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CHAPTER 4

MECHANICAL BEHAVIOUR OF COMPOSITES

4.1 INTRODUCTION

The basic idea of developing metal matrix composites is to derive high strength

materials. Large number of products have been designed and manufactured for

various applications. Many of the investigations have shown improved mechanical

properties, but limited with low & poor ductility. In the present investigation, an

attempt has been made to achieve a good combination of strength & ductility

properties with composites.

Fracture surface morphology of discontinuously reinforced metal matrix composites

exhibit characteristic features of ductile rupture mechanism. This failure process can

be conveniently split into there stages: void nucleation, growth and coalescence.

Ductile fracture of monolithic alloys to MMCs imply that the onset of void nucleation

is the dominant process; controlling the ductility in these materials. With high volume

fractions of reinforcements; found mostly with commercially attractive MMCs;

nucleation process to dominate, if void nucleation is at the reinforcing phase. Void

growth and coalescence have been much neglected in the study of MMCs because of

experimental difficulties. Ductility of MMCs cannot be uniquely correlated with the

void nucleation rate at the reinforcing particles.

4.2 LITERATURE REVIEW

The attractive physical and mechanical properties that can be obtained with metal

matrix composites, such as high specific modulus, strength and thermal stability, have

been documented extensively [1-2]. Various factors controlling the properties for

particulate MMC properties have been reviewed by several investigators [3].

Improvement in modulus, strength fatigue, creep and wear resistance has been

demonstrated for a variety of reinforcements [4-6]. Improvement in modulus,

strength, fatigue, creep and wear resistance has been demonstrated for a variety of

reinforcements [7, 8]. Of these, tensile strength is the most convenient and widely

quoted measurement and is of central importance in many applications.

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It is apparent from literature that parameters controlling the mechanical properties of

particulate reinforced composites are still to be understood in detail. However, some

of the important factors are:

• Strength of composites is observed to be most strongly dependent on the

volume fraction and size of the reinforcement.

• Dislocation strengthening will play a more significant role in the MMCs than

in the unreinforced alloy due to the increased dislocation density.

• Of greatest concern appears to be the introduction of defects and

inhomogeneties in the various processing stages, which has been found to

result in considerable scatter in the mechanical properties [9].

JJ Lewandowski et al [10] reported the effects of matrix microstructure and particle

distribution on fracture of al metal matrix composites. Apart from the reinforcement

level, the reinforcement distribution also influences the ductility and fracture

toughness of the MMC and hence indirectly the strength. A uniform distribution of

the reinforcement is essential for effective utilization of the load carrying capacity of

the resultant composite. Non-uniform distribution of reinforcement in the early stages

of processing was observed by MG Mckimpsm et al [11] to persist to the final product

in the forms of steaks or clusters of reinforcement with their attendant porosity, all of

which lowered ductility, strength and toughness of the material.

Nair et al [12], Nieh at al [13], Flom and Aressenault et al [14] assume that fracture in

DRMMCs follows the same sequence as dispersion–strengthening alloys, namely,

nucleation at the second phase particles followed by failure in the matrix through void

coalescence. You et al [15], Roebuck [16] contended that, increased levels of stress

and high levels of plastic constraint; imposed by the reinforcing particles on the

matrix, lead to void nucleation in the matrix as the initiation step, with the final stage

of fracture being the de-cohesion or cracking of the particles.

The two dominant void nucleation modes have been observed in DRMMCs particle

cracking by Lloyd [17], Davidson D L [18] and de-cohesion at the particle / matrix

interface by Crowe et al [19], Stephens et al [20], Manoharan M and JJ Lewandowski

[21]. Mummery PM and Derby B [22], Vasudevan et al [23], Yang et al [24] reported

that the mode is sensitive to a number of microstructural parameters, such as size and

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volume fraction of reinforcement, and in studies where a systematic variation of these

parameters has been made, transition between these modes have sometimes been

observed.

Two parameters have the greatest influence on the mode of nucleation: the size of

reinforcing phase and interfacial bond strength. A change from interfacial de-cohesion

to particle cracking has been observed on increasing the particle size. Particle / matrix

interfacial bond strength has been altered by a number of methods. Man et al [25],

Manoharan M and JJ Lewandowski [26], Strangwood et al [27] used a change in the

composition of matrix / alloy composition as a first method. Segregation of the

alloying element to the interface, or its reaction with the reinforcement has been used

to vary the bond strength. The second method changed the surface properties of the

reinforcement by baking it in a surface [28]. This encouraged the formation of a

brittle face at the interface through which failure proceeded. A different reinforcement

type with n the same matrix has also been used by Stephens et al [20]. Other

parameters that have been shown to affect the nucleation mode are volume fraction by

Mummery [29], aspect ratio of reinforcement by Whitehouse et al [30], matrix heat

treatment by Manoharan M and JJ Lewandowski [26], and strain rate by Pickard SM

et al [31].

Mc Danals [32], Lloyd [20], Miller WS and FJ Humphreys [33] reported that for a

given matrix alloy, the elongation to failure is reduced by increasing volume fraction

and the size of the reinforcement. Kamat SV et al [34], Liu C et al [35], Girot FA et al

[36], England J and Hall IW [37] reported that composites of high strength alloys

have low ductility than those of low-strength alloy matrices and with decreasing

ductility on aging to peak matrix strength, Mc Danals [32], Lewandowski et al [38,

39], Papazian and Adler [40] and Lloyd DJ [41].

Ductility of MMCs is not simply related to the rate of void nucleation. Elongation to

the failure of the composites can be increased by suppressing void nucleation at the

reinforcing phase. Composites ductility is governed by matrix processers that will be

affected by the presence of the reinforcements. This is evidenced by the decrease in

ductility while increasing reinforcement volume fraction.

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4.3 EXPERIMENTAL DETAILS

4.3.1 Synthesis of Composites

Composites were fabricated (page no. 42) using average particle size of 125�m as

reinforcement with varying weight fractions varying between 5 and 15%.

Subsequently, billets were hot extruded to 14mm rods (extrusion ratio 18:1). All the

extrudates were thoroughly homogenized with industrial furnace at 100oC for 24

hours.

4.4 RESULTS AND DISCUSSION

4.4.1 Physical Properties

4.4.1.1 Density of Composites

Table 4.1 summarizes the bulk densities of the alloy and composites in as-cast and

extruded conditions. In the cast condition, composites show a lower density values

than the calculated values (based on Rule of mixture), figure 4.1. Further, the

difference in densities found to be increasing with increasing reinforcement

concentrations. Since composites were prepared by stir cast technique entrapped gases

due to vortex formation, were the reasons for lower densities than the calculated ones.

And increased stir times with increasing reinforcement contents is a signature of the

above discussion, resulting decreased densities [42]. Also, loss of magnesium may be

the other reason for the drop in density with increasing reinforcement content. The

composites show a relative increase in density in the extruded condition minimizing

the porosity.

Table 4.1 Density values of the AA 2024 alloy and its composites in the as cast and

extruded conditions

CompositeTheoretical density

(g/cc)

Measured density (g/cc) Difference (g/cc)

������ Extruded ������ Extruded

0 2.820 2.820 2.820 0.000 0.000

5 2.872 2.833 2.866 0.038 0.006

10 2.923 2.876 2.914 0.048 0.009

15 2.975 2.918 2.961 0.057 0.014

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Figure 4.1 Density variations of composites

4.4.1.2 Resistivity Studies

Figure 4.2 shows the resistivity of the alloy and composite. Resistivity found to be

decreasing with reinforcement contents.

Figure 4.2 Electrical resistivity of composites

The drop in resistivity is due to the presence of reinforcements with lower resistivity

values. Resistivity values ranging between 36 for the alloy and 22 for that of

composites with 15% reinforcement contenent. Though, the rule of mixture (figure

4.3) calculations pertaining to alloy and the reinforcement show a nominal decrease,

the decrease in measured resistivity values were high. A good interface between the

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alloy and reinforcement and a uniform distribution of the reinforcement may be

reasons for the drop in resistivity. The dissolution of reinforcement at the interface

enhances copper & magnesium concentrations at the matrix-reinforcement interface.

This causes, increased concentrations of CuMgAl2. Presence of CuMgAl2 decreases

the resistivity (Chapter 3, figure no 3.6). Hence, the resistivity of the resultant

composites decreases with increasing concentrations of reinforcement.

Figure 4.3 Effect of reinforcement on electrical resistivity of composites

4.4.2 MECHANICAL PROPERTIES

4.4.2.1 Metallographic Studies

Figure 4.4 shows the SEM images of the composites with 5 & 10% reinforcements.

Structure shows the uniform distribution of the particulates. Though all the

composites were prepared with particulate material of 125 µm size, the average

particle size of the resultant composite found to be decreasing with increasing

reinforcement content, figure 4.5.

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(a)

(b)

Figure 4.4 SEM Image of a) AA 2024-5% and b) AA 2024-10% HEAp composites

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(a)

(b)

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(c)

Figure 4.5 SEM images (a) AA 2024-5% (b) AA 2024-10% and (c) AA 2024-15%

HEAp composites

Table 4.2 shows the average particle size of the reinforcement with increasing weight

fraction.

Table 4.2 reinforcement size of the resulting composite

% of reinforcement Average reinforcement

particle (�m)

Surface area to volume ratio

5% 8.42 0.7126

10% 6.85 0.8759

15% 3.60 1.6667

Since particulate addition times in composite making increases with increasing weight

fraction, interface dissolution increases with time. This has resulted in the decrease of

particle size figure 4.6.

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Figure 4.6 Effect of reinforcement on particle size

4.4.2.2 Hardness Studies

Figure 4.7 shows the effect of reinforcement content on the hardness of the

composites. Hardness increases with the increase of the amount of reinforcement

contents.

Figure 4.7 Hardness variations of composites

Similar behaviour has been reported by Kumar et al. [43] in a study on Al7075-Al2O3

metal matrix composites concluded that hardness of the composites increased with

increased filler content. Howell, et.al [44] and Vencl et.al [45] reasoned the

improvement of the hardness of the composites to the increased particle volume

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fraction. Wu [46] and Deuis [47] attributed this increase in hardness to the decreased

particle size and increased specific surface of the reinforcement for a given volume

fraction. Sug Won Kima et.al. [50] reasoned the increase in hardness of the

composites to the increased strain energy at the periphery of particles dispersed in

the matrix. Deuis et.al concluded that the increase in the hardness of the

composites containing hard ceramic particles not only depends on the size of

reinforcement but also on the structure of the composite and good interface

bonding. J Babu Rao et.al [51] reported, the hardness improvement in aluminium

alloys by incorporating flyash as reinforcement from 5 to 15 wt%, This could be due

to the presence of fly ash particulates which consists of majority of the alumina and

silica which are hard in nature.

An increment of 62% in hardness has been achieved. The increase may be attributed

to the reinforcement effect, interparticle distance, interfacial bond between

reinforcement and matrix & particle solubility in the matrix.

Figure 4.8 shows the relation between the reinforcement content and the surface area

to the volume ratio of the particulates measured as shown in table 4.2. The decrease in

particle size with increasing reinforcement content enhances the surface area to the

volume ratio of the resultant reinforcement, figure 4.9. This further enhances the

bonding between the matrix and the reinforcement.

Figure 4.8 Effect of reinforcement on surface area to volume ratio

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Figure 4.9 particle size vs. surface area to volume ratio

Figure 4.10 reports the comparison between the theoretical and measured values of

hardness of the investigated composites. Measured values found to be more compared

to the projected values by the rule of mixtures, this could be due to refined grain size

of the matrix, restricted dislocation mobility, enhanced dislocation density, and

constrain to the localized matrix deformation during indentation as a result of the

presence of reinforcement.

Figure 4.10 Rule of mixture

The cumulative effect of all the above mechanisms, stimulate the hardness to higher

values. Compared to the linear path of rule of mixture (ROM), the measured values

took an exponential path, figure 4.10.

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Since density plays an important in the selection of material, a comparison has been

made between the specific hardness and measured hardness of the matrix material and

the composites against their increasing weight fraction, figure 4.11. Though alloy

shows lower specific hardness compared to the measured hardness, reinforcing the

matrix with the particulate material enhances the specific hardness of the resultant

composite right from the lower weight percentages of reinforcements itself. And the

specific hardness found to be increasing with reinforcement content.

Figure 4.11 Effect of reinforcement content on specific hardness

4.4.2.3 Tensile Behaviour

Figure 4.12 shows the fractured specimens of the alloy and its composites.

(a)

(b)

(c)

(d)

Figure 4.12 Tensile fractured specimens a) Alloy b) 5% c) 10% d) 15% composites

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Table 4.3 shows the tensile behaviour of the alloy and the composites with

reinforcements between 5 and 15%. Composites show improved strength properties

compared to the base matrix. Increased reinforcement content enhances the strength

properties further. The tensile properties of composites found to be increasing with

reinforcement content of the composites.

Figure 4.13 Tensile strength vs. tensile strain of alloy and composites

Table 4.3 Summary of yield strength, UTS, and modulus of composites

Composite

Yield

Strength,

(MPa)

Ultimate

Tensile

Strength

(UTS), MPa

Young’s

Modulus of

Elasticity,

(GPa)

%

Elongation

AA 2024 alloy 207.13 330.07 78.14 16.53

A2024-5% 311.3 401.14 87.75 12.58

A2024-10% 380.41 493.71 94.86 10.85

A2024-15% 405.78 563.65 102.69 8.64

Rohatgi [50] reports that the increases in tensile elastic modulus with increase in

volume percent (3–10) of fly ash. Aghajanian et al. [51] have studied the Al2 O3

particle reinforced Al MMCs, with varying particulate volume percentages, and

report improvement in elastic modulus, tensile strength, compressive strength with

increase in reinforcement content. Composites behave normally up to the yield point

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under both tensile and compressive loads. However, compression samples (52 vol%

Al2O3 reinforced Al-10Mg MMC) were able to accommodate far more strain before

failing than tensile samples.

Surappa et.al [52] reported that the ductility of the composite decreased with the

increase in weight fraction of the fly ash. This is due to the hardness of the fly ash

particles or clustering of the particles. The various factors including particle size,

weight percent of reinforcement affect the percent elongation of the composites even

in defect free composites. Lorca, et.al, [53] proposed that at the initial stages of

plastic deformation the increase in load carried by the particles is mainly due to the

progressive strain hardening of the surrounding matrix, which is relatively ductile. As

the matrix strain hardening capacity is saturated relaxation of stresses from fractured

particles result in the stress transfer to nearby particles causing greater particle

fracture. They further inferred that the final fracture of the composites takes place by

a ductile mechanism involving the nucleation and growth of voids in the matrix,

which contributes to the final coalescence of the larger voids originating around

broken particle.

Khalid A Al-Dheylan et al [54], reported that, The yield strength, UTS and youngs

modulus of composites increased with the increase in volume fraction of the

reinforcement, while the ductility decreased. Due to the constraints imposed on the

deformation caused by the presence of the hard and brittle Al203 particles in the soft

and ductile 6061 Al alloy matrix higher applied stress is required to initiate plastic

deformation in the matrix. This in turn results in the increase in the elastic modulus

and strength of the composite.

With increasing weight percentage of the reinforcement more load was transferred to

the reinforcement resulting in a higher ultimate tensile strength values. The increase

in work hardening rate with increase in reinforcement content enhanced the modulus

values. Since both the matrix and reinforcement used were of similar nature of the

materials, the good compatibility between them offered lower rate of resistance

towards deformation resulting decelerated increase in modulus values. Yield strength

shows a similar trend as that of tensile strength depicting an increase of 95%, while

compared to 70% increase of that of ultimate tensile strength.

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As reported by several authors, there was only a 50% drop in % elongation compared

with the matrix material. The drop in ductility is due to the increased resistance

offered by the reinforcement and the intermetallics present at the matrix-

reinforcement interface as explained in earlier paragraphs. Composite with 15%

reinforcement has shown 8.6 % ductility which is quit high compared to any of the

metal matrix composites reported.

The specific properties of the ultimate tensile strength, yield strength, young’s

modulus of elasticity and ductility have been shown from figure 4.14 to 4.21. In all

the cases compositing has shown improved specific properties compared to the alloy.

Similarly, the specific property interms of ductility has been proved better compared

to that of matrix alloy.

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4.14 Effect of reinforcement content on yield strength

Figure 4.15 Effect of reinforcement content on specific yield strength

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4.16 Effect of reinforcement content on UTS

Figure 4.17 Effect of reinforcement content on specific ultimate tensile strength

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4.18 Effect of reinforcement content on youngs modulus of elasticity

Figure 4.19 Effect of reinforcement content on specific modulus of elasticity

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Figure 4.20 Effect of reinforcement content on % elongation

Figure 4.21 Effect of reinforcement content on specific % elongation

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4. 4.2.4 Toughness Studies

Toughness is the measure of the ability to absorb energy in the processes of

deformation till failure. Toughness measurements have been made by calculating the

area under the tensile stress-strain curves (figure 4.13, page no 65). Figure 4.22 shows

the toughness calculated against the increased reinforcement contents of the

reinforcements.

Figure 4.22 Effect of reinforcement content on toughness

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4.5 CONCLUSIONS

1. Specific hardness of the resultant metal-metal composites is much superior than

conventional MMCs.

2. The decrease in particle size with increasing reinforcement content enhances the

surface area to volume ratio of the resultant particulates.

3. Increased reinforcement contents enhance all the mechanical properties such as

yield strength, tensile strength and Youngs modulas of elasticity.

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