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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