite particulates act as barriers
to the movement of the dislocations within the matrix.
As in the case of the UTS described above, the most spectacular increase in the hardness
occurs when the albite particulate content is increased from 0 to 1 wt %. Quantitatively, this
increase in hardness is more than 40 per cent. Subsequent equivalent additions of albite
particulates, however, do not result in such great increases in hardness. The bar graph (Fig 8)
shows the comparative changes in hardness values, the albite and graphite reinforcements
have on the on the matrix alloy AL6061.
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
01234
Percentage of Albite and Graphite
content
Percentage variation in Hardness in BHN
AL6061/GRAPHITE
AL6061/ALBITE
51.41
44.58 40.6439.98 37.38
51.41
72.22 73.57 75.6
99.1
0
20
40
60
80
100
120
01234
Percentage of reinforcments
Hardness in BHN
AL6061/GRAPHITE
AL6061/ALBITE
Figure 7. Effect of the albite and graphite
content on Hardness
Figure 8. Bar graph of hardness values of
AL6061/Graphite and AL6061/Albite
metal matrix composites.
3.3 Young’s Modulus
Figure 9 is a graph showing the percentage variation in Young’s modulus and the bar graph
(Figure 10) provides a comparative insight in to the actual experimental results recorded for
the AL6061 albite and graphite particulate reinforced composites. The effect of graphite
content on the Young's modulus of cast AL6061/graphite particulate composites. As in the
cases of ductility, UTS and compressive strength described above, it can be seen that as the
graphite content increases, the Young's modulus of the composite material increases
monotonically by significant amounts. In fact, as the graphite content is increased from 0% to
4%, the Young's modulus increases by about 9%. As in the cases of UTS and compressive
strength described above, this increase in compressive strength may be due to the graphite
particles acting as barriers to dislocations in the microstructure [18]. Similar results have been
obtained in aluminium matrix composites where the Young's modulus has been reported to
102 A. Ramesh, J. N. Prakash, A. S. Shiva Shankare Gowda and Sonnappa Appaiah Vol.8, No.2
increase with increase in the content of the reinforcing material, regardless of the type of
reinforcement used [27]. Nevertheless, it must be emphasized at this point that the values of
Young's modulus presented in this paper are only useful for the purpose of comparison
among themselves.
Figure 9 is a graph showing the effect of albite particulate content on Young's modulus of
cast AL6061-albite particulate composites. As in the cases of the UTS and hardness described
above, it can be seen that, as the albite particulate content increases, Young's modulus of the
composite material increases monotonically and significantly by about 40 per cent as the
reinforcement content is increased from 0 to 4 wt%. This increase in Young's modulus is
broadly in line with the rule-of-mixtures prediction. McDanels [27] who obtained similar
results for particle-reinforced aluminium composites reported that Young's modulus increases
with increase in reinforcement content regardless of the type of reinforcement used.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
01234
Percentage of Albite and Graphite
content
Percentage variation in young's modulus
AL6061/GRAPHITE
AL6061/ALBITE
71.47 73.65 76.34 77.178.02
71.47
80.71
85.47 88
99.95
0
20
40
60
80
100
120
01234
Percentage of reinforcments
Youngs Modulus in Gpa
AL6061/GRAPHITE
AL6061/AL B ITE
Figure 9. Effect of the albite and graphite
content on Young’s Modulus
Figure 10. Bar graph of hardness values of
AL6061/Graphite and AL6061/Albite metal
matrix composites.
3.4 Ductility
Figure 11 is a graph showing the effect of graphite content on the ductility of cast
AL6061/graphite particulate composites. It can be seen that, as in the case of UTS described
above as the graphite content increases, the ductility of the composite material increases
monotonically by significant amounts. In fact, as the graphite content is increased from 0% to
4% the ductility increases by about 60%. This considerable increase in ductility is due to the
graphite additions, being an effective solid lubricant [22, 8-11], eases the movement of grains
along the slip planes. The effect of graphite is expected to be mechanical in nature since the
particles are unreactive with the matrix phase [19].
Vol.8, No.2 Comparison of Mechanical Properties 103
Figure 11 is a graph showing the effect of albite particulate content on the ductility (measured
in terms of percentage elongation) of the composites. It can be seen from the graph that the
ductility of the composites decreases monotonically and significantly with the increase in
albite particulate content. The ductility drops by about one-third as the albite particulate
content is increased from 0 to 4 wt %. These results tally with those obtained by other
researchers [29, 24] who also observed that the ductility of the composites decreases with
increase in the reinforcement content. This decrease in ductility in comparison with the
matrix alloy is a most commonly encountered disadvantage in discontinuously reinforced
MMCs [30].
The reduction in ductility can be attributed to the presence of a hard ceramic phase that is
prone to localized crack initiation and increased embrittlement effect due to local stress
concentration sites at the reinforcement-matrix interface. Hence, the introduction of this hard
secondary ceramic phase creates slip regions. Moreover, the reinforcing particulates resist the
passage of dislocations either by creating stress fields in the matrix or by inducing large
differences in the elastic behaviour between the matrix and the dispersoid. The bar graph (Fig
12 ) shows the comparative changes in ductility , the albite and graphite reinforcements have
on the on the matrix alloy AL6061.
-0.4
-0.2
0
0.2
0.4
0.6
0.8
01234
Percentage o f Albite and Graphite content
Percentage variation in ductility
AL6061/GRAPHITE
AL6061/ALBITE
5.9
7.31
8.47 8.6
9.4
5.9
5.25
4.57
4.1 3.95
0
1
2
3
4
5
6
7
8
9
10
01234
Percentage of reinforcments
Dutility in %
AL6061/GRAPHITE
AL6061/ALBITE
Figure 11. Effect of the albite and graphite
content on Ductility
Figure 12. Bar graph of ductility values of
AL6061/Graphite and AL6061/Albite metal
matrix composites.
Mummery et al. [30] are of the opinion that this loss in ductility is probably due to the voids
which nucleate during the plastic straining of the reinforcement. As in the cases of the UTS,
hardness and Young's modulus described above, the embrittling effect of albite particulates is
expected to be mechanical in nature since the inert albite particulates do not react with the
matrix phase [19].
104 A. Ramesh, J. N. Prakash, A. S. Shiva Shankare Gowda and Sonnappa Appaiah Vol.8, No.2
3.5 Compression Strength
Figure 13 is a graph showing the effect of graphite content on the compressive strength of
cast AL6061/graphite particulate composites. As in the cases of ductility and UTS described
above, it can be seen that as the graphite content increases, the compressive strength of the
composite material increases monotonically by significant amounts. In fact, as the graphite
content is increased from 0% to 4%, the compressive strength increases by about 44%. As in
the case of UTS described above, this increase in compressive strength may be due to the
graphite particles acting as barriers to dislocations in the microstructure [18]. Once again, this
dispersion-strengthening effect is expected to be retained even at elevated temperatures and
for extended time periods because the particles are unreactive with the matrix phase [19].
Figure 13 is a graph showing the effect of the albite particulate content on the compressive
strength of the composites. It can be seen that the compressive strength of the composite
increases monotonically by about, 5 per cent as the albite particulate content is increased
from 0 to 4 wt %.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0123
Percentage of Albite and Graphite content
Percen ta g e va riatio n in c o m p res s ion s tren g th
4
AL6061/GRAPHITE
AL6061/ALBITE
743.03
847.9
946.1
1028.72
1067.21
743.03 753.41 757.5 767.86 780.01
0
200
400
600
800
1000
1200
01234
Percentage of reinforcments
Ultimate Compression Strength in Mpa
AL6061/GRAPHITE
AL6061/ALBITE
Figure 13. Effect of the albite and graphite
content on Ultimate Compression Strength
Figure 14. Bar graph of UCS values of
AL6061/Graphite and AL6061/Albite metal
matrix composites.
Similar results were observed by earlier researchers such as Webster [31] and Awerbuch et al.
[13] when they conducted tests on whisker-reinforced composites. Towle and Fried [32]
compared the compressive and tensile properties of magnesium-based MMCs and observed
similar trends. The increase in compressive strength is chiefly due to the decrease in the inter-
particle spacing between the albite particles, since albite is much harder than aluminium alloy
6061. The presence of the albite particles resists deforming stresses, thus enhancing the
compressive strength of the composite material. The increase in compressive strength is not
as spectacular as those seen in the UTS, hardness and Young's modulus because the
compressive strength of the unreinforced matrix material itself is already very high, in fact
Vol.8, No.2 Comparison of Mechanical Properties 105
several times the UTS. There is only a very marginal increase in the compressive strength
when a secondary ceramic phase is introduced. Nevertheless, the addition of hard ceramic
particulates has caused the MMCs to behave as brittle rather than ductile materials, as is
evident from the above results. The bar graph (Fig 14) shows the comparative changes in
UCS, the albite and graphite reinforcements have on the on the matrix alloy AL6061.
4. CONCLUSIONS
The mechanical properties of the cast AL6061 /graphite particulate composites are
significantly changed by varying the amount of graphite therein. It was found that increasing
the graphite content within the aluminum matrix results in significant increases in the
ductility, UTS, compressive strength and Young's modulus, but a decrease in the· hardness. A
compromise is necessary when deciding how much graphite should be added to enhance the
mechanical properties of the composite without sacrificing too much of its hardness and
hence its wear resistance
The mechanical properties of the cast aluminium alloy 6061-albite particulate composites are
significantly altered by varying the amount of albite particulates therein. It was found that
increasing the albite particulate content within the aluminium alloy 6061 matrix results in
significant increases in the UTS, hardness and Young's modulus but a decrease in the
ductility. There is only a slight improvement in its compressive strength. A compromise is
necessary when deciding the amount of albite particulates to be added to the aluminium alloy
matrix to enhance the UTS, hardness and Young's modulus of the composite without
sacrificing too much of its ductility, bearing in mind also that the improvement to its
compressive strength is only very marginal
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