tensile behavoiur of squeeze cast am100 magnesium alloy and its al2o3 fibre reinforced composites
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Tensile behaviour of squeeze cast AM100 magnesium alloy and its
Al2O3 fibre reinforced composites
S. Jayalakshmi, S.V. Kailas, S. Seshan*
Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012, India
Received 22 November 2001; accepted 22 April 2002
Abstract
Magnesium alloys are increasingly used in automotive and aerospace applications mainly due to their light weight combined with
reasonably high tensile properties. In addition to providing a large reduction in weight, magnesium alloys exhibit excellent machinability and
good damping capacity. However, their low mechanical properties when exposed to elevated temperatures limit their usage. Making
composites out of these magnesium alloys by reinforcing them with ceramic particles or fibres appears to be a viable alternative for
improving their thermal stability. The work reported here involved experimental studies on the tensile behaviour of AM100 magnesium alloy
and its composites at different temperatures. Fractographic studies justify the effect of temperature on the tensile behaviour. q 2002
Published by Elsevier Science Ltd.
Keywords: Magnesium alloys; A. Metal matrix composites (MMCs); B. Mechanical properties; D. Fractography
1. Introduction
In recent times, magnesium alloys have gained a
prominent position in automotive, aerospace and electronic
industries where weight reduction is an important require-
ment. Magnesium alloys offer light weight, high stiffness,
excellent machinability, good dimensional stability and
damping capacity [1]. Among the currently used mag-
nesium alloys, the Mg–Al systems (with and without zinc)
offer reasonably high strength properties at room tempera-
ture and are in wide use. Though other alloy systems such as
those containing zirconium, rare earth elements and thorium
exhibit slower drop in properties up to 250 8C, their high
cost and difficulties in casting have limited their use to
certain specific applications. The Mg–Al system accounts
for about 90% of the total applications involving mag-
nesium alloys and find usage in computer housings, portable
telecommunication instruments, passenger seat frames and
instrument panels [2]. However, Mg–Al alloys can be used
safely only up to about 150 8C; even at this temperature,
considerable loss in strength is evident [3]. Hence, it is
obvious that their application as structural materials is
restricted due to the reduction in their mechanical properties
at increasing temperatures. Therefore, a need has been felt
to improve the mechanical properties of these alloys at high
temperatures.
Literature review [4–7] indicates that making compo-
sites out of the alloys would provide a workable solution to
the above. The base magnesium alloys (matrix) may be
reinforced with ceramic constituents (in the form of
particles, fibres or whiskers) that would provide improve-
ment in strength properties at high temperatures. Earlier
investigations [8–11] on the composites of aluminium and
copper alloys point out that in addition to improvement in
strength and thermal stability, appreciable enhancement in
hardness, stiffness and wear resistance was also realized.
The squeeze casting process (that refines grains and
eliminates porosity) is suitable to produce premium quality
castings. An extension of the above, the squeeze infiltration
method, is a simple and economical method for producing
short fibre reinforced metal matrix composites [10,12,13].
Literature available on magnesium alloy castings and their
composites produced using the squeeze casting technique is
rather limited.
In this work, magnesium alloy AM100 and its compo-
sites (reinforced with saffil alumina short fibres) were
processed through the squeeze casting technique. The alloy
and composite castings produced were characterized for
their tensile behaviour at different temperatures and the
fractured specimens were examined using a scanning
electron microscope.
1359-835X/02/$ - see front matter q 2002 Published by Elsevier Science Ltd.
PII: S1 35 9 -8 35 X( 02 )0 0 04 9 -0
Composites: Part A 33 (2002) 1135–1140
www.elsevier.com/locate/compositesa
* Corresponding author. Fax: þ91-80-3600648.
E-mail address: seshan@mecheng.iisc.ernet.in (S. Seshan).
2. Experimental details
The magnesium alloy AM100 (Mg–9.3 to 10.7Al–
0.13Mn) castings and composites were produced using the
squeeze casting and squeeze infiltration techniques, respect-
ively. The squeeze pressure was maintained at 40 MPa for
both the alloy and its composites. Saffil alumina short fibres
were used as the reinforcement material (fibre length:
200 mm and mean fibre diameter: 3 mm). Three volume
fractions (viz. 15, 20 and 25%, respectively), of fibre
preforms were used. The cylindrical preforms were of
diameter 70 mm and height 30 mm and the initial preform
temperature was 850 8C. The base alloy castings and the
composites produced were heat-treated to the T6 condition
for attaining peak hardness. For microstructural studies,
specimen preparation included initial dry polishing on SiC
abrasive sheets (220, 320, 400 and 600 grits) followed by
wet alumina and diamond polishing. The polished speci-
mens were etched in a glycol etchant [14] for 30–60 s.
Tensile tests of the peak hardened test specimens were
conducted in a 2-ton Monsanto tensometer (with a high
temperature furnace attachment) at a controlled strain rate
of 0.001 s21. Tensile tests were conducted at four different
test temperatures, viz. 25 (room temperature), 100, 150 and
200 8C, respectively. Fracture analysis was done using a
Jeol scanning electron microscope.
3. Results and discussions
3.1. Microstructure
Typical microstructures of the unreinforced alloy and
its composites (20% Vf) are shown in Figs. 1 and 2,
respectively. Microstructural analysis of the unreinforced
alloy (Fig. 1) indicates the presence of eutectic along the
grain boundaries, within which is present the b-Mg17Al12
precipitates [2,14–18]. The precipitates are hard and brittle
[2] and contribute to the high hardness values. Fig. 2 shows
the cross-section of the composite cut in a direction normal
to the thickness direction of the preform. The short fibres are
distributed in a planar, isotropic orientation as a result of
the method of production of the preform [15,19]. In the
composites, the fibres are uniformly distributed throughout
the matrix. Earlier works [20,21] indicate that the fibre/
matrix interface acts as nucleation sites for precipitation to
occur during aging, thereby increasing the hardness with
increasing fibre volume fraction. It is observed that the
highest volume fraction (25%) resulted in a hardness value
(165 BHN) that is nearly twice that of the unreinforced base
alloy (85 BHN). In an earlier work reported elsewhere [22]
it was observed that the aging behaviour of AM100
magnesium alloys and its composites exhibited a rapid
reduction in aging time with increase in aging temperature,
indicating the acceleration in the aging kinetics. Such a
behaviour has also been observed earlier in aluminium
composites [23,24].
3.2. Tensile behaviour
3.2.1. Unreinforced alloy
The ultimate tensile strength of the unreinforced base
alloy at different test temperature is shown in Fig. 3. The
unreinforced alloy is very sensitive to temperature and
undergoes a drastic reduction in strength as the test
temperature increases. At the highest test temperature of
200 8C, the strength drops to almost one-third of its room
temperature value. Fig. 4 shows the variation of %
elongation with test temperature. It is observed that the %
elongation of the alloy increases initially with increasing
test temperature; it reaches a maximum value at 150 8C and
reduces there after. The highest test temperature (200 8C)
being very close to the aging temperature (225 8C), gross
precipitation coarsening would occur leading to a reduction
in strength as well as ductility of the alloy [24].
The fractographic evidences of the alloy specimens are
shown in Fig. 5. Fig. 5(a) shows dominant brittle
intergranular fracture at room temperature. The presence
of brittle Mg17Al12 precipitates along the grain boundaries
contributes to this. As the temperature increases, flow of the
Fig. 1. Scanning electron micrograph of unreinforced AM100 alloy
showing: (A) matrix, (B) eutectic along grain boundaries, (C) Mg17Al12
precipitates within the eutectic network.
Fig. 2. Scanning electron micrograph of AM100 composite (20% Vf).
S. Jayalakshmi et al. / Composites: Part A 33 (2002) 1135–11401136
material would introduce ductility into the alloy with the
fracture surface showing large dimples. This change in the
fracture mode may be due to the introduction of additional
slip planes [25] in the alloy that occurs with increasing
temperature. At 150 8C, the alloy thus shows prominent
ductile failure (Fig. 5(b)) that is in consensus with the high
elongation values obtained. But, at 200 8C, the highest test
temperature, the fracture behaviour changes and the alloy
again fails by intergranular failure (Fig. 5(c)) indicating the
weakening of the grain boundaries due to precipitate
coarsening [2].
3.2.2. Composites
Fig. 3 shows the variation of tensile strength of the
composites (of different volume fractions) with tempera-
ture. At room temperature, composites of all volume
fractions exhibit similar behaviour as the base alloy,
indicating that the room temperature behaviour is largely
controlled by the inherent brittle matrix. However, with
increase in temperature, unlike the base alloy, the
composites do not exhibit any monotonic decrease in
strength. This is attributed to the load bearing capacity of the
fibres at higher temperatures. Fig. 4 shows the variation of
% elongation of the composites with temperature. The
composites exhibit lower ductility in comparison to the base
alloy, the reason being the presence of brittle ceramic fibres.
Therefore, with increase in fibre volume fraction, the %
elongation of the composites continues to decrease. As the
temperature increases, the difference in the elongation
values amongst the MMCs reduces such that at the highest
test temperature, all the composites exhibit almost the sameFig. 4. Variation of % elongation of AM100 composites with temperature.
Fig. 3. Variation of ultimate tensile strength of AM100 composites with
temperature.
Fig. 5. Scanning electron micrographs of fractured alloy specimens tested at
different temperatures: (a) 25, (b) 150, (c) 200 8C.
S. Jayalakshmi et al. / Composites: Part A 33 (2002) 1135–1140 1137
values of % elongation. In comparison, the base alloy
exhibits elongation values that are almost an order of
magnitude greater than that of the composites, indicating the
influence of fibres on the mechanical behaviour.
Fig. 6(a)–(d) show the representative scanning electron
micrographs of the fractured surfaces of the composites
(20% Vf) tested at different temperatures. At room
temperature, MMCs with all volume fractions exhibit
similar behaviour as that of the base alloy. Fig. 6(a)
provides the evidence of large matrix cracking depicting
that the inherently brittle matrix material largely controls
the room temperature behaviour of the composites. The
absence of plastically induced load transfer to fibres, due to
the limited ductility of the matrix causes the early failure of
the matrix. The few fibre pull-outs seen in Fig. 6(a) further
indicate that the interfacial strength of the composite is good
[24]. As the temperature increases to 100 8C, the load is
taken up by the fibres due to the plastic flow of the matrix,
thus increasing the absolute value of UTS at elevated
temperatures. Fig. 6(b) shows the reduction in matrix
cracking and an increase in the flow of the matrix, as seen
from the dimples on the fracture surface. At this
temperature, fibre pull-outs are also observed. At 150 8C,
overaging of precipitates occur [22,24], in addition to the
large flow of matrix that leads to fibre cracking. As the
precipitates are largely present along the fibre/matrix
interface [15,21], the interface is weakened resulting in
large fibre pull-outs (Fig. 6(c)). In these randomly oriented
fibre composites, fibre/matrix debonding and fibre failure
occur in fibres that are oriented normal to the loading
direction, whereas fibre pull-outs are dominant in fibres that
are aligned along the loading direction. Such occurrences
have been earlier observed by many investigators [5,24,26,
27]. From Fig. 6(c), it can be seen that fibre pull-outs largely
occur in the fibres that are oriented parallel to the loading
direction. With increase in test temperature (200 8C),
overaging process accelerates leading to further reduction
in the strength of the composites. Earlier works [22,24] also
indicate that higher the fibre volume fraction and aging
temperature shorter is the attainment of peak aging time.
Hence, higher volume fraction composite overages more
rapidly than its lower volume fraction counterparts. The
reversal in trend observed in the strength values of the
composites at higher temperatures (Fig. 3) is attributed to
this rapid overaging process. In addition to overaging, the
softening of the matrix at these temperatures transfers large
load to the fibres resulting in fibre cracking (Fig. 6(d)).
Chawla [28] suggests that the flow of the matrix that occurs
at high temperatures causes large local stresses on the fibres
resulting in fibre deformations and fibre breakage. Any flow
of the matrix surrounding the fibre would result in a highly
directional, local stress field on the fibre. As the temperature
increases, this stress increases and such a high local stress
would cause local plastic deformation. When the fibre that is
carrying this high stress reaches its fracture strain, it would
crack resulting in complete failure, as seen in Fig. 7.
Fig. 6. Scanning electron micrographs of fractured composite specimens tested at different temperatures: (a) 25, (b) 100, (c) 150, (d) 200 8C.
S. Jayalakshmi et al. / Composites: Part A 33 (2002) 1135–11401138
It is clear that the changes in the microstructure such as
the coarsening of precipitates that occur during the test play
a very important role in determining the tensile strength of
the composite. In any attempt to estimate the strength using
ROM methods (rule-of-mixtures) for randomly oriented
short fibre composites [27,29–31], it would be difficult to
take into account (apriori) the microstructural changes such
as precipitation coarsening. Moreover, assuming the
strength of the matrix material as that of the unreinforced
alloy in those estimations would be too simplistic, as
residual stresses are created at the interface due to thermal
mismatch, which would significantly affect the mechanical
behaviour of the composites [28]. More importantly, both of
these (overaging and residual stresses) are accelerated with
increase in fibre volume fraction. Devoid of such consider-
ations, the theoretical estimates could be far away from the
real experimental values.
4. Conclusions
1. The inherent brittle nature of AM100 alloy, determined
by the nature and distribution of precipitates, dominate
its tensile behaviour at all test temperatures.
2. The room temperature strength of the composite is
dominated by the brittle alloy matrix. At higher
temperatures, the strength is attributed to the load
carrying capacity of the fibres.
3. The composites exhibit high thermal stability when
compared to that of the unreinforced alloy. Though
strength reduction with temperature occurs in the
composites, they still exhibit reasonably high values for
many applications.
4. The strength reduction is mainly caused by overaging
and softening of the matrix alloy. This indicates that in
addition to matrix flow properties due consideration
should be given to the influence of fibres on the
precipitation.
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