G. KARTHIKEYAN, G. R. JINU: TENSILE BEHAVIOUR AND FRACTOGRAPHY ANALYSES OF LM6/ZrO2 COMPOSITES
549–553
TENSILE BEHAVIOUR AND FRACTOGRAPHY ANALYSES OF
LM6/ZrO2 COMPOSITES
OBNA[ANJE IN ANALIZA PRI NATEZNI OBREMENITVI
PRELOMA KOMPOZITOV LM6/ZrO2
Govindan Karthikeyan1, Gowthanithankachi Raghuvaran Jinu2
1University College of Engineering Pattukottai, A Constituent College of Anna University Chennai, Department of Mechanical Engineering,
Rajamadam, Pattukottai-614701, Tamilnadu, India
2University College of Engineering, Department of Mechanical Engineering, Nagercoil-629004, Tamilnadu, India
p_gkarthikeyan@yahoo.co.in, gr_jinu1980@yahoo.com
Prejem rokopisa – received: 2015-10-21; sprejem za objavo – accepted for publication: 2016-06-15
doi:10.17222/mit.2015.319
Aluminium-based metal matrix (LM6) composite materials were prepared using various percentages of 0 %, 3 %, 6 %, 9 % and
12 % of zirconia by the weight fraction method and fabricated by the stir casting method. Five samples were prepared as per the
ASTM B-557-M-94 standard, and then the prepared sample was subjected to an energy-dispersive spectrometry (EDS), hard-
ness and tensile tests. The hardness and tensile test results revealed that the maximum hardness, maximum ultimate strength,
yield strength was obtained for the composite containing 12 % of mass fractions of ZrO2. The percentage of increase in zirconia
increases the hardness, ultimate strength, yield strength. The minimum percentage of zirconia gives the minimum hardness and
ultimate strength. Then EDS test was conducted on prepared samples. It was confirmed that aluminium and zirconia particles
are present in the metal matrix composites. Fractography analysis was also employed in this work to understand the fracture
mechanics of the surface. The fractography analysis was conducted with a scanning electron microscope (SEM). From the
fractography analysis it was revealed that the nature of the fracture surface was changed from ductile with a large dimple surface
to brittle fracture.
Keywords: metal matrix composites, aluminium LM6 alloy, zirconia (ZrO2), tensile test, fractography
Kovinski kompoziti na osnovi aluminija (LM6) so bili pripravljeni z razli~nimi masnimi dele`i 0 %, 3 %, 6 %, 9 % in 12 %
cirkonijevega dioksida, z metodo ulivanja z me{anjem taline. Pet vzorcev je bilo pripravljenih skladno s standardom ASTM
B-557-M-94 in nato pregledanih z energijsko disperzijsko spektrometrijo (EDS), izmerjena je bila trdota in izvedeni so bili
natezni preizkusi. Iz meritve trdote in nateznih preizkusov sledi, da je bila najve~ja trdota in najvi{ja natezna trdnost dose`ena
pri kompozitu z 12 masnimi % ZrO2. Nara{~ajo~a vsebnost cirkonijevega dioksida povi{uje trdoto, natezno trdnost in mejo
plasti~nosti. Najmanj{a vsebnost cirkonijevega dioksida daje najmanj{o trdoto in najni`jo natezno trdnost. Na pripravljenih
vzorcih je bila izvedena tudi EDS analiza, ki je potrdila, da sta aluminij in delci iz cirkonijevega dioksida prisotna v kovinski
osnovi kompozita. Analiza prelomov je bila tudi uporabljena v tem delu, da bi razumeli mehaniko preloma povr{ine. Analiza
prelomov je bila izvedena z vrsti~nim elektronskim mikroskopom. Analiza prelomov je odkrila, da se narava povr{ine preloma
spreminja iz duktilnega z velikimi jamicami, v krhek prelom.
Klju~ne besede: kompoziti s kovinsko osnovo, aluminijeva LM6 zlitina, cirkonijev dioksid (ZrO2), natezni preizkus, analiza
preloma
1 INTRODUCTION
Metal matrix composites offer various advantages in
applications where high specific strength, stiffness, and
resistance are required.1 Aluminium-based composites
containing ceramic whiskers, fibers and particles have
been used for automobile parts such as pistons and
cylinder liners in engines due to their superior wear
properties.2 Stir casting techniques are the conventional
and economic way of producing AMC. But with the
conventional stir casting techniques, it is difficult to
produce a particulate reinforced composite.3 They are
usually reinforced by Al2O3, SiC, C, and in addition
SiO2, B, BN, B4C may also be considered.4 In this field,
aluminium is the most popular matrix for the metal
matrix composites. Most of the research works deal with
aluminium matrix because of their lightweight nature.
The Al alloys are quite attractive due to their low density,
good corrosion resistance, processing flexibility, high
thermal and electrical conductivity, improved elastic mo-
dulus, strength and their high damping capacity.
Different processing techniques are used for the produc-
tion of aluminium metal matrix composites. A lot of re-
searchers have concentrated on developing new compo-
sites such as Al 6061, LM25, reinforcing with various
elements such as TiB2, SiC, TiC, ZrB2, AIN, Si3N4,
Al2O3 and their tensile properties were analyzed in
earlier researchers.3-6 Very few works were identified in
analyzing the effect of ZrO2 on the tensile behavior.
Zirconia is a refractory material with a melting point of
about 2680 °C. ZrO2 possesses good properties, such as a
low coefficient of thermal expansion, good thermal
shock resistance, high melting point, low thermal con-
ductivity and excellent thermal stability. Its density,
Young’s modulus, and hardness are 5.71 g/cm3, 190 GPa
and 1200 HV, respectively.
Materiali in tehnologije / Materials and technology 51 (2017) 3, 549–553 549
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 67.017:620.172:620.173.24 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 51(3)549(2017)
No works have been identified in analyzing the effect
of ZrO2 on LM6 base material while subjected to tensile
and fractography analysis. Hence, the works mainly
concentrated on developing new composite material by
taking LM6 as the base material and varying the
percentage of ZrO2 to identify the influence of ZrO2 on
the tensile and fracture behavior. The tensile strength and
fractography analysis of these materials were also
studied. The present research is an attempt to test and
analyze the mechanical properties of LM6 reinforced
with (0, 3, 6, 9 and 12) % various by weight fractions of
ZrO2 particulate produced by the stir casting method.
2 MATERIALS AND METHODS
2.1 Experimental part
The LM6 aluminium alloy was taken as the base
material and ZrO2 powder (particle size 1–10 μm) was
chosen as the reinforcement material. The corresponding
chemical compositions are given in Table 1. In this
work, five cast samples were prepared by keeping LM6
as the base material at varying weight percentage of
ZrO2, such as 0 %, 3 %, 6 %, 9 %, and 12 %. The casting
was carried out using the stir casting process, as shown
in Figure 1. Initially, the aluminum LM6 alloy was
melted in a pot by heating in a blower furnace at 850 °C
for 15 min. The ZrO2 powder was preheated at 575 °C in
a separate muffle furnace. The furnace temperature was
raised above the liquid temperature of LM 6 at about
850 °C to melt the LM6 completely and then it is slowly
added to the preheated ZrO2 powder. The stirring was
carried out with the help of the drilling machine for
about 15 min and the stirring rate maintained a speed of
about 950 min–1. This combination was then transferred
into the mould cavity, and it is cooled to a room tem-
perature. This procedure was followed for the prepara-
tion of five specimens with different compositions of
ZrO2 as shown in Figure 1.
G. KARTHIKEYAN, G. R. JINU: TENSILE BEHAVIOUR AND FRACTOGRAPHY ANALYSES OF LM6/ZrO2 COMPOSITES
550 Materiali in tehnologije / Materials and technology 51 (2017) 3, 549–553
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Table 1: Chemical composition of aluminum LM6 alloy, in mass fractions (w/%)
Tabela 1: Kemijska sestava aluminijeve zlitine LM6 v masnih odstotkih (w/%)
Cu Mg Si Fe Mn Ni Zn Pb Sn Ti Al
0.1 0.10 10-13 0.6 0.5 0.1 0.1 0.1 0.05 0.2 Bal
Figure 1: a) Schematic view of stir casting setup, casting of MMC by
using stir casting method, b) image of stir casting setup, casting of
MMC using the stir casting method
Slika 1: a) Shematski prikaz sestava za ulivanje z me{anjem, ulivanje
MMC z uporabo metode me{anje-ulivanje, posnetek sestava za
ulivanje z me{anjem, ulivanje MMC z uporabo metode me{anje-
vlivanje
Figure 2: SEM images and EDS patterns of various composites:
a) SEM image of LM6, b) EDS pattern of LM6, c) SEM image of
LM6-3% ZrO2 composites, d) EDS pattern of LM6-3% ZrO2
composites, e) SEM image of LM6-6% ZrO2 Composites, f) EDS
pattern of LM6-6% ZrO2 composites, g) SEM image of LM6-9% ZrO2
composites, h) EDS pattern of LM6-9% ZrO2 composites
Slika 2: SEM-posnetki in EDS-analiza razli~nih sestav: a) SEM-po-
snetek LM6, b) EDS-analiza LM6, c) SEM-posnetek kompozita LM6
– 3 % ZrO2, d) EDS-analiza kompozita LM6 – 3 % ZrO2, e) SEM-po-
snetek kompozita LM6 – 6 % ZrO2, f) EDS-analiza kompozita LM6 –
6 % ZrO2, g) SEM-posnetek kompozita z 9 % ZrO2, h) EDS-analiza
kompozita LM6 – 9 % ZrO2
2.2. SEM-EDS analysis of LM6/ZrO2 composite speci-
men
Energy-dispersive spectrometry analyses are applied
in the present work, which helps to find out information
of the type of element present in the samples. From the
above Figures 2b, 2d, 2f and 2h indicate the energy-dis-
persive spectrum of aluminium LM6 with their elements.
The Figures 2a, 2c, 2e and 2g indicates the topo-
graphical scanning electron microscope (SEM). The
microstructure and EDS analysis of ZrO2 particles are
shown in Figures 2e, 2f, 2g and 2h. From the graph after
finishing EDS test, results are confirmed that the
chemical composition of LM6/ZrO2 composites like
aluminium, zirconium, iron, copper, magnesium, zinc,
silicon, and others all the elements are present in the
prepared composites as shown in Figures 2f and 2h. The
shrinkage and porosity were not identified in the
micrograph. This is evidence of the good quality of the
casting. Finally, EDS analyses confirmed the presence of
aluminium, ZrO2 particles, which showed in Figures 2f
and 2h. The areas or points of the EDS analyses were
taken from the SEM images given in Figures 2a, 2c, 2e
and 2g.
3 RESULTS AND DISCUSSION
3.1 Hardness measurements
Microhardness tests were conducted on the polished
samples of aluminium LM6 alloy and its composites by
adopting a standard testing procedure. The hardness of
the composite was measured using a Vickers microhard-
ness tester as per ASTM: E384-10. All the samples were
applied with a load of 300 g for a period of 10 s. The test
was carried out at three different locations to avoid the
possible effect of the indenter resting on the hard rein-
forcement particles. The averages of all the five readings
were reported. The hardness of the composite depends
on the reinforcement of the matrix material. As the
coefficient of thermal expansion of zirconia is less than
the aluminium alloy, an enormous amount of disloca-
tions are generated at the particle-matrix interface during
the solidification process. This makes a further increase
in the hardness of the matrix.5 The results are graphically
shown in Figure 3. It is observed that the hardness
values of the material are directly proportional to the pre-
sence of ZrO2, and with the increase in ZrO2, the hard-
ness increases.
3.2 Tensile test
The 15 tensile test sample specimens were prepared
as per the ASTM standard (ASTM B-557-M-94) as
shown in Figure 4, and they were tested in a Universal
Testing Machine (FIE Pvt.Ltd., model: unitek 94100).
Before measuring the tensile strength, the 15 samples
were machined into a cylindrical shape. Three samples
were tested for each composition to obtain the best
tensile test result. The tensile tested sample of the five
specimens LM6 reinforced with 0 %, 3 %, 6 %, 9 %, and
12 %, respectively, are given in Figure 4. From Figure 5
it is clear that the maximum ultimate tensile strength is
obtained for the sample 5, which contains 12 % of ZrO2.
Therefore, this clearly reveals that the ultimate tensile
strength increases with an increasing percentage of ZrO2,
as shown in Figure 5. Similar trends have been observed
by many other researchers.6–8
G. KARTHIKEYAN, G. R. JINU: TENSILE BEHAVIOUR AND FRACTOGRAPHY ANALYSES OF LM6/ZrO2 COMPOSITES
Materiali in tehnologije / Materials and technology 51 (2017) 3, 549–553 551
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Figure 4: Photograph of tensile-tested specimen
Slika 4: Posnetek izgleda nateznih preizku{ancev
Figure 3: Vickers hardness values of various LM6/ZrO2 composites
Slika 3: Vrednosti za Vickers trdoto razli~nih kompozitov LM6/ZrO2
Figure 5: Tensile strength of various LM6/ZrO2 composites
Slika 5: Natezna trdnost razli~nih kompozitov LM6/ZrO2
3.3 SEM and fractography analysis of tensile specimen
From these SEM images (Figures 6 to 8) it is clear
that the distribution of particles throughout the matrix
was found to be fairly uniform as a dark region. It is also
observed that as the weight percentage of reinforcement
of ZrO2 increases the area fraction also increases as
shown in Figures 7 and 8 as a dark region. It can also be
noticed that the average grain size of the aluminium
LM6 matrix decreases with an increase in the weight
fraction of ZrO2 reinforcement.
From these Figures 7 and 8 it is clear that the homo-
geneous distributions of ZrO2 reinforced particles with
aluminium alloy occurred. Further, these figures reveal
the homogeneity of the cast composites. The properties
of the aluminium MMCs depend not only on the matrix
particle and the weight fraction but also on the distri-
bution of the reinforcing particles and the interface
bonding between the particle and the matrix.
Fracture surface analysis revealed different topo-
graphies for the composites containing different weight
percentages of zirconia particles. The results of the
fracture surface analyses conducted on fracture tough-
ness specimens of FCC structured LM6 alloy samples
revealed large dimples, along with a large amount of
plastic deformation, indicating a ductile fracture. The
fracture surfaces also exhibit fine and shallow dimples,
indicating that the fracture is ductile, as shown in Fig-
ure 9.
Fracture surfaces of the MMCs tested are shown in
Figures 9 to 11. An examination of these fracture sur-
face features in the SEM at high magnification is to
identify the fatigue and final fracture region, to identify
areas of micro-crack initiation and early crack growth,
and the over-loaded region to identify the fine scale frac-
ture features.
G. KARTHIKEYAN, G. R. JINU: TENSILE BEHAVIOUR AND FRACTOGRAPHY ANALYSES OF LM6/ZrO2 COMPOSITES
552 Materiali in tehnologije / Materials and technology 51 (2017) 3, 549–553
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Figure 9: Fractured surface of 0% zirconia and 100% LM6
Slika 9: Slika preloma pri 0 % ZrO2 in 100 % LM6
Figure 7: SEM image of LM6–6% of ZrO2
Slika 7: SEM-posnetek LM6 – 6 % ZrO2
Figure 8: SEM image of LM6–12% of ZrO2
Slika 8: SEM-posnetek LM6 – 12 % ZrO2
Figure 6: SEM image of LM6 alloy
Slika 6: SEM-posnetek zlitine LM6
Figure 11: Fractured surface of composite with 12 % of zirconia and
88 % LM6
Slika 11: Povr{ina preloma kompozita z 12 % ZrO2 in 88 % LM6
Figure 10: Fractured surface of composite with 6 % zirconia and
94 % LM6
Slika 10: Povr{ina preloma kompozita z 6 % ZrO2 in 94 % LM6
Fracture surfaces revealed different topographies for
the composites containing a different weight percentage
of zirconia particles. The fracture surface in the case of
MMCs containing 12 % of mass fractions reinforcement
revealed a cleavage type of fracture (Figure 11) due to
the presence of excessive zirconia particulates. It can
also be seen that regions of clustered particles for
reinforcement above 6 % of mass fractions (Figure 10)
are sensitive to premature damage in the composites and
the large particles seem to be prone to fracture and hence
registered a reduction in the fracture-toughness value.
The fracture mode of the matrix alloy which changed
from ductile to cleavage type (in the case of MMCs) was
dominated with microcrack nucleation and propagation,
as shown in Figures 9 to 11. Scanning electron
microscope observations of MMCs suggest that, at
higher reinforcement content (12 % of mass fractions,
Figure 11), void nucleation may also take place at large
participates in addition to the matrix/particle interface,
while at lower reinforcement (0, 6 % of mass fractions,
Figures 9 and 10) rates the fracture seems to occur
through the breakage of the particles.
Fracture surface analysis of MMCs containing 12 %
of mass fractions dispersoid (Figure 11) exhibit pre-
dominantly that fractured particles (dispersoid) and the
matrix material with coarse dimples, suggests that
fracture is of mixed mode type. Close examination of the
fractured surface indicates that most dimples were
associated with the matrix material (Figure 11). Massive
separations of the particle indicate that failure of the
material is initiated by the fracture of the particles rather
than debonding between the matrix and the reinforce-
ments. The microstructural features are shown in
Figures 10 and 11, which are a sample containing 12 %
of mass fractions dispersion casting of 10 mm thick,
which indicates that zirconia particles have fractured
during straining and fracture could have occurred in a
brittle manner as a result of stress concentrations and the
propagation of the cracks. Finally, a composite contain-
ing 12 % of mass fractions dispersoid showed cleavage
indicating that the fracture is towards brittle.
4 CONCLUSIONS
The aluminium-based composite was successfully
fabricated by the stir casting method with a uniform
distribution of ZrO2 particles.
EDX studies confirmed the presence of aluminium,
zirconium, iron, magnesium, silicon in the prepared
composite material.
The hardness of the reinforced composites was
improved compared with the unreinforced alloy and
12 % of mass fractions of ZrO2 combination achieves the
high hardness value.
There is an improvement in the tensile strength,
ultimate tensile strength with the addition of ZrO2
particle.
The SEM images of various weight percentage of
ZrO2 reinforcement reveal that the SEM image for 12 %
of mass fractions of ZrO2 contains more presence of
ZrO2 particles. It was clearly seen as a dark region.
The fractography results show that the increase in
weight percentage of the ZrO2 changes the mode of
failure from ductile to brittle, which is revealed from the
occurrence of large dimple tiny particles with a large
amount of plastic deformation present in the low weight
percentages of ZrO2 reinforced composites.
Fracture surface observation of the samples shows
that the failure of the LM6 /zirconia composite is similar
to the unreinforced LM6 alloy one, which was controlled
by inter-dendrite cracking of the matrix. In addition, a
number of dimples were observed on the fracture surface
of all the samples, which could be the result of the void
nucleation and self-subsequent coalescence during the
fracture process.
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G. KARTHIKEYAN, G. R. JINU: TENSILE BEHAVIOUR AND FRACTOGRAPHY ANALYSES OF LM6/ZrO2 COMPOSITES
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