37

CHAPTER 2

LITERATURE REVIEW

2.1 STIR CASTING OF METAL MATRIX COMPOSITES

Conventional stir-casting technology has been employed for

producing particulate reinforced metal matrix composites for decades. So far,

only a few researches have been reported on the successful casting of it, as

details of the casting techniques are always considered proprietary and rarely

reported by the manufacturers.

Hashim et al (2000), evaluated a relatively low cost stir casting

technique for use in the production of silicon carbide and aluminium alloy

metal matrix composites. The technical difficulties associated with attaining a

uniform distribution of reinforcement, good wettability between substances

and a low porosity material were presented and discussed.

The factors influencing the homogeneous distribution of

reinforcement have been listed as Particle size, density, size shape and

volume fraction, which influences the particle and settling rate. Surface

properties of the reinforcement affect the wetting rate. Reaction between the

reinforcement and the matrix affect the rheological behaviour of the

composites. Porosity and casting defects affect the particle distribution in

general the reinforcement particles occupy inter dendrite or between

secondary dendrite arm spacing, therefore the finer the spacing or the finer the

matrix grain size the better the particle distribution.

38

Surappa (2003) presented an overview of aluminium matrix

composites material systems on aspects relating to processing, microstructure,

properties and applications. Several challenges must be overcome in order to

intensify the engineering usage of aluminium matrix composites. Design,

research and product development efforts and business development skills are

required to overcome these challenges.

The science of the primary processing of aluminium matrix

composites need to be understood more thoroughly, especially factors

affecting the micro structural integrity including agglomeration in the

aluminium matrix composites. There is need to improve the damage tolerance

properties, particularly fracture toughness and ductility, in aluminium matrix

composites. Work should be done to produce high quality and low cost

reinforcements from industrial wastes and by products. Efforts should be

made on the development of aluminium matrix composites based on non-

standard aluminium alloys as matrices. There is a greater need to classify

different grades of aluminium matrix composites based on property profile

and manufacturing cost. There is an urgent need to develop simple,

economical and portable nondestructive kits to quantify undesirable defects in

aluminium matrix composites. Secondary processing is an important issue in

aluminium matrix composites.

Work must be initiated to develop simple and affordable joining

techniques for aluminium matrix composites. Development of less expensive

tools for machining and cutting aluminium matrix composites is of great

necessity. Work must be done to develop re-cycling technology for

aluminium matrix composites. There must be more consortium or networking

type approaches to share and document the wealth of information on

aluminium matrix composites. There exist tremendous opportunities to

disseminate several high profile success stories on the engineering

39

applications of aluminium matrix composites amongst the materials

community. Aluminium matrix composites must be looked upon as materials

for energy conservation and environmental protection. These twin issues must

create awareness at the government and policy formulators’ level and work to

increase market acceptance by disseminating information on the outstanding

potential of aluminium matrix composites.

Balasivanandha Prabu et al (2005), successfully synthesized an

high silicon content aluminium alloy silicon carbide metal matrix composite

material, with 10% silicon carbide using different stirring speeds and stirring

times. The microstructure of the produced composites was examined by

optical microscope and scanning electron microscope. The Brinell hardness

tests were performed on the composite specimens. It was stated that the

results revealed that stirring speed and stirring time influenced the

microstructure and the hardness of composite. Microstructure analysis

revealed that at lower stirring speed with lower stirring time, particle

clustering was increased. Increase in stirring speed and stirring time resulted

in a better distribution of the particles. The hardness test results also revealed

that stirring speed and stirring time have their effect on the hardness of the

composite. The uniform hardness values were achieved at 600 rpm with

10 min stirring. But beyond certain stir speed the properties degraded again.

Rajan (2007), investigated the effects of three different stir casting

routes on the structure and properties of fine fly ash particles (13µm average

particle size) reinforced Al .7Si .0.35Mg alloy composite was evaluated. It

was found that among liquid metal stir casting, the modified compocasting

followed by squeeze casting routes have resulted in a well dispersed and

relatively agglomerate and porosity free particle dispersed composites.

Interfacial reactions between the fly ash particle and the matrix leading to the

40

formation of MgAl2O4 spinel and iron intermetallics are more in liquid metal

stir cast composites than in compocast composites.

The surface treatment of the reinforcement was a prerequisite for

getting acceptable dispersion. Separation was more in compocasting than in

liquid stir casting. Modified compocasting cum squeeze casting resulted in

best distribution of the particles. Interfacial reactions were more in stir casting

than in compocasting.

2.2 FORGING OF METAL MATRIX COMPOSITES

Ismail O È zdemir et al (1999), studied composites of an aluminium

and silicon alloy (Al±5%Si±0.2%Mg) containing different volume fractions

of particulate silicon carbide reinforcement. Samples were produced by

permanent die casting technique. The cast ingots were cut into blanks to be

forged in two steps to obtain rectangular plate-shaped samples. At each step

of closed-die hot forging approximately 50% reduction in thickness was

obtained.

The microstructures and mechanical properties of the matrix alloy

and the composite samples were investigated in the as-cast state and after the

forging operation. It was found that the forged microstructures had a more

uniform distribution of the silicon carbide particles and the eutectic silicon in

comparison to the as-cast microstructures. Evaluation of the mechanical

properties showed that the forged samples had strength values superior to

those of the as-cast counterparts. It was stated that after forging, the yield

strength of the matrix alloy and composite samples was increased by about

80 %, and the improvement in tensile strength was about 40 %. The addition

of increasing amounts of particulate silicon carbide decreased the ductility

and increased the yield and tensile strength up to an optimum reinforcement

volume fraction over which a decrease in strength and ductility was obtained.

41

The yield strength in all the specimens in the as-cast and in the forged states

increases with the addition of up to about 17 volume percentage of silicon

carbide. But starts decreasing after the additions of silicon carbide were above

this amount.

Narayana Murty et al (1999) derived a simple instability condition

applicable to a general flow stress versus strain at any temperature. The study

was done to delineate the regimes of unstable material flow during hot

deformation of aluminium reinforced with alumina or silicon carbide

particles. It was found from the processing maps that increasing particle

volume fraction increases the instability regions.

Hirokuni Yamamoto et al (2000) fabricated super plastic aluminum

alloy composite sheets reinforced with silicon carbide particles (of 5 or 20 µm

in diameter) by the hot pressing method. They have discussed the effects of

silicon Carbide particle size and hot pressing conditions on the mechanical

properties of the composite sheets. Their formability under tensile stress fields

(uniaxial, plane strain and balanced biaxial) and deep drawability using the

local heating and cooling deep-drawing method were estimated at 803 and

808 K temperatures respectively.

The maximum and minimum limits of stretch formability were

revealed in the areas of uniaxial tension and balanced biaxial tension.

However, in gas pressure stretch forming (balanced biaxial tension) without

friction, the stretch formability was equal to or greater than that under

uniaxial tension.

Ganesan et al (2004) studied the hot working characteristics of

Al 6061/15%Silicon carbide particulates produced by stir casting. The maps,

based on dynamic materials model, generated through stress data obtained

through hot compression tests. Shear band formation and particle fracture

42

were noticed at high strain rates and lower temperature, thereby defining the

flow instability domain.

Cavaliere and Evagelista (2003) investigated the hot formability of

two aluminium alloys 6061 and 2618 reinforced with 20% Al2O3 particulate

for isothermal forging of automotive components. The isothermal forging of

aluminium alloys by torsion and hot compression tests was investigated to

find out the formability of the materials. The results were analysed with the

equations relating flow stress, temperature and strain rates. Optical

micrographs and electron microscopy observations were performed to

quantify the damage in terms of fracture of the particles. Processing maps

were constructed. A finite element model was proposed to predict the

mechanical changes during isothermal deformation.

2.3 AGEING OF METAL MATRIX COMPOSITES

Doel et al (1993) studied the aluminium 7075 reinforced with

silicon carbide particles, in underaged, peak aged and overaged conditions.

The composites were produced by a co-spray deposition technique. The

tensile properties and fracture toughness were investigated at room

temperature. It has been found that the coarse particle reinforced composites

have poorer strength than composites reinforced with fine particles. The

coarse particulate reinforced composites have reasonable toughness similar to

composites with finer particles. A model to predict the fracture toughness

from tensile ductility and nominal particle spacing was proposed to explain

the observed results.

Bekheet et al (2002) tested 2024 aluminium reinforced with silicon

carbide particles produced by a squeeze casting technique. It was stated that

the effects of reinforcement and cold working before artificial ageing had

accelerated interface reaction between particles and matrix. The peak

43

hardness of these composites was slightly higher than the unreinforced alloys.

The fatigue strength of the composites with 5% of Silicon carbide was

increased by 100 % compared to unreinforced alloys.

Sug Won Kim et al (2003) investigated the effect of alloying

elements and heat treatment on the hardness and wear characteristics of

composites manufactured by a duplex process. The duplex process consists of

squeeze infiltration followed by squeeze casting. The heat resistance

characteristics on hardness and wear properties were improved by the addition

of Ni element. In aluminium composites reinforced with 10 wt% silicon

carbide particles, the amount of wear decreased with an increase in sliding

speed. Wear resistance of 10 µm Silicon carbide reinforced aluminium

composite was improved more than 3 times that of 5 µm silicon carbide

reinforced aluminium composites.

Srivatsan et al (2004) studied the cyclic stress strain response of

under aged and peak aged aluminium alloy 7034 discontinuously reinforced

with silicon carbide particulates. The specimen were cyclically deformed

using fully reversed tension-compression loading under total strain amplitude

control, at both ambient and elevated temperatures. The cyclic stress response

and stress versus strain response characteristics, cyclic strain resistance, low-

cycle fatigue (LCF) life, and final fracture behavior of the composite, for both

the under aged and peak aged microstructures, at the two temperatures, were

compared. The influences of cyclic strain amplitude and stress were studied to

gain an insight of the intrinsic micro structural effects, deformation

characteristics of the composite constituents, and macroscopic aspects of

fracture.

The microscopic examination of the fractured surfaces was

reminiscent of locally ductile and brittle mechanisms. Fracture was dominated

by cracking of the reinforcing silicon carbide particulates and decohesion at

44

the matrix particle interfaces. Constraints in mechanical deformation, induced

in the plastically deforming aluminum alloy metal matrix, by the hard, brittle

and elastically deforming silicon carbide reinforcement phase, coupled with

local stress concentration effects at the matrix–particulate interfaces promotes

silicon carbide particulate failure through the conjoint influences of cracking

and decohesion at its interfaces. Final failure occurs by fast fracture through

the composite matrix.

Mahadevan et al (2005) investigated the effects of delayed ageing

on aluminium 6061 reinforced with silicon carbide particles. The delayed

aged composites were subjected to hardness and fatigue tests. It was reported

that the mechanical properties were degraded for a delay of up to 12hours

before ageing, however for delay beyond 16hours the properties were similar

to zero delayed composites. The results were discussed with scanning electron

micrographs of fatigue-fractured surfaces.

Muratoglu and Aksoy (2006), investigated, the influence of

temperature (in the range 20-200o C) on the abrasive wear of 2124 Aluminum

reinforced with silicon carbide particles produced by powder metallurgy

technique. Some specimens were artificially aged to T6 condition to

determine ageing effects. The worn surfaces were examined using scanning

electron microscopy, EDS and optical microscopy. Wear tests showed the

weight loss of the aged specimen was less than that of the non-aged

specimens. There was little or no change in wear rate above 50o C in both

aged and non-aged specimens.

Murato lu et al (2006) investigated the joining characteristic of

silicon carbide particulate reinforced aluminum metal matrix composites with

pure aluminum by diffusion bonding process. The joining quality of the

aluminium silicon carbide metal matrix composites was studied to determine

the influences of silicon carbide particulates with homogenization and age

45

hardening on bonding properties. The experimental result indicated that

application of aging before and after diffusion bonding decreases silicon

carbide particulate accumulation and increases other elemental concentration

at interface. The application of aging treatment before the diffusion bonding

of aluminium silicon carbide metal matrix composites to pure aluminium

increased copper percentage concentration at interface.

2.4 MECHANICAL AND FATIGUE CHARACTERISTICS OF

METAL MATRIX COMPOSITES

Mc Kimpson and Scott (1989) summarized the scope of operations

for both cast and powder based processing issues. Squeeze infiltration, vortex

stir casting, powder processing and deposition processing were analyzed. It

was suggested that reinforcing material was the control factor. Quantify the

differences in reinforcement distribution and morphology between the control

and experimental samples. The bulk materials must be tested after the

extrusion or other mechanical working of the materials and the results should

accompany with low magnification micrograph of the samples.

Doel et al (1993) studied the mechanical properties of aluminium

alloy 7075 reinforced with silicon carbide particles in the underaged, peak

aged and over aged conditions. Three grades of reinforcement were used

(average particle size of 5µm, 13µm and 60µm). The volume fractions were

11% for 5µm reinforcement and 17% for the remaining particle size

reinforcements. It was stated that the tensile and fracture toughness of the

matrix increases with the addition of reinforcement and it tend to vary with

the ageing condition. With the increase in particle size the ductility was

decreased but the toughness was increased.

Hall Jody et al (1993) studied the effects of particle size (2, 5, 9 and

20µm) and volume fractions (10%, 20% and 30%) on the aluminium 2124

46

reinforced with silicon carbide particulates. The composites were

manufactured by powder metallurgy technique. The extruded composites

were tested in underaged peak aged and overaged conditions. The tensile and

yield strength and fatigue life of the composites were determined. It was

stated that strength and fatigue life were increased as the reinforcement

particle size was decreased and volume fraction loading was increased. The

frequency of particle fracture depends upon the particle size, volume fraction

and maximum stress intensity. Fatigue cracks were initiated from large

intermetallic inclusions and clusters of silicon carbide particles typically near

the surface.

Papakyriacou et al (1996) investigated the fatigue properties of

aluminium 6061 reinforced with alumina particles in the high cycle regime

under fully reversed loading conditions (R= -1). Fatigue investigations were

done for four types of specimens, pure aluminium 6061, and aluminium 6061

reinforced with 12 vol%, 15 vol% and 21 vol%. The fatigue limits were

145MPa for unreinforced 6061-T6 and 115 MPa for the reinforced alloys.

Large broken particles were observed as preferential sites for fatigue crack

initiation. Brittle fractures of the particles were also observed. It was

suggested that for optimal fatigue properties, fine particles should be used as

reinforcement.

Vaidya and Lewanowski John (1996) conducted high cycle fatigue

tests on monolithic AZ91D and AZ91D magnesium alloy composites

manufactured by squeeze casting and extrusion process. The silicon carbide

particles were either 15 µm or 52 µm size, at both the 20% and 25% volume

fraction reinforcement level. The effect of changes in silicon carbide particle

size and volume fraction in the high cycle regime was investigated. It was

stated that the addition of silicon carbide particle increases the strength and

modulus provided by utilizing the finer reinforcement.

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Damage quantification of the failed fatigue specimens revealed that

a larger percentage of cracked silicon carbide particles were found both on the

fracture surface as well as beneath the fracture surface for the composites

containing 52 µm silicon carbide particulates. In all composites a greater

percentage of cracked silicon carbide particles were found in the overload

region of failure. The large grained, commercial purity magnesium exhibited

the poorest fatigue properties in all the analyses conducted.

Han et al (1997) investigated the cyclic stress response

characteristics and low cycle fatigue endurance of powder metallurgy

processed pure aluminum composites reinforced with silicon carbide particles

of size 10 µm and 43 µm. The tests were conducted at 441K. Tensile

properties of the composites were also examined. It was stated that the

addition of particulate silicon carbide to the commercially pure aluminum

increases both the elastic modulus and tensile strength at elevated

temperature. The composite containing large particles showed a higher elastic

modulus but a lower tensile strength in comparison with the small-particle-

reinforced composites.

The cyclic stress response characteristics of the composites and its

aluminum matrix, in the as extruded condition, were similar to each other at

elevated temperature. The materials showed continuous cyclic softening

behavior except for the composites, which displayed slight cyclic hardening

in the first loading cycle. The silicon carbide particle size has no apparent

influence on the evolution of cyclic softening for the composites under

constant plastic strain loading except for the composite containing small

silicon carbide particles, which gives a slightly higher cycle stress response.

All of the composites and the unreinforced aluminum followed the Coffin-

Manson law at elevated temperature. The low-cycle fatigue resistance of the

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composites was lower than that of the unreinforced aluminum under higher

cyclic strain ranges.

Srivatsan and Vasudevan (1998) studied the role of composite

microstructure on failure mechanisms governing the quasi-static and cyclic

fracture behaviour of X2080 aluminium discontinuously reinforced with

silicon carbide particles. Composites with two different volume fractions

15vol% and 20vol% were produced by powder metallurgy technique. The

billets were extruded with the extrusion ratio 19:1 and age hardened to get T6

hardness. The fatigue tests were conducted on a fully automated closed servo

hydraulic test system under fully reversed loading (R = -1). The fractured

surfaces were then analyzed using scanning electron microcopy to determine

the macroscopic fracture mode and to characterize the fine scale topography

and microscopic mechanisms governing the fracture.

The initial microstructure of the composite revealed the uniform

distribution of reinforcement in the matrix in the three orthogonal extrusion

directions. An agglomeration or clustering of the particles was seldom seen.

The presence of hard and brittle silicon carbide particles in the soft aluminium

matrix caused micro cracks to initiate at low values of stress. Fractography

revealed little ductility on a macroscopic scale, but microscopically features

were reminiscent of locally ductile and brittle mechanisms. Fracture was

coupled with cracking of particles and decohesion at the interfaces allowing

the microscopic cracks to grow through metal matrix resulting in macroscopic

failure and low tensile ductility.

Han et al (1999) investigated the low cycle fatigue lives and cyclic

stress response characteristics of silicon carbide particulate reinforced

aluminium 2024 at 22o C and 190o C. The 15vol% composites were fabricated

by casting followed by extrusion with an extrusion ratio of 20:1. The

composites were then heat treated by artificial ageing to T6 condition. The

49

test results showed that the cyclic stress response characteristics of the

composite and the 2024 aluminium alloy were similar to each other in spite of

changing the test temperature. The composite and its unreinforced counterpart

generally exhibited cyclic hardening at 22oC and cyclic softening at 190oC.

An increase in the low cycle fatigue resistance for both the

composite and the aluminium alloy was observed as the test temperature rose

from 22 to 190oC. For a given temperature the low-cycle fatigue endurance of

the composite was lower than that of the unreinforced matrix alloy in the high

and middle strain regions, however, at low strains the difference in fatigue

endurance between the composite and the aluminum alloy decreased. The

mechanism of strain in the composite, i.e. the strain in the composite was

nearly completely sustained in the soft matrix, and the strain concentration

adjacent to the reinforcement were two important factors that led to the

shorter strain-fatigue life for the composite.

Koh et al (1999) investigated the low cycle fatigue behaviour of

silicon carbide particulate reinforced aluminium silicon cast alloy with two

different volume fractions 10% and 20% with average particle size of 15µm.

Heat treatment to T4 condition was performed on the composites. Tensile and

Fatigue tests were performed on a 50kN servo hydraulic test system, in

accordance test standards ASTM E8 ad E606 respectively. The composites

and the unreinforced alloy showed strain hardening behaviour. For the tensile

mean strain tests, the initial high tensile mean stress relaxed to zero for the

ductile Al-Si alloy, resulting in no influence of the tensile mean strain on the

fatigue life of the matrix alloy. However, tensile mean strain for the

composite caused tensile mean stresses and reduced the fatigue life.

The pronounced effects of mean strain on the low-cycle fatigue life

of the composite compared to the unreinforced matrix alloy were attributed to

the initial large prestrain causing non relaxing high tensile mean stress in the

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composite with limited ductility and cyclic plasticity. Fatigue damage

parameter using strain energy density accounted for the mean stress effects

quite satisfactorily. Predicted fatigue life using this damage parameter

correlated fairly well with the experimental life within a factor of 3.

Moreover, the fatigue damage parameter indicated the inferior life in the low-

cycle regime and superior life in the high cycle regime for the composite,

compared to the unreinforced matrix alloy.

Davidson and Regener (2000) studied 10wt% silicon carbide

particulate reinforced aluminium 6061 composites. The average sizes of the

silicon carbide particles were 7µm and copper coated silicon carbide particles

were also used. Double sided cold compaction method was used for the

manufacture of the composites. In-situ scanning electron microscope tensile

testing revealed enhanced failure strains in specimens containing copper

coated silicon carbide reinforcements compared with their non-coated

equivalents. In the non-coated specimens, there was evidence of decohesion

between the particulates and the matrix and no evidence of fractured particles.

There was intense deformation of the matrix beside and ahead of the main

crack and a dimpled fracture surface confirmed the ductile nature of the

failure.

That et al (2001) investigated the fatigue properties of silicon

carbide particles reinforced aluminium 5083 which specifically developed for

improving forgeability, on smooth specimens by rotating bending test.

Aluminium 5083 reinforced by 10% volume fraction of silicon carbide

particles exhibits fatigue strength of 95 MPa at 107 cycles and this value was

less than that of the unreinforced aluminium 5083 by 25 MPa. However, by

increasing the volume fraction of silicon carbide particles to 15%, high cycle

fatigue strength was almost equal and low cycle fatigue strength (from 105

51

cycles to 5*105 cycles) was apparently superior to those of the unreinforced

aluminium 5083.

By analyzing the observation results of fatigue crack, it was

revealed that fatigue crack initiation resistance was deteriorated on the surface

but crack propagation was suppressed in the bulk of the specimen, particularly

in low cycle fatigue. The cause of this fatigue crack behavior was attributed to

the clustering structure of reinforcement mechanically formed by the forging

process.

Montanari et al (2001) showed that it was possible to increase the

fatigue life and endurance limit of 20% silicon carbide particulate reinforced

aluminium 6061 by means of titanium coatings sputtered at room

temperature. The composites were produced by powder metallurgy technique

and extruded in the form of bars. The coating thickness of the three groups of

probes was 1.0, 1.5 and 2.0 µm. Both coated and uncoated probes were tested

using a rotating bending machine with a single end cantilever (R = -1,

frequency = 17 Hz).

Fatigue behaviour of the aluminum 6061/20% silicon carbide

particulate composite in rotating bending tests is highly affected by the

surface conditions of the probes. A relevant improvement of fatigue life and

endurance limit was observed in probes previously coated by sputtered

titanium. Titanium films, which are considerably harder than the substrate,

exhibit good adhesion for a large part of their fatigue life. Fatigue life and

endurance limit increase by increasing coating thickness until the surface

roughness is no more affected by the morphology of the substrate. This

condition is reached with a deposition thickness of 1.5 mm. Thicker coatings

do not modify surface roughness and thus do not improve composite fatigue

behaviour.

52

Umit Cocen and Kazim Onel (2002) studied the effect of hot

extrusion on the strength and ductility of particulate silicon carbide reinforced

aluminium alloy (Al–5% Si–0.2% Mg) composites. Cast ingots of the matrix

alloy and the composites were extruded at 500oC at an extrusion ratio of 10:1.

The microstructures and mechanical properties of the composite samples and

the matrix alloy have been investigated in the as cast state and after extrusion

and compared with the mechanical properties of hot forged composites of the

same composition.

The extruded microstructures have a more uniform distribution of

the silicon carbide particles and the eutectic silicon by comparison with as-

cast microstructures. The microstructures of the as cast composites exhibit

fairly uniform distribution of silicon carbide particles with some regional

clusters of smaller silicon carbide particles, and contain some porosity. With

the application of extrusion the clusters of silicon carbide particles disappear

and the porosity content was substantially reduced to very low levels. The

yield strength and tensile strength of the composites increased with the

volume fraction silicon carbide up to 17 vol.% and then decreased with

further additions of reinforcement. With the application of extrusion, the yield

strength and the tensile strength values were improved by approximately

40%. In the extruded samples the yield and tensile strength increases

continuously with the volume fraction of reinforcement. The ductility of the

composites was decreased with the increasing amounts of silicon carbide.

With the application of extrusion a substantial improvement in

ductility was obtained. The elongation to fracture of the composites with and

up to 22 vol.% silicon carbide was observed to be above 10%. The extruded

samples of high reinforcement composites exhibited better ductility levels

than the forged samples and this observation was explained by the reduction

53

in reinforcement particle size, the absence of particle decohesion and the

improvement of particle matrix interfacial bond during extrusion process.

Hartmann et al (2002) investigated the cyclic deformation

behaviour of three metal matrix composites (Aluminium 6061-T6 reinforced

with 20 vol.% alumina particles and short fibers and pure aluminium

reinforced with 20 vol.% short fibers) at temperatures between T= -100°C and

T= 300°C. The study was focused on the dependence of stress response

during strain-controlled cyclic deformation on the different matrix strengths,

on the reinforcement morphology at a given volume fraction and on test

temperature.

All composites exhibit initial cyclic hardening at and below room

temperature, which becomes more pronounced at higher strain amplitudes due

to a higher amount of dislocation multiplication in the matrix. Both

composites with Aluminium 6061 matrix exhibit pronounced cyclic softening

at the highest temperature of T=300°C. This was correlated with coarsening

of precipitates. Initial cyclic hardening was most pronounced for the short

fibre reinforced composite with the unalloyed matrix and less pronounced in

the case of particle reinforcement. The comparison of monotonic with cyclic

stress strain curves exhibits higher cyclic strength at higher strain amplitudes.

With decreasing strain amplitude, the cyclic strength was similar or smaller

than that obtained in monotonic tensile tests. The shape of the reinforcement

phase (particles vs. fibres) influences the cyclic stress strain curves only to a

minor degree.

Srivatsan et al (2002) studied, the quasi-static and cyclic fatigue

fracture behavior of aluminum alloy 6061 discontinuously reinforced with

fine particulates of silicon carbide. The discontinuous particulate reinforced

aluminum 6061 alloy was cyclically deformed to failure at ambient

temperature under stress amplitude controlled conditions. The influence of

54

volume fraction of particulate reinforcement on high cycle fatigue response

was presented. The underlying mechanisms governing the fracture behavior

were discussed.

The cracks initiated both at and near the particulate-matrix

interphases and in regions of particulate agglomeration. The quasi-static

fracture surfaces revealed limited ductility or brittle appearance on a

macroscopic scale, but at microscopic level features reminiscent of locally

ductile and brittle rupture mechanisms. The fracture surface revealed

combinations of tear ridges, cracked particulates and separation through

decohesion at the matrix particulate interphases.

Increasing the volume fraction of the silicon carbide particulates

resulted in higher fatigue strength. In particular 15vol% resulted in highest

strength as compared to unreinforced alloy. With an increase in particulate

content the fracture was dominated by particulate cracking and decohesion at

the particulate interfaces.

Llorca (2002) reviewed the fatigue behaviour of discontinuously

reinforced metal matrix composites at high temperature. The effect of high

temperature on the micro mechanisms of deformation, crack nucleation and

crack propagation were dealt. The overall performances of these composites

under isothermal and thermo mechanical fatigue loading have been examined.

It was stated that high temperature exposure might also induce

severe micro structural changes in the composite matrix. In particular, stable

precipitates can be formed at the particle/interface. These brittle precipitates

cannot accommodate the large plastic strains in the matrix and may lead to

interface fracture. In addition, the growth of the stable precipitates depletes

the matrix of solute in the near reinforcement areas, which become weaker,

localize the deformation, and finally nucleate cracks.

55

Both phenomena contribute to changing the preferential sites for

crack nucleations at high temperature from inter metallic inclusions and

particle clusters to failure in the matrix near the interface. Most of the damage

during thermo mechanical loading was accumulated during the high

temperature part of the cycle but the compressive hydrostatic stresses in the

matrix tended to suppress creep damage, improving the thermo-mechanical

fatigue life of the composite. The influence of the thermal strains increased

with the reinforcement volume fraction and with the difference between the

maximum and minimum temperature in the cycle. This mechanism was not

operative, however, under in-phase loading because mechanical and thermal

stresses were in opposition, and similar fatigue lives were measured in the

composite and in the unreinforced alloy.

Borrego et al (2004), performed low cycle fatigue tests on two

AlMgSi aluminium alloys with different chemical composition, namely

6082-T6 and 6060-T6 alloys, using standard round specimens and tube

specimens. The tests were undertaken in strain control with a strain ratio

R = -1. The cyclic stress strain curves were determined using one specimen

for each imposed strain level. The low cycle fatigue results were used for the

characterisation of the cyclic plastic response and the fatigue live of the

alloys. Moreover, the geometry of the hysteresis loops and the occurrence of

Masing behaviour were also analysed. The observed behaviour was discussed

in terms of the chemical composition of the alloys (Mg2Si hardening particles

and Mn dispersoid content) and fracture mechanisms. Alloy 6060-T6 exhibits

nearly ideal Masing behaviour, while alloy 6082-T6 presents significant

deviations from the Masing model. The type of cyclic deformation behaviour

in AlMgSi alloys seems to be influenced by the dispersoid phase.

Cyclic softening and hardening for axial strain amplitudes

respectively lower and higher than 0.82% were observed for alloy 6082-T6,

56

whereas alloy 6060-T6 presented stable cyclic behaviour. The ductility and

strength properties of both alloys were experimentally determined. The

transition fatigue life was found to be about 744 and 1030 cycles for alloys

6082-T6 and 6060-T6, respectively. Alloy 6060-T6 exhibits nearly ideal

Masing behaviour and alloy aluminium 6082-T6 non-Masing behaviour.

However, for alloy 6082-T6, the Masing model can still be used for strain

ranges up to 1.5%. The type of deformation behaviour in AlMgSi alloys

seems to be influenced by the dispersoid phase. This phase was enhanced by

particle/dislocation interaction and, thus, promotes non-Masing behaviour.

Xu et al (2004) fabricated metal matrix composite, gradually

distributed silicon carbide particulate reinforced aluminium matrix

composites by powder metallurgy processing. Fatigue crack growth tests of

the composite were conducted in crack growth direction of from 5% to 30%

Silicon carbide volume fraction layers under sinusoidal waveform with stress

ratios of 0.1, 0.3, 0.5 and 0.7, respectively.

The fatigue crack growth rates increases with an increase in stress

ratio. This was interpreted by crack closure mechanism. The retardation of

fatigue crack growth was found when crack propagated from low volume

fraction of silicon carbide layer to high-volume fraction of silicon carbide

layer. The crack deflection and branching at interfaces were observed, which

decreased crack growth rates. In the functionally graded metal matrix

composites, the fatigue crack growth rate was increased with an increase of

stress ratio, which was interpreted by crack closure mechanism just like in

metal matrix composites. The crack deflection and branching at interfaces

were observed, which reduced crack growth rates. The functionally graded

metal matrix composites displayed better crack resistance than metal matrix

composites with a stress ratio of 0.1 in intermediate region.

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The retardation of fatigue crack growth was found when crack

propagated from low Silicon carbide volume fraction layer to Silicon carbide

high volume fraction layer. The crack deflection and branching occurred at

transition region between the two adjacent layers, which decreased crack

growth rates.

Srivatsan et al (2005) studied the influence of discontinuous

ceramic particulate reinforcements on cyclic stress response, cyclic stress

versus strain response, cyclic strain resistance, deformation and fracture

behavior of 2009 aluminum alloy discontinuously reinforced with silicon

carbide particulates. The cyclic strain amplitude controlled fatigue properties

and fracture characteristics of the aluminium 2009 reinforced with silicon

carbide particulates composite specimens were discussed for a range of cyclic

strain amplitudes and at two different temperatures. The conjoint influence of

test temperature and strain amplitude on cyclic stress response, cyclic stress

versus strain response, and cyclic strain resistance was highlighted. The

intrinsic mechanisms governing stress response, cyclic deformation and

fatigue fracture characteristics were presented and discussed.

An increase in test temperature decreased the elastic modulus and

the strength of the 2009/SiCp/15p-T42 composite and increased ductility,

quantified both by elongation to failure and reduction in area. The 2009/SiCp

composite exhibited a linear trend for the variation of elastic strain amplitude

with reversals to failure and plastic strain amplitude with reversals-to-fatigue

failure. At equivalent plastic strain amplitudes an increase in test temperature

enhanced cyclic plasticity and improved cyclic fatigue life. The improvement

in cyclic strain resistance and resultant fatigue life was far more noticeable at

the lower cyclic strain amplitudes. Cyclic stress response of the discontinuous

particulate-reinforced 2009 composite revealed hardening to failure at all

cyclic strain amplitudes. The response is tested at both ambient and elevated

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test temperatures. The hardening was more pronounced at the higher test

temperature. At a given test temperature, the degree of hardening was greater

at higher cyclic strain amplitudes and resultant higher response stress

resulting in shorter cyclic fatigue life than observed at the lower cyclic strain

amplitudes.

For the volume fraction of the SiCp reinforcement phase in the 2009

aluminum alloy metal matrix, fracture morphology was essentially similar

over the range of cyclic strain amplitudes. Macroscopic observations revealed

fracture to be essentially brittle with microscopic features suggesting local

ductile and brittle mechanisms. The intrinsic brittleness of the reinforcing

SiCp coupled with the propensity for it to fracture due to localized

inhomogeneous deformation and local stress concentration results in

particulate cracking and interfacial failure through debonding being the

dominant damage modes.

Constanza et al (2005) studied the influence of Titanium coatings

on the different aluminium matrices (6061, 2618 and A359) reinforced with

alumina or silicon carbide particles. The composites were made either by

powder metallurgy technique or proprietary molten metal process. The

composites used were of extruded bars. A coating thickness of 2µm has been

chosen for all materials. Fatigue tests were performed on a rotating bending

machine (R= -1). Experimental results showed that Ti coatings improve the

fatigue behaviour of all the examined composites at room temperature. The

effect was observed also in tests at 200oC on composites 6061 and A359.

SEM observations on fracture surfaces showed the same features in

samples with and without coatings thus crack propagation takes place in the

same way. Since coatings did not affect crack propagation, the improvement

of fatigue behaviour was connected to delayed crack initiation. The possible

presence of compressive stresses in the matrix near the interface with the

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coating and the quality of the surface of coated samples were may be the

reasons for improvement in fatigue life.

The results of XRD measurements indicate that the residual stresses

induced in the matrix by the coatings can be considered negligible. The

improvement is ascribed to the retarded initiation of fatigue cracks, may be

due to

Ti films which are considerably harder than the substrates,

Ti films seal the defects always present on the surface of metal

matrix composites decreasing the silicon carbide roughness

They maintain good adhesion to the substrates for large parts

of fatigue life.

More homogeneous the particle distribution is in the substrate; the

better is the effect of coating.

An-Long Chen et al (2005) studied the effects of thermal cycling

on the monotonic and cyclic deformation behaviors of a cast aluminium alloy

discontinuously reinforced with fine particulates of silicon carbide. The

discontinuous particulate reinforced aluminium alloys were monotonically

and cyclically deformed to failure at room temperature under strain rate or

strain-amplitude controlled conditions. The underlying mechanisms

governing the deformation and fracture behavior of the materials with and

without thermal cycling during monotonic and cyclic loadings were studied.

The availability of superposition and the measured interactions between the

effect of reinforcement, thermal cycling and mechanical cyclic loading on the

mechanical properties were examined quantitatively.

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The amount of increase in the yield strength due to the increase in

silicon carbide particulate reinforcement was predicted by Tanaka–Mori

method. The combination of Tanaka–Mori and Manoharan–Gupta methods,

can rationalize the change of the work hardening rate induced by the presence

of the reinforcing particulates at relatively small plastic strain qualitatively.

The quantitative prediction of the amount of increase in the work hardening

rate can be available for the thermally cycled materials at relatively small

plastic strain.

The decrease in fracture strain of the reinforced aluminium alloys

compared with the unreinforced aluminium alloys was due to the existence of

the particle fracture or the interfacial debonding between the silicon carbide

particulates and the matrix alloy and the high tri-axial stresses of matrix

around the reinforcing particle causing by the plastic constraint of matrix

alloy.

Ceschini et al (2006) studied the tensile properties and the low-

cycle fatigue behavior of the 7005 aluminum alloy reinforced with 10vol% of

alumina particles and 6061 aluminum alloy reinforced with 20vol% of

alumina particles. The micro structural analyses showed clustering of alumina

particles, irregularly shaped and with a non-uniform size. A significant

increase of the elastic modulus and tensile strength in the metal matrix

composites, respect to the unreinforced alloys, was evidenced by the tensile

tests, while the elongation to fracture decreased. The temperature effect on the

tensile properties was not relevant up to 150oC, while strength significantly

decreased at 250oC, mainly in the composite with the lower content of the

ceramic reinforcement.

The low cycle fatigue tests showed no evidence of isotropic

hardening or softening for the 7005 aluminium metal matrix composites, and

a slight cyclic softening for the 6061 composites. SEM analyses of the

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fracture surfaces showed that both the tensile and fatigue fracture was

controlled by interfacial decohesion (especially for the 7005 aluminium metal

matrix composites), fracture of reinforcing particles (mainly for the 6061

aluminium metal matrix composites composite), and void nucleation and

growth. Also the presence of the MgAl2O4 spinel, probably, played a

significant role in the mechanisms of failure in the 6061 aluminium metal

matrix composites, by promoting void nucleation at the particles–matrix

interfaces, interfacial decohesion, and also failure of the particles. These

effects can be responsible of the slight softening observed in the 6061

aluminium metal matrix composites under the low cycle fatigue conditions.

Aigbodion and Hassan (2006), studied the effects of silicon carbide

particles on the as-cast microstructure and properties of Al–Si–Fe alloy

composites produced by double stir-casting method. A total of 5–25 wt%

silicon carbide particles were added. The microstructure of the alloy

particulate composites produced were examined, the physical and mechanical

properties measured including densities, porosity, ultimate tensile strength,

yield strength, hardness values and impact energy.

The results revealed that, addition of silicon carbide reinforcement,

increased the hardness values and apparent porosity by 75 and 39%,

respectively, and decreased the density and impact energy by 1.08 and 15%,

respectively, as the weight percent of silicon carbide increases in the alloy.

The yield strength and ultimate tensile strength was increased by 26.25 and

25% respectively up to a maximum of 20% silicon carbide addition. These

increases in strength and hardness values were attributed to the distribution of

hard and brittle ceramic phases in the ductile metal matrix. The microstructure

obtained reveals a dark ceramic and white metal phases, which resulted into

increase in the dislocation density at the particles matrix interfaces. These

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results show that better properties are achievable by addition of silicon

carbide to Al–Si–Fe alloy.

Yoshiaki Akiniwa et al (2006) studied the fatigue under four point

bending, a smooth specimen of an aluminum alloy 2024-T6 reinforced with

20 vol% of silicon carbide particles The X-ray diffraction method was used to

measure the loading and residual stresses in each constituent phase. The phase

stresses were determined from the diffractions of Aluminium 2 22 and silicon

carbide 116. The compressive residual stress in both phases increased with

increasing stress cycles.

The half value breadth increased with number of stress cycles

before final fracture. Difference between the phase stresses measured at the

maximum load and zero loads was examined during fatigue. The difference of

the macro stress calculated from both phase stresses decreased with the

number of stress cycles. The behavior can be divided into four regions. A lot

of cracks in the matrix and decohesions at the interface were observed on the

specimen surface. Decreasing of the difference of the macro stress was caused

by the initiation and propagation of fatigue cracks. The effect of the stress

relaxation by fatigue cracks was calculated on the basis of the crack density

and the distribution of crack length.

The compressive residual phase stress and the macro residual stress

increase with the number of stress cycles. The value of the half value breadth

also increased just before final fracture. The difference between the macro

stress measured at the maximum load and zero loads were examined. The

value decreased with the number of stress cycles. The decreasing behaviour

can be divided into four regions. A lot of matrix cracks and decohesions at the

interface were observed on the specimen surface. When the density of the

fatigue crack increased, the difference of the macro stress became small. The

macro stress at the specimen surface is released by the initiated and grown-up

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fatigue cracks. The maximum macro stress evaluated on the basis of the

effects of the change of the residual stress and the relaxation due to initiated

and grown-up fatigue cracks agreed very well with the experimental data.

Olivier Beffort et al (2007) studied the mechanical properties of

high volume fraction Silicon carbide particle reinforced aluminum based

metal matrix composites produced by means of pressurized liquid metal

infiltration. The mechanical properties were triggered by matrix alloying and

heat treatment procedures. It was distinguished between the effect of those

alloying elements that only act on matrix strengthening, leaving the interface

unaffected, and those alloying elements that interact with both (i.e. Mg).

Among the first category a further sub-division was made between pure solid

solution and precipitation hardening elements (i.e. Zn and Cu, Zn and Mg

respectively). In particular, this study addresses the effect of alloying and age

hardening for AlCu3 and AlZn6Mg1 as well as the specific role of Mg

additions to aluminium silicon carbide metal matrix composites on interface

microstructure formation, mechanical properties and fracture mode.

It was shown that single additions of Mg catalyse the formation of

Al4C3 whereas additions of Cu as well as (Zn + Mg) provide opportunities to

enhance the composites strength. Infiltration of silicon carbide particle

preforms with high purity aluminium, the formation of Al4C3 is widely

prevented, owing to the peculiarities of the squeeze casting process, which

does not provide favourable thermodynamic and kinetic conditions for

preceding the associated reaction.

Under identical process conditions, additions of Mg lead to the

formation of both Al4C3 and Mg2Si. Silicon which is released from the direct

reaction between aluminium and silicon carbide reacts with Mg to form

Mg2Si; this reaction, in turn, decreases the silicon activity and, thus favours

the formation of Al4C3 in squeeze cast AlMg /Silicon carbide composites.

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Any potential positive effect of enhanced interfacial bonding strength due to

the Mg addition on mechanical properties was counterbalanced by the

embrittling effect of the interfacial reaction products. No evidence of oxides

or oxygen containing phases has been found, meaning that the silicon carbide

particles did not contain any significant amounts of SiO2 at the origin. In

contrast to the composite flow stress, the elastic modulus of AlXX /Silicon

carbide composites is not significantly influenced by matrix alloying and heat

treatment; instead, it is dictated by the silicon carbide volume fraction and its

values of 200–210 GPa fall within the bounds proposed by Hashin and

Shtrickman.

The best compromise between maximum bending strength and

composite ductility was obtained after the addition of 3-wt% of Cu to the Al

matrix and subsequent T6 heat treatment. Comparable strength values were

obtained with the combined addition of 6-wt% of Zn and 1 wt% of Mg after

T6 heat treatment. Pure linear elastic behaviour was observed until ultimate

composite rupture, without evidence of significant matrix plastic deformation.

For both T6 aged composites, the dominant failure mechanism was silicon

carbide intra particulate fracture, while inter particulate matrix plastic

deformation and shearing, with unaffected silicon carbide particles, was the

dominant fracture mode in the other composite systems.

Cheng Nan-Pu et al (2007) studied the effects of the matrix

properties, particle size distribution and interfacial matrix failure on the

elastoplastic deformation behavior in A1 matrix composites reinforced by

Silicon carbide particles. The average size of the particles was 5µm and

volume fraction of 12% were quantitatively calculated by using the expanded

effective assumption (EMA) model. The particle size distribution naturally

brings about the variation of matrix properties and the interfacial matrix

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failure due to the presence of silicon carbide particles. The theoretical results

coincide well with those of the experiment.

The current research indicates that the load transfer between matrix

and reinforcements, grain refinement in matrix, and enhanced dislocation

density originated from the thermal mismatch between silicon carbide

particles and aluminium matrix increased the flow stress of the composites,

but the interfacial matrix failure is opposite. It was stated that the load

transfer, grain refinement and dislocation strengthening were the main

strengthening mechanisms, and the interfacial matrix failure and ductile

fracture of matrix were the dominating fracture modes in the composites. The

mechanical properties of the composites strongly depend on the metal matrix.

Shubin Ren et al (2007) investigated the effect of adding Mg and Si

to aluminum on the thermo mechanical properties of pressureless infiltrated

silicon carbide particulate reinforced aluminium matrix composites.. The

results showed that, when the Si content was lower than 6-wt% or the Mg

content was lower than 4-wt%, the composites showed poor thermo-physical

properties because of higher porosity in the composites resulting from the

poor wettability between aluminium and silicon. Increasing the silicon content

to the aluminum can enhance the elastic modulus, thermal dimensional

stability and thermal conductivity of the composites and reduce the coefficient

of thermal expansion of the composites.

However, excessive Si beyond 12-wt% can reduce the thermal

conductivity and bending strength of the composites. An optimum content of

Mg addition to aluminum was found to be 4–8 wt%, at which the composites

exhibited good thermo-mechanical properties. However, as the Mg content

was increased beyond 8-wt%, the higher porosity in the composites resulting

from the lower pressure of the magnesium led to lower thermo-mechanical

properties. The microstructures of the composites were studied by SEM and

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TEM to better understand the effect on their properties by the addition of

Si and Mg. Silicon addition to aluminum alloy can prevent or retard the

potential for chemical reactions between the aluminum alloy and Silicon

carbide.

The detrimental interfacial reaction was prevented as the Si content

was over 12-wt% at 1000oC. Si and Mg addition to the aluminum can

improve the wettability of silicon carbide by the aluminum. When the Si

content added to aluminum is lower than 6 wt% or the Mg content is lower

than 4 wt%, the infiltrated Silicon carbide/ aluminium composites exhibited

poor thermo-mechanical properties because the wettability between

aluminium and Silicon carbide was so poor that the relative density of

infiltrating silicon carbide particulate reinforced aluminium matrix

composites was very low. The Si addition into the aluminum above 6wt% can

improve the elastic modulus, thermal dimensional stability and the thermal

conductivity of the composites and reduce the coefficient of thermal

expansion of the composites. However, excessive Si beyond 12wt% could

reduce the thermal conductivity and bending strength of the composites.

Cheng et al (2008) studied the preparation, microstructures and

deformation behavior of 12 vol. % silicon carbide particulate reinforced

aluminium6066 composites fabricated by a powder metallurgy route. The

experimental results indicated that silicon carbide particles were distributed

homogeneously in the aluminum matrix and that the constituents of the matrix

were Al, needle-shaped ’-Mg2Si phases and a small amount of dispersoids

(Fe, Mn, Cu)3 Si2Al15 (BCC structure with lattice parameter a 12.8 A ).

A well-bonded silicon carbide/aluminium interface consisting of a thin and

clean layer of polycrystalline structure of metal matrix with segregation of

Mg element has been observed. The Silicon carbide particle cracking and the

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ductile-tearing of Silicon carbide/Al interfaces caused the rupture of the

composites.

The experimental data coincided well with the theoretical results

predicted by an extended effective model assumption (EMA). The current

study indicates that load transfer between the matrix and reinforcements,

grain refinement of metal matrix, and dislocation strengthening are the main

strengthening mechanisms of silicon carbide /aluminium matrix composites.

The ductile-tearing of silicon carbide /aluminium interfaces and the silicon

carbide particle cracking were the dominating failure modes and the

deformation behavior of silicon carbide/aluminium composites strongly

depends on the properties of matrix alloy.

Kyuhong Lee et al (2008) investigated the correlation of

microstructure with mechanical properties and fracture toughness of three cast

A356 aluminum alloys fabricated by low-pressure-casting, rheo-casting, and

casting-forging. Micro fracture observation results showed that eutectic

Si particles were cracked first, but that the aluminum matrix played a role in

blocking crack propagation. Tensile properties and fracture toughness of the

cast-forged alloy were superior to those of the low-pressure-cast or rheo-cast

alloy. A simple fracture initiation model based on the basic assumption that

crack extension initiated at a certain critical strain developed over some

microstructurally significant distance interpreted these results.

In the low pressure-casting alloy, eutectic Si particles were released

from network type solidification cells, and were distributed in a more

homogeneous shape. Observation of micro fracture process revealed that

micro cracks were first initiated at eutectic Si particles in solidification cell

regions, but that the crack initiated at the notch tip propagated in a repeated

process of momentary crack stopping when meeting with the matrix, crack

blunting under reapplied loading, and continued propagation. These micro

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fracture processes seemed to be hardly affected by the presence of few

micropores, which were harmful to ductility. In the rheo-casting alloy,

eutectic Si particles were cracked to initiate micro cracks under a relatively

low load, which were then connected to form longer cracks. The CF alloy had

superior mechanical properties including hardness, strength, elongation, and

fracture toughness to those of the LP and RC alloys according to the effects of

matrix strengthening and homogeneous distribution of eutectic Si particles.

2.5 SUMMARY OF THE LITERATURE REVIEW

Porosity of the samples was found to be increased with an

increase in volume fraction of silicon carbide particulates.

Appropriate matrix alloying elements such as Mg and Cu in

the aluminium silicon carbide system and reinforcement

coatings such as Cu coating on silicon carbide significantly

reduce the contact angle, enhance wettability at the interface,

and could be effective in suppressing porosity formation.

The porosity was increased with an increase in shell

temperature and hydrogen content. Low shell and low pouring

temperature generally produced high mechanical properties.

Test specimens with greater porosity were observed to have

lower fatigue life. The size and amount of inclusions, and the

size and shape of the porosity near the surface seems to have

the greatest influence in decreasing fatigue life.

The increase in porosity content decreases both the yield and

ultimate tensile strength values of the produced samples. The

average porosity content is not a reliable parameter to predict

the mechanical results.

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It has been found that secondary processing reduces the

porosity and increases the mechanical and fatigue properties

of the composites as well as matrix alloy.

The forging process results in more uniform distribution of

particulates and eutectic silicon as compared to cast

composites.

The ductility of the composites is decreased with the addition

of silicon carbide particulate, and with the application of

forging a substantial improvement in ductility was obtained.

The addition of silicon carbide particulates and ageing of the

composites increases the mechanical and fatigue strength of

the composites.

The decrease in particle size increases the mechanical and

fatigue properties of the composites.

The changes in the mechanical and fatigue properties due to

secondary processing and ageing was attributed to the changes

in the microstructure of the matrix and the composites.

The presence of hard and brittle silicon carbide particulates in

the soft and ductile aluminum alloy metal matrix caused fine

micro cracks to initiate at low values of applied stress.

The brittle fracture of the particulates and the separation at the

interface are the modes of failure of the composites.

2.6 OBJECTIVES AND SCOPE OF THIS WORK

Recent widespread publicity on the increasing use of light metals in the

transport sector replacing steel has possibly overlooked the tried and proven

70

technology of forging aluminium in a wide range of applications. MMCs are

made by dispersing a reinforcing material into the matrix. Reinforcing light

metal with abrasive material like silicon carbide or alumina improves the

mechanical and thermal properties.

The effect of reinforcing an aluminium alloy depends upon the

following factors (i) processing method (ii) reinforcement type (whisker,

particulate etc.,) (iii) geometrical constituents (shape, size and volume

fraction) (iv) ageing or heat treatment and (v) reinforcement / matrix

interphases. Several related studies have focused on understanding the

mechanical and fatigue properties of MMCs. However there exists a complex

relationship between the mechanical, fatigue response and fracture

characteristics of forged MMCs, which needs to be investigated.

This research aims to investigate the effect of particle percentage

fraction and particle size on the mechanical and fatigue behaviour of cast,

forged and age hardened MMCs. A stir casting setup was fabricated to cast

the MMCs. Three different percentages of SiCp particles 5%, 10% and 20%,

by percentage weight of Al6082 were studied. Three different particle sizes of

average particle sizes of 22µm, 12µm and 3µm were used to study the effect

of particle size variation. The mechanical properties were characterized by

porosity test, hardness test and tensile tests. The fatigue test was conducted on

a rotating beam testing machine (R= -1) to find the number of cycles to

fracture for the applied stress. The tensile and fatigue fracture surfaces were

studied using SEM micrographs to investigate the mechanism of their failure