Journal of Minerals & Materials Characterization & Engineering, Vol. 11, No.2 pp.159-168, 201 2
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159
Thermal, Hardne ss and Microstr uct ural Char acterization of
Al-Si-SiCp Composites
Alo, O. A., Umoru, L. E., Ajao, J. A.* and Oluwasegun K. M.**
Department of Materials Science and Engineering, O.A.U., Il e -Ife. Nigeria.
*Center for Energy, Research and Development, Obafemi Awolowo University, Ile-Ife.
Nigeria.
**Department of Materials and Metallurgy, University of Birmingham, B15 2TT, United
Kingdom.
Abstract
This study investigated the effects of silicon and silicon carbide particles contents on the
thermal, hardness and microstructural behaviour of Al-Si-SiCp composites. 16 samples of the
composite produced by stir casting technique were of silicon contents of 1, 2, 3 and 4% by
weigh, and silicon carbide contents of 0.5, 1, 1.5 and 2% by weight for each composition of
silicon. Each of the samples were subjected to homogenizing annealing heat treatment.
Differential thermal analysis (DTA), hardness test and microstructural analysis were then
performed on the samples from each composition. The results obtained showed that the
hardness of the composite increased gradually as the silicon and silicon carbide particles
content increased. The micrographs obtained revealed the presence of silicon carbide, silicon
precipitates and aluminium carbide (Al 4C3) within the metallic matrix. The amounts of these
phases varied with the silicon and silicon carbide content. All the samples gave DTA curves
with major endothermic peaks between 550 – 570 oC and two sets of exothermic peaks
between 580 – 610oC for the first set and between 565 – 570oC for the second set. It was
inferred from the study that although varied silicon and silicon carbide contents affected the
thermal, hardness and microstructural behaviour of the Al-Si-SiCp composites, the variation
of the SiCp content had a more pronounced effect on the hardness value of the Al-Si-SiCp
composite.
1. INTRODUCTION
A metal matrix composite (MMC) is a composite material with at least two constituent parts,
one being a metal. The other material may be a different metal or another material, such as
160 Alo, O. A., Umoru, L. E., Ajao, J. A. Vol.11, No.2
ceramic or organic compound (Kelly and Davies, 1965). Metal-matrix composites (MMCs)
are of interest today because the y offer opportunity to tailor a material with a combination of
properti es unavail able in an y single material. For instance, combining various types of fibres
of very high tensile strength and modulus of elasticity with the low density metals such as
aluminium, titanium or magnesium often lead to a composite material with a higher strength-
to-wei ght or modulus-to-weight ratio than any single known alloy. Despite the fact that
aluminium and its alloys have found remarkably wide applications in the field of engineering,
a major limitation to their applicability is their poor wear resistance property and the inability
to retain their strength under high temperature service conditions (Pay and Deborah, 1995).
This is probably responsible for the development and characterization of various aluminium
based composites using various reinforcements that can impart good wear resistance and
improved high temperature strength.
By carefully controlling the relative amount and distribution of the ingredients of a
composite, its properties can be further improved. Hot pressed Zr2(Al(Si))4C5/SiC
composites containing 0 30 vol.% silicon carbide (SiC) particles have been reported to have
improved hardness value, flexural strength, and fracture toughness with increasing SiC
volume contents (Guiqing et al., 2010). Furt hermore, it has been reported that in Al–Si
Fe/SiC particulate composites, the addition of silicon carbide reinforcement increased the
hardness values and apparent porosity by 75 and 39%, respectively. However, density and
impact energy decreased by 1.08 and 15% respectively, with increased SiC content in the
alloy (Aigbodion, and Hassan, 2006).
Due to the wide industrial application of Al-Si allo ys, the effect of varying t he amounts of Si
and SiC on the thermal, hardness and microstructural properties of the alloys has been
investigated in this study.
2. EXPERIMENTAL DETAILS
The major materials used during the course of the stud y are aluminium scraps, sili con, silicon
carbide pa rticles and etch ant. The Al-Si-SiCp compos ite materi al was pre pared at the F ederal
Institute of Industrial Research Oshodi (FIIRO), Lagos, Nigeria, by stir casting method.
Sixteen different compositions of the composite was cast by varying the silicon and silicon
carbide particle (600 grits) contents. The silicon contents were 1, 2, 3 and 4% by weights, and
for each composition of silicon, the silicon carbide content was varied from 0.5 to 2.0% by
weight at interval of 0.5% wt. The melted composite materials were then cast into cylindrical
rods. The furnace charge calculations are as shown in Table 1.
The cast samples were annealed in order to homogenize the composition. They were heated
in an OMSZOV electrical furnace which was set to a temperature of 420oC. They were then
soaked at this temperature for five hours and furnace cooled.
Vol.11, No.2 Thermal, Hardness and Microstructural Characterization 161
The spectrometric analysis of the Al-Si-SiCp composites was carried out at the quality
control department of Manaksia Industries Nigeria Limited, Sango Ota. The analysis was
carried out to determine the final elemental composition of the composites, after stir casting.
Table 1 : Furnace Charge Calculation for AlSi-SiCp Composite with 4% Silicon
Composite Al(kg) Si (kg) SiC(kg) Tm(kg)
Al -4%Si – 0.5%SiC
p
Al -4%Si – 1.0%SiCp
Al -4Si – 1.5%SiCp
Al -4%Si – 2.0%SiCp
1.4325
1.4625
1.4175
1.4100
0.060
0.060
0.060
0.060
0.0075
0.0150
0.0225
0.0300
1.5
1.5
1.5
1.5
The differential thermal analysis was carried out using the NETZCH DTA404PC differential
thermal analyser. Each of the samples was put in the sample holder, and the sample and an
inert reference material were made to undergo identical thermal cycles by heating to a
temperature of 650oC and then cooling to room temperature at a rate of 10oCmin-1. The
differential temperature and temperature signals were then recorded and the plot of ∆T
against T was obtained by using a computerized data processing unit.
Samples of the composite materials were subjected to hardness test, using the Brinell
Hardness test accessory of the Monsanto Hounsfield Tensometer. Also parts of the heat
treated cas t samp les wer e cut t o sui table siz es. The samples were ground and then polished to
a mirror-like surf ace. The prepared polished surface o f each sample was etched with sodium
hydroxide solution (10 g NaOH dissolved in 90 ml water), immersed in the etchant for 20
seconds, and then dipped in concentrated HNO 3 to remove stains. The etched samples were
mounted on an ACCUSCOPE metallographic microscope and examined using a
magnification of X400. Attached to the microscope was an ocular camera and a computer
system through which the micrographs were viewed and captured.
3. RESULTS AND DISCUSSION
The results of our investigation have been compiled in three subsections 3.1, 3.2 and 3.3 with
respect to the Differential Thermal Analysis, hardness and microstructure, respectively. The
elemental composition of the samples with 2% silicon is presented in Table 2.
Table 2: The Chemical Composition of the Al-Si-SiCp Composite Containing 2%Silicon
Element Al Si Fe Ti Zn
Composition(%) 95.900 3.808 0.111 0.061 0.014
162 Alo, O. A., Umoru, L. E., Ajao, J. A. Vol.11, No.2
3.1 Differential Thermal Analysis (DTA)
The DTA curves obtained from the differential thermal analysis are presented in Figure 1.
The thermograms show plots of ∆T against T for each composite composition. The red
curves (dark background) show the heating process while the purple curves (lighter
background) show the cooling process. All the composites gave two exothermic peaks on
cooling. However some gave one endothermic peak while others gave more than one. In most
of the sam ples, t he major endotherm ic peak and the fir st exot hermic peak occur b etween 58 5
- 595oC.
Al–Si-SiCp composites containing 1% silicon and SiC p contents ranging from 0.5 to 2.0% all
gave a major endothermic peak each on heating with onsets between 550 and 560oC.
However, samples of the same silicon content betw een 0.5% and 1.5% SiCp exhibited two
other small endothermic peaks with onsets at 555 and 605oC, respectively. Al–Si-SiCp
composite containing 2% silicon and SiCp contents ranging from 0.5 to 2.0% all gave a
major endothermic peak with onsets at about 550oC (Fig. 1). However, sample of this silicon
content and 2.0% SiCp also gave two other small endothermic peaks with onsets at 555oC and
615oC (Fig. 1d). Al–Si-SiCp composite containing 3% silicon and SiCp cont e n t s r an gi ng f rom
0.5 to 2.0% all gave a major endothermic peak with onsets between 555oC – 575oC.
However, samples of this silicon content and 1.0% and 1.5% SiCp also both gave two other
endothermic peaks with onsets at 555oC and 610oC respectively while that with 2.0% SiCp
also gave two endothermic peaks at 555oC and 595oC. Al–Si-SiCp composite containing 4%
silicon and SiC p contents ranging from 0.5 to 1.5% all gave a major endo thermi c peaks wit h
onsets at about 560oC. Also, these samples all gave two other endothermic peaks with onsets
at 555oC and 600oC.
Al–Si-SiCp composite containing 1% silicon and SiCp contents ranging from 0.5 to 2.0%
gave the first exothermic p eaks with onsets between 590 - 610oC , and th e seco nd ex othermi c
peaks with onsets between 565 - 570oC. Al–Si-SiCp composite containing 2% silicon and
SiCp contents ranging from 0.5 to 2.0% gave the first exothermic peaks with onsets between
580 - 590oC, and the sec ond exothermic peaks with onsets between 565 – 570oC (Fi g. 1 ). Al–
Si-SiCp composite containing 3% silicon and SiCp contents ranging from 0.5 to 2.0% gave
the first exothermi c peak s with onsets between 585 - 610oC, and the second exothermic peaks
with onsets between 565 – 570oC. Al–Si-SiCp composite containing 4% silicon and SiCp
contents ranging from 0.5 to 1.5% gave th e firs t ex othermic peaks with onsets between 605
610oC, and the second exothermic peaks with onsets between 565 – 570oC.
Vol.11, No.2 Thermal, Hardness and Microstructural Characterization 163
100 200 300 400 500 600
Tem perature /°C
-2
-1
0
1
2
3
DSC /(mW /mg)
[1.2]
[1.4]

exo
a: Composite with 0.5% SiCp
100 200 300 400 500 600
Tem perature /°C
-2
-1
0
1
2
3
DSC /(mW /mg)
b: Composite with 1.0% SiCp
100 200 300 400 500 600
Tem perature /°C
-3
-2
-1
0
1
2
3
4
DSC /(mW /mg)
[1.2]
[1.4]

exo
c: Composite with 1.5% SiCp
100 200 300 400 500 600
Tem perature /°C
-2
-1
0
1
2
3
DSC /(mW /mg)
d: Composite with 2.0% SiCp
Fig. 1 Thermograms for Al-Si-SiCp Composites with 2% Silicon Content
The peaks on the DTA curves indicate phase transformations in the composite samples
during the heating and cooling cycles. The phases formed during these transformations can be
obtained using the aluminium-silicon, aluminium-carbon and silicon-carbon binary phase
diagrams. All the samples gave DTA curves with a major endothermic peak during heating
with onsets between 550 – 570oC. This could be due to formation of f.c.c Al(α) phase.
All the samples gave two exothermic peaks during cooling. The first exothermic peaks with
onsets between 580 – 610oC, could be attributed to the formation of solid solution of Al(α)
and Al4C3 while the second peaks with onsets between 565 – 570oC, could be linked with the
solidification of f.c.c Al(α) phase.
A close look at the DTA curves revealed that composites with 2% silicon content gave
thermograms with single endothermic peaks. Composites with 3% silicon content gave
thermograms with three endothermic peaks while all the composites with 4% silicon content
gave thermograms with multiple endothermic peaks. Therefore, the silicon contents of the Al-
Si-SiCp composite could be said to have affected the number of peaks obtained in the DTA.
3.2 Hardness
164 Alo, O. A., Umoru, L. E., Ajao, J. A. Vol.11, No.2
The resul ts of the Brinell h ardness tes ts are pres ented in Fi gures 2 and 3. Figure 2 shows the
variation of hardness values of Al-Si-SiC composites with varying SiC contents, while Figure
3 shows the variation of hardness values of Al-Si-SiC composites with varying silicon
contents. Observation revealed that for all the composites produced, hardness values varies
with increase in SiCp content (Fig.2) and silicon content (Fig.3). For a particular silicon
content, hardness of AlSi-SiCp composite increased gradually when the SiCp content was
increased from 0.5 to 2.0% (Fig. 2). Also, for a particular SiCp content, hardness of the
composite increased gradually when the silicon content was increased from 1 to 4%. (Fig. 3).
The slopes of the graphs of hardness values against the SiCp and silicon contents were
obtained from the equations of the trend li nes of variation of hardness wit h S iCp content s an d
silicon contents, respectively, with a view to determining which of Si and SiC p has produced
more effect on th e hardness of the composite matrix . Table 3 shows the slopes of the graphs
of hardness values against the SiC P contents wh ile Table 4 shows the slopes of the graphs of
hardness values against the silicon contents. The tables reveal that increase in SiC p content
has a more positive effect on the hardness values of the Al-Si-SiCp composite than increase
in silicon content.
As observed in Figure 2, the hardness value increases as the silicon carbide particle content
increases. This agrees with the results obtained by Aigbodion and Hass an (2006) in a study to
invest i gate th e eff ects of sil icon carbide reinforcement on the microstructure and properties of
cast Al–SiFe/SiC particulate composites. The increase i n hardness values is attributed to the
distribution of hard and brittle ceramic phases in the ductile metal matrix. The
microstructures obtained reveal a dark ceramic and light metal phases, which probably
resulted to increase in the dislocation density at the particle – matrix interfaces.
Fig. 2 Variation of Brinell Hardness of Al–Si-SiCp composite with SiCp Content
Vol.11, No.2 Thermal, Hardness and Microstructural Characterization 165
Fig. 3. Variation of Brinell Hardness of Al–Si-SiCp composite with Silicon Content
Table 3: Slopes of graphs of hardness value against SiCp contents
Silicon Content(%)
Slope
1
2
3
4
6.036
7.904
7.972
9.124
Averag e( m1)
7.759
Table 4: Slopes of graphs of hardness value against Silicon contents
SiC Content(%) Slope
0.5
1.0
1.5
2.0
3.019
3.114
3.583
4.418
Averag e( m
2
)
3.5335
In the hardness test, severe plastic flow has been concentrated in the localized region directly
below the indentation, outside of which material still behaves elastically. Directly below the
indentation, the density of the particles increased locally, compared to the regions away from
the depression (Manoj et al., 2009).
As the indenter moves downward during the test, the pressure has been accompanied by non
uniform matrix flow along with localized increase in particle concentration, which tends to
increase the resistance to deformation. Consequently, the hardness value increases due to
local increase in particle concentration associated with the indentation.
Also, the in crease i n the h ardness v alue o f the co mp osit es as th e sil icon con tents increase can
be attributed to the fact that silicon is a harder phase than aluminium. Tables 3 and 4 show
that increase in SiC p content has a more pronounced effect on the ha rdness values of the Al-
Si-SiCp composite than increase in silicon content. This can be attributed to the fact that
silicon carbide is harder than silicon.
166 Alo, O. A., Umoru, L. E., Ajao, J. A. Vol.11, No.2
3.3 Microstructure
The micrographs of the cast Al–Si-SiCp composites are shown in Plates 1a through 1d. The
Plates depict micrographs essentially consisting of SiC particles dispersed in a matrix of
primary α Al dendrites and interdendritic α Al + Si eutecti c. Othe r phas es sus pected to be
present are silicon and Al4C3 which are formed as a result of the reaction between molten
aluminium and SiC according to the equation:
a: Composite wit h 0.5% SiCp
b: Composite with 1.0% SiCp
c: Composite with 1.5% SiCp
d: Composite with 2.0% SiCp
Plate 1: Micrographs of Al–Si-SiCp composites with 1% silicon after etching with NaOH
solution X40 (Dark region represents SiC, while light region represents α Al)
Vol.11, No.2 Thermal, Hardness and Microstructural Characterization 167
Plate 1 shows microstructures of cast Al–Si-SiCp composites with 1% silicon content and
SiCp contents var ying from 0.5 to 2.0%. The mic rographs reveal homogenous di stribution of
SiC particles in the aluminium matrix in composites with 0.5 and 1.0% SiC. However, some
clustering of SiC were observed in composites with 1.5 and 2.0% SiC.
The micrographs of AlSi-SiCp composites with 1% silicon content and 0.5 to 1.0% SiCp
are characterized b y homogenous distribution of SiC particles in the aluminium matrix(Plate
1). Therefore, the properties of the composites are expected to be isotropic. Micrographs of
Al Si SiC composites with 1% silicon content and 1.5% and 2.0% SiC show some
clustering of S iC in som e part of the composit e matrix. T his can be ex plai ned b y the fact t hat
SiC particles have a lower thermal conductivity and heat diffusivit y than those of aluminium
and therefore, may have been unable to cool down as the aluminium melt. As a result, the
temperature of the particles was somewhat higher than that of the liquid alloy. The hotter
particles may have heated up the liquid in their s urroundings, and thus de lay solidification of
the surrounding liquid alloy.
Nucleation of α Al phas e st art s i n t he l iq ui d at a di st ance away from the p art icl es, wher e t he
temperature was lower. The growth of α Al nuclei lead to enrichment of Si in the melt. The
enrichment of Si in the melt around the particles leads to heterogeneous nucleation. Another
effect of thermal lag was that the melt around the particles would solidify in the last stage.
This would make the particles located between dendrites. In other words, the interdendritic
clusters of SiC particles have been partially inherited from inhomogeneous distribution of
particles in the original slurries (Manoj et al., 2009).
The micrographs of Al–Si-SiCp composites with 2% silicon content and SiCp contents
varying from 0.5 to 2.0% show a more pronounced presence of Si precipit ates as rev ealed b y
clusters of silicon in the microstructures. This may have resulted from the chem ical react ions
between SiC and molten aluminium, which has been reported in the work of Viala et al.
(1990). The authors found that Al does not react with SiC up to its melting temperature (933
K), but molten Al reacts promptly with SiC, giving different products depending on
temperature. Between 940 and 1620 K, Al4C3 is produced according to the reaction
represented by equation 1. Also, according to Lai and Chung (1994), the reaction is only
significant at elevated temperatures, which are encountered during composite fabrication by
liquid metal infiltration or stir casting, or during remelting after composite fabrication.
The micrographs of AlSi-SiCp composites with 3 to 4% silicon and SiCp contents varying
from 0.5 to 2.0% revealed a less pronounced presence of silicon and Al4C3 precipitat es. This
may be traced to the fact that, introduction of silicon into the molten Al would displace
reaction 1 to the left hand side (Viala et al., 1990). The higher silicon contents in these
composites accounts for how the reaction represented by equation 1 was suppressed to a
significant level.
4Al
(l)
+ 3SiC
(s)
Al
4
C
3(s)
+ 3Si (1)
168 Alo, O. A., Umoru, L. E., Ajao, J. A. Vol.11, No.2
4. CONCLUSIONS
Based on the strength of the results obtained in the study, it can be concluded that varying the
silicon and silicon carbide contents affects the thermal, hardness and microstructural
behaviour of the Al-Si-SiCp composite. Although, increase in silicon and SiCp contents
increased the hardness value of the composite, the SiCp content has produced a more
pronounced effect on the hardness value of the Al-Si-SiCp composite than silicon.
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