fabrication and characterization of iron aluminide reinforced aluminiun matrix composite

7/29/2019 Fabrication and Characterization of Iron Aluminide reinforced Aluminiun matrix composite

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Development of Fe - Aluminide Reinforced Al Matrix Nanocomposite

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CONTENT

Chapter 1: Introduction

1.1 Metal Matrix Composite a brief Page 4

1.2 Ex-situ Metal Matrix Composites Page 5

1.3 In-Situ Metal Matrix Composites Page 6

1.4 Processing of MMC Page 6

Chapter 2: Objective One Of The Possible Ways

2.1 Dispersion Strengthening Mechanism of Strengthened Composite

Page 8

2.2 Strengthening Mechanism of Particulate Composite Page 8

2.3 Problems Associated with the Preparation of Ex-Situ Metal

Matrix Composites Page 8

2.4 In-Situ Metal Matrix Composites an overview Page 9

2.5 Iron Aluminide Reinforced Al Matrix Composite as the choice

Page 10Chapter 3: Literature Survey

3.1 Current Status of Research and Development of Metal Matrix

Composite Page 12

3.2 Preparation of Particulate Composites Page 13

3.3 Processing Methods of In-Situ Metal Matrix Composites

3.3.1 Application of Reinforcement/ Matrix Pre-Treatment

Strategy Page 14

3.3.2 Reactive Gas Injection Process Page 15

3.3.3 Displacement Reaction Page 16

3.3.4 XD and LANXIDE Insitu Composite Materials Page 16

3.3.5 Combustion Synthesis Process or Self Propagating High-

Temperature Synthesis Page 17

3.3.6 In situ formation of metal-ceramic composites byReduction Reaction Page 18


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3.3.7 Reactive Infiltration Technique for Manufacturing of Net

Shaped Al-Matrix Composites Page 18

3.3.8 Stir Casting Method of Fabrication of Mmcs Page 19

3.4 Processing Of Insitu Intermetallic Composite Page 21

3.5 Interaction between Matrix and Precipitate at High

Temperature Page 23

3.6 Metal-Matrix Composite Spectromicroscopy Page 23

Chapter 4: Experimental Procedure

4.1 Preparation of Insitu Metal Matrix Composite by MeltCast

Route Page 264.2 Characterizations of Metal Matrix Composite

4.2.1 Tensile Test Page 27

4.2.2 Micro hardness test Page 28

4.2.3 Charpy Impact Testing Page 30

4.2.4 Microstructural study Optical Microscopy Page 31

Chapter 5: Results and Discussions

5.1 Prediction through Optical Microscopy Page 34

5.2 Micro Hardness Measurements and Analysis Page 38

5.3 Tensile Testing Page 40

5.4 Charpy Impact testing Page 41

Chapter 6: Conclusions Page 43

Chapter 7: References Page 45


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Metal Matrix Composite a brief Ex-situ Metal Matrix Composites In-Situ Metal Matrix Composites Processing of MMC


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CHAPTER 1: INTRODUCTION

A composite material is a macroscopic combination of two or more distinct materials, having a

recognizable interface between them. Composite is a multiphase material that exhibits a

significant proportion of the properties of both constituent phases such that a better combination

of properties is realized. This is termed as the principle of combined action.

The composites industry has begun to recognize that the commercial applications of composites

promise to offer much larger business opportunities than the aerospace sector due to the sheer

size of transportation industry. Thus the shift of composite applications from aircraft to other

commercial uses has become prominent in recent years.

The various reasons for the use of composites are due to

To increase stiffness, strength and dimensional stability. To increase tough and impact strength. To increase heat deflection temperature. To increase mechanical damping. To reduce permeability to gases and liquids. To modify electrical properties. To reduce cost. To decrease thermal expansion. To increase chemical wear and corrosion resistance. To reduce weight. To maintain strength/stiffness at high temperatures while under strain conditions in a

corrosive environment. To increase secondary uses and recyclability, and to reduce negative impact on the

environment.

1.1. Metal Matrix Composite a brief:Metal matrix composites constitute a new class of materials, now starting to make a major

industrial impact in fields as diverse as aerospace, automotive and electronics. This wider interest

is because of the fact that these composites have following attractive characteristics:

Higher temperature capability Fire resistance Higher transverse stiffness and strength No moisture absorption Higher electrical and thermal conductivities Better radiation resistance No out gassing Fabric ability of whisker and particulate-reinforced MMCs with conventional metalworking

equipment.

The matrix in a metal matrix composite (MMC) is usually an alloy, rather than a pure metal, and

there are three types of such composites, namely, Dispersion-strengthened, in which the matrix contains a uniform dispersion of very fine

particles with diameters in the range 10100 nm,


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Particle-reinforced, in which particles of sizes greater than 1 m are present, and Fiber-reinforced, where the fibers may be continuous throughout the length of the

component, or less than a micrometer in length, and present at almost any volume

fraction, from, say, 5 to 75%.

It has been reported that the properties of the MMCs are influenced to a great extent by the

nature of the reinforcements and their distribution in the host metal matrix.

Ceramic whiskers, fibers and particles reinforced Aluminium alloys are now considered as

candidate materials to replace some of the existing structural components, mostly made of Fe

based alloys. For example, pistons and cylinder liners in automotive engines are currently being

fabricated from Al-based composites.[1]

Recently, the technique of reinforcing metal matrices by in situ reaction has gained considerable

attention. In this technique, the reinforcing phase(s) is (are) formed in the host matrix via in situ

chemical reaction between the matrix and the precursor element(s)/compound(s) during the

composite fabrication. These composites, termed as in situ metal matrix composites and oftenreferred as second generation metal matrix composites, offer many advantages over the

conventional composites. The most important advantage among many is that the reinforcements

so formed by the in situ reaction are finer in size and their distribution is more uniform, resulting

in better mechanical properties of composites. However, here it would be worthwhile to mention

that in most of the cases due to the high initiation temperature of the in situ reaction(s),

formation of the reinforcements within the host matrix necessitates high processing temperature.

But processing the composites at high temperatures involves the risk of oxidation of the matrix

and may also cause agglomeration and coarsening of the reinforcements, which will cast an

adverse influence on the mechanical properties of the composite.

Based on the method of preparation metal matrix composite are classified into two types:

Ex-situ metal matrix composite and In-situ metal matrix composite.

1.2. Ex-situ Metal Matrix CompositesHere, the reinforcement materials are prepared separately prior to composite fabrication and

there after incorporated into the host metal matrix without any reaction between the reinforcing

materials and the host matrix.

In composites produced by conventional means it is necessary to achieve uniform and

homogeneous distribution of reinforcing phases within the composite matrix by some mechanical

techniques for combining the various phases. The only conventional composites in which this has

been relatively easy are those employing continuous fibers. However, Ex-situ composites, while

offering many advantages are structurally efficient materials, suffer from the following significant

drawbacks, e.g.

Property anisotropy

Expensive constituents

Costly process of fabricationOther conventional ex-situ composites, which are produced by some mechanical mixing of the

constituents, typically have far from ideal microstructure with random rather than uniform


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distribution of the reinforcing phases. Though little theoretical modeling has sought to address the

effects of such non-uniform distribution of reinforcements there is substantial evidence that it

causes significant deleterious effects, e.g. the damaging effects of matrix rich areas in fiber

composites, which results in lowering of strength and toughness of these composites. Similar

effects are also noted in a variety of other composites including those reinforced by particles,plates, or whiskers. In some cases e.g. whisker reinforced metal and ceramics there are additional

difficulties associated with the mixing processes itself and with consolidation of the mixed

constituents. Therefore, it becomes very difficult indeed to add more than 20-30 volume% of

whiskers to a metal or ceramic powder and achieve any substantial degree of mixing or

subsequent densification.

1.3. In-Situ Metal Matrix CompositesInsitu metal matrix composites are defined as multiphase materials whose reinforcing phases are formed

in situ during the fabrication of the metal by the reaction between the precursors materials used.[5]

1.4. Processing of MMCsAccordingly to the temperature of the metallic matrix during processing the fabrication of MMCs

can be classified into three categories:

(a) Liquid phase processes,

(b) Solid state processes,

(c) Two phase (solid-liquid) processes.

In modern industry, it is more and more enforced to develop new composites, such as high

resistant, alternative materials of low density, superior mechanical property in order to realize

multifunctional pieces.


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Dispersion StrengtheningMechanism of Strengthened

Composite

Strengthening Mechanism ofParticulate Composite

Problems Associated with thePreparation of Ex-Situ Metal

Matrix Composites

In-Situ Metal Matrix Composites an overview

Iron Aluminide Reinforced AlMatrix Composite as the choice


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CHAPTER 2: OBJECTIVE ONE OF THE POSSIBLE WAYS

The project deals with the synthesis of a metal matrix composite, having Al matrix with the prime

reinforcement of Iron Aluminide and some oxide ceramic particles. Now the structure of IronAluminide should be such controlled that the composite reveals superior mechanical property.

The composite also consists of intermetallic of Aluminium and Magnesium as the matrix metal as a

large amount of Mg is supposed to be added to decrease Al loss as discussed further. The Al-Mg

intermetallic also strengthens the matrix of the composite imparting a higher mechanical

property.

Hence our objective is to produce an insitu metal matrix composite having Al-Mg intermetallic

matrix and the primary reinforcements of Iron Aluminide along with some ceramic particles like

magnesite, alumina etc.

Although many processes have been developed to incorporate reinforcing particles of ironaluminide in matrix, an in-situ technique in which a thermodynamically stable reinforcing phase

is produced during processing can be the most economical on synthesis of composites. A typical

example will be the conventional melt processing based on melting and casting, by which in-situ

composites can be prepared at low production costs and high efficiency. Furthermore, this

processing has an advantage that surfaces of reinforcements are not contaminated with oxidation

films or other detrimental surface reactions.

2.1 Dispersion Strengthening Mechanism of Strengthened CompositeIn the dispersion strengthened composite the second phase reinforcing agents are finely dispersed

in the soft ductile matrix. The strong particles restrict the motion of dislocations and strengthen

the matrix. Here the main reinforcing philosophy is by the strengthening of the matrix by the

dislocation loop formation around the dispersed particles. Thus the further movement of

dislocations around the particles is difficult. Degree of strengthening depend upon the several

factors like volume % of dispersed phase, degree of dispersion, size and shape of the dispersed

phase, inter particle spacing etc.[4]

In this kind of composite the load is mainly carried out by the

matrix materials.

2.2 Strengthening Mechanism of Particulate CompositeIn the particulate reinforced composite the size of the particulate is more than 1 m, so itstrengthens the composite in two ways. First one is the particulate carry the load along with the

matrix materials and another way is by formation of incoherent interface between the particles

and the matrix. So a larger number of dislocations[2]

are generated at the interface, thus material

gets strengthened. The degree of strengthening depends on the amount of particulate (volume

fraction), distribution, size and shape of the particulate etc.[3]

2.3 Problems Associated with the Preparation of Ex-Situ Metal

Matrix Composites

Higher cost of some material system. Relatively immature technology Complex fabrication methods for fiber reinforced systems (except for casting)


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Limited service experience. Numerous combinations of matrices and reinforcements havebeen tried since work on metal matrix composite began in the late 1950s. However, metal

matrix composite technology is still in the early stages of development.

Since metal matrix composites are man -made material their constituents are rarelythermodynamically stable, consequently during processing or extended high temperatureoperation extensive fiber matrix inter action occur.

Fiber damage. Micro structural non-uniformity. Fiber-to-Fiber contact. The high cost associated with currently available whiskers. Health hazards associated with high aspect ratio of the particulate.

2.4 In-Situ Metal Matrix Composites an overview

In situ composites are multiphase materials where the reinforcing phase is synthesized within thematrix during composite fabrication. This contrasts with ex-situ composites where the reinforcing

phase is synthesized separately and then inserted into the matrix during a secondary process such

as infiltration or powder processing. In-situ processes can create a variety of reinforcement

morphologies ranging from discontinuous to continuous and the reinforcement may be either

ductile or ceramic phases. The potential advantages of in-situ composites as compared to

discontinuous metal ceramic composites produced by ex-situ methods include:

1. Smaller reinforcement particle size with higher strength (a contribution from compositestrengthening mechanism) and improved fatigue resistance and creep.

2. Small, single crystal reinforcements (lower propensity for particle fracture).3. Clean, un-oxidized particle matrix interfaces with higher interfacial strength (higher

ductility and toughness) and improved wettability.

4. Thermodynamically stable particles that are weldable and castable do not dissolve athigher temperatures (vis--vis age-hardened alloys), and do not have a reaction layer

(higher interfacial strength, improved corrosion and long term stability.)

5. Better particle size distribution (improved mechanical properties).6. More conventional processing with the potential for lower cost and production with

conventional equipment.

In conclusion, the objectives to develop the in-situ particulate composites can be summarized as

follows:

1. The interfaces of the particle matrix can be cleaner, as the particles separate out of thematrix. It can lead to better interfacial bonding.

2. Very fine particles may form within the matrix to produce dispersion hardened particlecomposites.

3. The problem of non-wettability of particles and interface degradation by chemicalreaction, as observed in synthetic composites can be ruled out.

4. The in-situ formed particles may be coherent with the matrix that may improve themechanical properties of the composites.


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2.5 Iron Aluminide Reinforced Al Matrix Composite as the choice

Ordered intermetallics based on aluminides of transition metals such as iron, nickel, niobium,

titanium and cobalt have been evaluated for their potential as high temperature structural

materials for the last several decades. The amount of Aluminium in these alloys exceeds that usedin conventional alloys and can range from 10 to 30 wt. %. This concentration of Aluminium allows

the formation of an impervious oxide layer which is responsible for the excellent oxidation,

sulfidation and carburization resistance at temperatures of 1000oC or higher. In particular, iron

aluminide intermetallic alloys have a lower density than most stainless steels. They possess high

oxidation resistance and are unequalled in their resistance to sulfidation in H2S and SO2 gases. Iron

aluminides have also been found to be resistant to corrosion in certain molten salts. They are

potentially less expensive than many currently used high-temperature alloys, as they contain no

nickel and only minor amounts of other alloying elements.

A significant improvement in the room temperature properties has resulted from recent

development efforts. However, the strength of iron aluminides decreases above 873 K and thus

the advantages they offer in terms of corrosion resistance have not been fully exploited. In order

to improve the high-temperature strength of intermetallic alloys, ceramic particles can be utilized

as reinforcements. In particular, FeAl and Fe3Al alloys have been the subject of investigations

where ceramic particles have been introduced into the matrix in attempts to increase the high

temperature creep strength.

Mixing of dissimilar materials such as the reinforcement and the matrix phases may lead to

interfacial reactions during service at high temperatures, due to the lack of a thermodynamic

equilibrium between the two phases. These interfacial reactions may produce brittle phases at the

interface leading to premature failure. On the other hand, reinforcement phases that are

thermodynamically compatible with the matrix can be produced by in situ processing. One of the

many available in situ processing techniques includes fabrication using displacement reactions.

Processing by displacement reactions allows the in situ growth of reinforcements in the composite

matrix resulting in composites with unique combinations of reinforcements and matrices. Near

net-shape metal and intermetallic oxide composites can be obtained by appropriately selecting

the reactants and processing parameters, taking care to minimize the volume changes resulting

from the reaction. Displacement reactions are phase transformations between two or more

elements or compounds resulting in the formation of new product compounds that are

thermodynamically more stable than the starting reactants. The product phases exhibit specific

morphologies that depend on the relative stabilities of the growth interfaces of the product

phases. While some systems demonstrate stable planar or layered growth features throughout

the reaction, other systems exhibit morphological instabilities that cause the product phases tointerpenetrate the parent phase during growth from initially planar interfaces. Therefore,

displacement reactions exhibiting morphological instabilities have the potential to grow

interpenetrating reinforcement phases in situ.


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Current Status of Research AndDevelopment Of Metal Matrix

Composite

Preparation of ParticulateComposites

Processing Methods of In-SituMetal Matrix Composites

Processing Of Insitu IntermetallicComposite

Interaction between Matrix andPrecipitate at High Temperature

Metal-Matrix CompositeSpectromicroscopy


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CHAPTER 3:LITERATURE SURVEY

3.1 Current Status of Research and Development of Metal Matrix

Composite

During the last few years a lot of attention has been paid to the development and application of

metal matrix composites and inter-metallic matrix composites.[6]

Traditionally these composites

were produced by such processing technique as powder metallurgy,[7]

rapid solidification[6]

and

diverse casting techniques, the reinforcing phase (AL2O3 particulate for instance) is first mixed with

the matrix material. The scale of the reinforcing phase is consequently constrained by the starting

powder size, which is typically of the order of few microns to that of few tens of microns & rarely

below 1m.[8]

In the last ten years, new in-situ processing technology for fabricating metal and ceramic

composite has immerged as a very promising technique. Here it is to be noted that in-situ metalmatrix composites is generally defined as multiphase materials whose reinforcing phases(s) is(are)

synthesized in the metal matrix by reaction during the composite fabrication.[5]

This is in sharp

contrast to the conventional metal matrix composites which are termed as Ex-situ metal matrix

composites and whose reinforcing phase(s) is (are) prepared separately prior to the composites

fabrication. Conventional processing of Ex-situ metal matrix composites has some drawbacks that

have to be overcome, such as interfacial reaction between reinforcement and matrix, etc. this has

led to the development of in-situ metal matrix composites. Some researchers logically regard in-

situ metal matrix composites as second-generation metal matrix composites.[9]

Indeed this group

of material is new and under active studies worldwide because of the potential advantages it has

over the conventional metal matrix composites. Furthermore, the Ex-situ metal matrix compositesreported in the open literature are mainly discontinuously reinforced. The following are the

advantages of the in-situ metal matrix composite fabrication technique over the discontinuously

reinforced metal matrix composites produced by Ex-situ method.[9-11]

The in-situ form reinforcement is thermodynamically stable in the matrix, leading to lessdegradation in high temperature service.

The reinforcement matrix interfaces are clean contributing to an improvement inwettability.

Fabrication cost is lower because it is a one step process. The In-situ form reinforcing particle is finer in size and their distribution is more. The In-situ form reinforcing particle are finer in size and their distribution is more uniform

resulting in better mechanical properties of the metal matrix composites.

Because of the great potential that the In-situ metal matrix composites offered for widespread

application, many techniques are recently developed to produce them. Although important

processing details are not generally reported in the open literature the number of publication in

this area has been decreasing worldwide.

In most of the cases in-situ techniques use chemical reaction for the formation of the

reinforcement; these technologies include Self-propagating High Temperature Synthesis (SHS),

Direct Metal Oxidation Method (DIMOX), Exothermic Dispersion (XD) Mechanical alloying[12-17]

and reactive infiltration[18-21]

or reactive powder metallurgy.[22-23]

Because of the fineness and

thermodynamic stability of the reinforcing phase, it is expected that these in-situ composites


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should offer excellent dispersion of fine reinforcing particle and generate nascent interface,

resulting in better mechanical properties and high temperature performance.

Another advantage of in-situ technique is their capability of processing materials or composites

that could be difficult to obtain by other conventional methods. Such as Al-Fe based composites

Fe3Al has lower density and very good oxidation resistance. Such attractive characteristic alsomakes Fe3Al the potential candidate for low density high temperature structure materials. But

due to its low cleavage strength, its application is so far limited. One way to improve the strength

is to introduce reinforcing phase into the Fe3Al matrix.

Reactive sintering is one of the many routs that have been used to develop in-situ metal matrix

composites. Basically it is a process that is similar to the well-established sintering process in

powder metallurgy. One of the most important aspect is that, here, the components are reactive

and during the process chemical reaction takes place leading to the formation and dispersion of

the reinforcement in the matrix. During the reactive sintering process the matrix metal particles

are usually melted to promote the reaction and to bind the reinforcing particle. This technique has

been applied to produce Al/TiB2 metal matrix composites from elementary powder.Another technique of the application of this technique is the synthesis of Al/ (Al2O3, TiB2) master

alloy that can be diluted in an Aluminium melt to process Al/ (Al2O3, TiB2) metal matrix

composites. The synthesized components were found to be free of agglomeration and therefore

suitable for being the master alloys.

In a related field the conventional combustion synthesis or SHS method can produce high purity

products but with high porosities.[24]

The relatively high porosity (30-70%) in the product is mainly

due to (i) Low grain density of the reactant mixture. (ii) Intrinsic value changes between the

reactant and product in the combustion synthesis reaction. If the grain density of the reaction

mixture is increased the relative density of the product is expected to improve but for the powder

SHS process it is difficult to achieve full density in the reactant mixture. Therefore, consolidationtechniques, such as hot isostatic pressing (HIP) & Hot Extrusion have to be employed to increase

the relative density of the final product. Although these techniques can provide high relative

density, they do in an economic penalty.[25]

3.2 Preparation of Particulate Composites

The particulate composition can be prepared by incorporating the reinforcing particles directly

into the matrix or by producing in-situ with the help of various techniques. The first approach is

more popular, where the reinforcing particles are injected into either solid or liquid matrix.

In case of powder metallurgy technique, alloy powders are blended with the ceramic particles and

the mixture are hot or cold compacted in controlled atmosphere to desire shape and degassed.

The compact are usually hot worked for final consolidation. In case of cold compact, sintering may

precede the hot working operation. But the liquid phase processes have been investigated in

greater detail in recent year due to its ability to produce massive component at lower costing

these processes, the ceramics particles are incorporating into metal using various techniques or

cast into ingots for secondary processing. The major difficulties in such processes are the non-

wettability of the particle by liquid metal and the consequent rejection of the particle from the

melt and also the non-uniform distribution of particles due to preferential segregation and

extensive interfacial reaction.

During solidification of composite, particle-interface interaction plays a major role in dictating the

particle distribution.


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The particle ahead of the interface may get pushed, engulfed, or entrapped in the moving

solidification front. In turn the presence of the particle may influence the morphology of the

advancing interface.

In the early stage of development of cast particle composite, particle were injected into molten

Aluminium through carrier gas. To achieve greater recovery, coating of particle has beensuggested than the uncoated particle e.g. nickels coating of graphite particle in case of Al matrix

composite. Nickel improved the wettability of the particle by Al melt. To ensure uniform

distribution of particles, stirring of melt after injection has been suggested.[26]

Pellet method was developed to incorporate ceramic particle in to the Al melt. In this method the

coarser particle of the base alloy and reinforcing ceramic particle were mixed and pressed to form

pellets. These pellets were subsequently plunged into the melt followed by the slow stirring

manually or mechanical. The distribution of particles in the cast particulate composites was not

satisfactorily uniform when it was prepared by injection technique or pellet method without

stirring.

Addition of Mg, Li, Si, and Ca into Al melt improved wettability either by changing the interfacial

energy through some interfacial reaction or by modifying the oxide layer on the metal surface. Mg

addition has an effect on the recovery of alumina particles in Al melt. It is also revealed in their

study, that fresh addition of Mg is more effective as compared to pre alloyed Mg. Addition of Ca

also improve the wettability of alumina particle in Al-4.5 Cu melt, but its effect on improvement of

the retention of particle is less than with magnesium addition.

Another approach to avoid the rejection of ceramic particle from Al melt is to add the particle into

partially solidified slurry of liquid alloy. A suitable temperature is selected to have about 40% of

solid in the alloy. The partially solid alloy is agitated and the particles are added to it. Initially the

ceramic particles are mechanically entrapped in the partially solidified slurry and with furthermixing, interaction between the ceramic particle and the liquid alloy promote binding. After

mixing of the ceramic particle, the semisolid composite slurry can be cast directly by rheocasting.

In another method, the composition slurry is heated to temperature above its liquid temperature

and subsequently can be cast. This process is known as compocasting. A similar method has been

used to prepare Al alumina particle composites by addition the particle into a mechanically

stirred Al alloy in the semi fused state. The process requires costly tooling and still, uniform

distribution of particle is difficult to obtain.

In this case we have decided to go for Liquid metal Technique to process the composite and Stir

Casting Method of Fabrication. The theoretical outline of the method is described briefly below.

3.3 Processing Methods of In-Situ Metal Matrix Composites

Advanced composite materials have been widely used in aerospace and military application. But

the present situation demands their commercial applications. The primary reason, which prohibits

their usage as commercial material, is their high cost, which is involved in their processing. In this

scenario processing of composites have taken a large part of the research activities.

3.3.1 Application of Reinforcement/ Matrix Pre-Treatment StrategyIn a recent investigation, reinforcement pretreatment was used to achieve desired interfacial de-

bonding performance in a system of practical engineering importance.[26]

The process includedcoupling reinforcement pretreatment with reactive hot compaction to synthesize a niobium

aluminide composite reinforced with ductile niobium filaments as a toughening phase. The pre-


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oxidized Nb filaments, niobium powder and Aluminium powder were blended and compacted. In

the presence of low melting point liquid Aluminium, niobium oxide on the surface of the pre-

oxidized niobium filaments were converted, in-situ, to niobium + alumina and the niobium powder

was converted by reaction sintering to niobium-aluminide. It has been previously determined that

alumina coating of niobium filament was necessary as a diffusion barrier to prevent formation ofstrongly bonded embrittling reaction layers at the filament matrix interface.

3.3.2 Reactive Gas Injection Process

The reactive gas injection process (RGI)[27]

process has been developed for the synthesis of fine

single crystal TiC platelets in Al and Ni matrix. The process involves the injection of carbon or

nitrogen bearing gas into a mother alloy and subsequently reaction of carbon/nitrogen containing

gas with the alloying elements to produce micro level reinforcement. The highly exothermic

process is moderated by means of the carrier gas and leads to fine homogeneous distribution of

the stoichiometric carbides. The in situ process produces TiC of size 0.1 to 5 m (Vis a Vis 5 to 100m for artificial mixing processes) with a narrower carbide size distribution. The minimum i is

limited b the nucleation process and corresponds to 0.1 m in diameter. he maximum size of

reinforcement is determined by diffusion controlled coarsening of carbides and varies from 5 to 15

m. The maximum volume fraction reinforcement is limited by the melt viscosity and does not

actually exceed 40 volume percent. The schematic of the in situ synthesis techniques is shown in

the figure below. An alumina crucible coated with Yittria (Y2O3) was used as the reaction vessel

and was placed within a graphite susceptor to ensure uniform heating over the length of the

crucible. A thermocouple placed in contact with the bottom surface of the graphite susceptor was

used for temperature monitoring purpose. Melting was achieved under vacuum and subsequently

the chamber was backfilled with purified Argon which also serves as the carrier gas. Upon reaching

the appropriate processing temperature, the carbonaceous gas was introduced into the melt via agas diffuser system. The reaction was conducted at a constant temperature for an appropriate

length of time to ensure complete conversion of Ti to TiC. After completion of the reaction the

Fig 3.1: Schematic of the RGI synthesis laboratory approach


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powder was turned off and the melt was allowed to solidify. The processing time and temperature

depend upon the gas partial pressure and alloy chemistry. Processing times and temperatures

ranged from 6 minutes to 8 hours and 1150oC to 1600

oC respectively, depending upon the matrix

chemistry as well as the melt quantity and volume fraction desired.

3.3.3 Displacement Reaction

Displacements reactions are the processes in which one element displaces another in a compound

by diffusional transport to form new product compounds that are more thermodynamically stable

than the starting reactants. For such reactions between metals and oxides, two distinct

morphologies have been observed, aggregate and layered. Which morphology forms depends on

the presence or absence of the growth instabilities governed by the principle of maximum

reaction rate.[28]

For systems in which aggregate structure occurs displacement reactions can be used to grow

reinforcement in-situ. An example of the system is SiO2 in molybdenum di-silicide matrix[29]

.The

processing involves reacting compacts of Si and molybdenum carbide in the range of 1600o

1800oC. It produces an initial layer of Mo5Si3C. The true displacement reaction occurs between this

ternary phase layer and silicon to form molybdenum disilicide + silicon carbide. In situ composites

are also being formed by displacement reactions between liquid and solids. A method involving

the displacement of silicon from silica or mullite by Aluminium has been demonstrated.[30]

When

silicon is placed in melted Aluminium at approximately 1100oC, the silica reacts with the

Aluminium to form a ceramic/metal composite consisting of co-continuous mixture of alumina and

Aluminium, with silicon precipitates in the metal.

3.3.4 XD and LANXIDE Insitu Composite Materials

Fig 3.2: Schematic of process for production of original Lanxide Dimox material


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Both the XD and LANXIDE are the process of development of in situ composite materials. The

LANXIDE belongs to the family of metal-toughened ceramics whereas XD belongs to the family of

particulate and whisker reinforced metal and intermetallics. Both the materials can be classified as

resulting from SHS (Self Propagating High Temperature Synthesis) type reactions which involve

exothermic process.[31]

Both of these materials are truly in situ composites with reinforcingphases produced during the processing and fabrication of the materials, not added to the matrix

through some mechanical dispersion scheme. The original LANXIDE materials consisted of an

Aluminium oxide matrix toughened by a minor amount of Aluminium metal at the Aluminium

grain boundaries and were capable of being produced to near net shape.

These materials were produced by controlled oxidation of Aluminium metal alloy performs of

particular composition as shown in figure 3.2.

In addition to the concerns about keeping the basic process proprietary there were also concerns

that the many possible ramifications of the process, such as the potential for net shape

processing,[32]

the formation of composites,[33]

and the preparation of materials such as nitrides[34]

or carbides

[35-37]

could be easily deduced once the basic process itself was understood.The original XD materials consisted of a matrix of various titanium aluminides and titanium,

reinforced by a dispersion of inter-metallic and ceramic particles produced by exothermic

reactions of constituents during billet formation. This material looks very promising when

compared to conventional titanium alloys. The XD Ti3Al+TiAl+TiB2 material showed much higher

elastic modulus than Ti-6Al-4V.[38-39]

The XD materials also showed improvements in yield stress

and creep resistance at temperatures over 1000K.

The XD process is also one where the ramifications are fairly obvious, once one has the basic

concept. If it is possible to produce TiB2 reinforced Ti3Al/TiAl, it should likewise be possible to

produce a wide range of other similar materials, e.g. ceramic particulate reinforced intermetallics

as well as ceramic and inter-metallic reinforced metals.

3.3.5 Combustion Synthesis Process or Self Propagating High-

Temperature Synthesis

The underlying basis of combustion synthesis relies on the ability of highly exothermic reactions to

Fig 3.3: Schematic representation of the temperature-time curve during SHS

reaction


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be self-sustaining and, therefore, energetically efficient. Thee exothermic self-propagating high

temperature synthesis (SHS) reaction is initiated at the ignition temperature Tig and generates

heat, which is manifested in a maximum or combustion temperature Tc, which can exceed 3000K.

The exothermic reaction mixture, normally in the form of powders is pressed into a pellet of

certain green density and is subsequently ignited, either locally at one point (propagating mode)or by heating the whole pellet to the ignition temperature of exothermic reaction (simultaneous

combustion mode). A schematic representation of a typical temperature-time slot for a

combustion reaction is given in figure below. The un-reacted reactant mixture, at initial

temperature T0, is heated to the ignition temperature T ig, whereupon the reaction is initiated.

One of the major limitations of combustion synthesis is the degree of porosity of the synthesized

products. Typical porosity levels which reach 50%,[40]

is quite acceptable and quite advantageous

if the resultant products are to be subsequently milled and used as powders. However if the main

objective is to produce dense components during the synthesis and processing operation, a

subsequent or simultaneous compaction process is required. Coupling the simultaneous

combustion mode of SHS with hot pressing and the use of diluents to control the combustiontemperature has been shown to produce dense ceramic-metal composites.

[41]

3.3.6 In situ formation of metal-ceramic composites by Reduction

Reaction [42]

The starting materials were manganous carbonate, MnCO3.xH2O and hematite (99%). The

MnCO3.xH2O was calcined overnight and then at 1100oC for 24 hours to obtain Mn3O4. The Fe3O4

and Mn3O4 powders were wet ball milled using Teflon balls. The mixed powder was dried and

compacted in an alumina die in a controlled atmosphere hot press at 1300oC, with a stress of

about 12 MPa under uniaxial loading at an oxygen activity of Log (aO2) = -10.35 maintained by aCO/CO2 (= 4 : 1) gas mixture diluted in 50% nitrogen. The sample was then allowed to remain in

the furnace for 5 hours to transform the initial oxide mixture to stable oxide solid solution (Fe1-

xMnx)O with the rock salt structure. The presence of this phase was confirmed by an X-Ray

diffraction powder experiment. The hot pressed material produced by this process was estimated

to have 95-96% of the theoretical density. For addition of dopants, such as ZrO2 a water soluble

compound, zirconyl nitrate (ZrO (NO3).9H2O) was added in appropriate amount during ball milling.

This allowed a uniform distribution of the dopant within the polycrystalline material. The samples

used for reduction were rectangular bars with cross section of 1mm 2 and length of 2-3 mm.

For a systematic comparison of the metal-ceramic microstructures obtained for different

reduction conditions, the following process parameters are supposed to be used-

Temperature ranging from 800oC to 1200oC. Two ratios of CO/N2: 5% O in N2 and 10% O in N2 (N2 denotes Nitrogen containing

Oxygen as an impurity at a level of 200 ppm).

Reaction time of 4 and 8 hours, and 0.5 wt% and 2wt% addition of ZrO2 as dopant.

3.3.7 Reactive Infiltration Technique for Manufacturing of Net

Shaped Al-Matrix Composites [43]

Reactive infiltration has potential for fabricating complex near net shaped MMC components. The

process consists of infiltrating a carbon or polymer precursor preform with a reactive alloy and

possible reactions are dependent on the relative thermodynamic stability and reaction kinetics.


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Al-Ti/Si + [C]* Al + TiC/SiC

Al-Ti/Si + [AlN] Al + TiN/Si3N4

Al Ti/Si + [C2H5]n Al + TiC/SiC

(Preform materials in parenthesis)

It is possible to tailor the preform to have graded reinforcement and controlled reinforcement

volume fractions. The polymeric performs can also be selected to have non-carbide

reinforcement. The matrix reinforcement i.e., Vf, size, gradient can be controlled by designing the

appropriate preform.

Reticulated graphite preform or carbon preform can be used but it is believed that it is preferable

to use carbon perform instead of the reticulated graphite due to higher reactivity of the carbon as

compared to the reticulated graphite. Three different Aluminium alloys were utilized forinfiltration; Al - 10wt% Ti, Al - 25wt% Si and Al - 22wt% Si - 1.5wt%Mg - 1.5wt%Zn. Mg and Zn were

added to the Al - Si alloy to study the effect of tertiary and quaternary addition on infiltration

kinetics. The infiltration was done in a pressure casting facility with carbon perform, 27.5 mm

diameter and 45 mm high. The preform was preheated to 1250oC and infiltrated with the selected

alloys at 1150oC; the casting was held at 1150

oC for 15 minutes and then cooled to room

temperature.

3.3.8 Stir Casting Method of Fabrication of Mmcs

Stir Casting is a liquid state method of composite materials fabrication, in which a dispersed phase(ceramic particles, short fibers) is mixed with a molten matrix metal by means of mechanical

stirring. The liquid composite material is then cast by conventional casting methods and may also

be processed by conventional Metal forming technologies.[44]

Fig 3.4: Melt Stirring Route

Reinforcement

Molten

Metal

Heating Coil

Stirrer

T>Tm


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Speed of Rotation:The control of mould speed is very important for successful production of casting. Rotational

speed also influences the structure, the most common effect of increase in speed being to

promote refinement and instability of the liquid mass at very low speed. It is logical to use thehighest speed consistent with the avoidance of tearing.

Pouring Temperature:Pouring temperature exerts a major role on the mode of solidification and needs to determine

partly in relation to type of structure required. Low temperature is associated with maximum grain

refinement and equiaxed structures while higher temperature promotes columnar growth in

many alloys. However practical consideration limits the range .The pouring temperature must be

sufficiently high to ensure satisfactory metal flow and freedom from cold laps whilst avoiding

coarse structures.

Pouring speed:This is governed primarily by the need to finish casting before the metal become sluggish;

although too high a rate can cause excessive turbulence and rejection. In practice slow pouring

offers number advantages. Directional solidification and feeding are promoted whilst the slow

development of full centrifugal pressure on the other solidification skin reduces and risk of

tearing. Excessive slow pouring rate and low pouring temperature would lead to form surface lap.

Mould Temperature:The use of metal die produces marked refinement when compared with sand cast but mould

temperature is only of secondary importance in relation to the structure formation. Its principal

signification lies in the degree of expansion of the die with preheating .Expansion diminishes the

risk of tearing in casting. In nonferrous castings, the mould temperature should neither be too low

or too high. The mould should be at least 25 mm thick with the thickness increasing with size and

weight of casting.

Mould Coatings:Various types of coating materials are used. The coating material is sprayed on the inside of the

metal mould. The purpose of the coating is to reduce the heat transfer to the mould .Defects like

shrinkage and cracking that are likely to occur in metal moulds can be eliminated, thus increasing

the die life. The role of coating and solidification can be adjusted to the optimum value for a

particular alloy by varying the thickness of coating layer. For Aluminium alloys, the coating is a

mixture of Silicate and graphite in water.

Mould Life:Metal mould in casting is subjected to thermal stresses due to continuous operation. This may

lead to failure of the mould. The magnitude of the stresses depends on the mould thickness and

thickness of the coating layer, both of which influence the production rate. Deterioration takes

place faster in cast iron mould than in steel mould.[45]

Stir Casting is characterized by the following features:

1. Content of dispersed phase is limited (usually not more than 30 vol. %).2. Distribution of dispersed phase throughout the matrix is not perfectly homogeneous:


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Fig 3.5: Laboratory Technique of Melt Cast Route to Process Insitu Composite

There are local clouds (clusters) of the dispersed particles (fibers); There may be gravity segregation of the dispersed phase due to a difference in the

densities of the dispersed and matrix phase.

3. The technology is relatively simple and low cost.[44]

3.4 Processing Of Insitu Intermetallic Composite

The conventional method of preparing IMCs is casting.

A casting mold includes a melt-receiving mold cavity having a preformed metallic or intermetallic

insert suspended therein by one or more first elongated, slender suspension members fixed at

one end to the insert and at another end to the mold to locate the insert in a first direction in the

mold and by one or more second elongated, slender suspension members fixed at one end to the

insert and abutting the mold at another end to locate the insert in a second direction in the mold.

A melt of metallic or intermetallic material is introduced into the mold cavity about the suspended

insert and the suspension members and is solidified to form a composite casting. The casting issubjected to elevated temperature/elevated isostatic gas pressure conditions wherein the

interface between the suspension members and the cast melt is effective to inhibit gas

penetration between the insert and cast melt A method is described for preparing a refined or

reinforced eutectic or hyper-eutectic metal alloy, comprising: melting the eutectic or hyper-

eutectic metal alloy, adding particles of non-metallic refractory material to the molten metal

matrix, mixing together the molten metal alloy and the particles of refractory material, and casting

the resulting mixture under conditions causing precipitation of at least one intermetallic phase

from the molten metal matrix during solidification there of such that the intermetallics formed

during solidification wet and engulf said refractory particles. The added particles may be very

small and serve only to refine the precipitating intermetallics in the alloy or they may be larger and

serve as reinforcing particles in a composite with the alloy. The products obtained are also novel.A discontinuously reinforced metal matrix composite wherein the reinforcing material is a

particulate binary intermetallic compound is described along with methods for preparing the


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same. Metal matrixes reinforced with discontinuous intermetallic particles may be prepared by

cooling a molten alloy, minimally comprising the primary matrix metal and a sufficient quantity of

the other metal, wherein the other metal is a metal capable of forming an intermetallic compound

with the primary matrix metal, to precipitate the intermetallic compound as a particulate. The rate

at which the molten alloys of desired composition are cooled determines the size of the resultantintermetallic particles dispersed in the metal matrix. The size of these intermetallic particles is

inversely related to the cooling rate of the alloy. That is high cooling rates produce small

intermetallic particle sizes while low cooling rates produce large intermetallic particles. Routine

experimental methods well known to those skilled in the art may be used to identify those cooling

rates that result in dispersed intermetallic particles having the desired particle size. The cooling

rates required to produce the intermetallic particle sizes of the invention are typically very rapid.

Such high cooling rates may be obtained using gas atomization of the alloy.[58]

Alternatively, it may be possible to utilize other rapid cooling methods such as, but not limited to,

splat cooling. The binary intermetallic compound includes the same type of metal as is theprincipal matrix metal in combination with one other metal. The particle size of the particulate

binar intermetallic compound ma be less than about 20 m and ma be between about 1 m

and about 10 m. he intermetallic particles ma be present in the discontinuousl reinforced

metal matrix composites in an amount ranging from about 10% to about 70% by volume. The

discontinuous reinforced metal matrix composites of the invention may be used in structures

requiring greater strength and stiffness than can be provided by matrix metal alone. Scott invents

an intermetallic composite and method of making an intermetallic composite is disclosed

comprising a porous titanate perform of the formula iOz, where represents an element

reducible by molten Aluminium to form an aluminate of the formula AjOk.[45]

An Aluminium based metal matrix composite is produced from a charge containing a rapidly

solidified Aluminium alloy and particles of a reinforcing material present in an amount ranging

from about 0.1 to 50 percent by volume of the charge. The rapidly solidified ribbon is the product

of a melt spinning process selected from the group consisting of jet casting or planar flow casting.

In such processes, which are conventional, the melt spun ribbon is produced by injecting and

solidifying a liquid metal stream onto a rapidly moving substrate. The ribbon is thereby cooled by

conductive cooling rates of at least about 105/sec and preferably in the range of 10

5to 10

7/sec.

Such processes typically produce homogeneous materials, and permit control of chemical

composition by providing for incorporation of strengthening dispersoids into the alloy at sizes and

volume fractions unattainable by conventional ingot metallurgy.[57]

A metal matrix composite is formed by contacting a molten matrix alloy with a permeable mass of

filler material or preform in the presence of an infiltrating atmosphere.

Under these conditions, the molten matrix alloy will spontaneously infiltrate the permeable mass

of filler material or preform under normal atmospheric pressures. Once a desired amount of

spontaneous infiltration has been achieved, or during the spontaneous infiltration step, the matrix

metal which has infiltrated the permeable mass of filler material or preform is directionally

solidified. The directionally solidified metal matrix composite may be heated to a temperature in

excess of the liquidus temperature of the matrix metal and quenched. This technique allows the

production of spontaneously infiltrated metal matrix composites having improved microstructures

and properties. The Aluminium- iron ore metal matrix composite was prepared by stir casting

route.


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3.5 Interaction between Matrix and Precipitate at High TemperatureThe product of interaction between Al and iron Fe3Al at elevated temperature has a wide range of

applications in refractory, structural and electronics. If the interfacial region between the reactant

compounds is examined using analytical techniques; the formation of Fe3Al as the interfacialcompound is described. The thermodynamics of the Al Fe O system is explained as it relates to

the particular conditions for the Al Fe3Al reaction research. Thermodynamic principles have been

used to demonstrate that the formation of Fe3Al is favored instead of other FexAly compounds for

the set of conditions outlined in this thesis. A study of the mechanism of interactions in the

interfacial region can help towards being able to determine the reaction kinetics that lead to the

control of microstructure and thus an improvement in the material performance. The formation of

Fe3Al at the interface is a result of the reduction reaction between Fe2O3 and Al. The O released

during the reduction of Fe2O3 has been investigated and demonstrated to partly remain dissolved

in Fe3Al at the interfacial region. Some O reacts with Al as well to form crystalline Al2O3 in the

interfacial layer.[53-56, 59]

The interaction between Al and Fe2O3 yields Fe3Al and possibly another compound at the

interface. The interfacial layer was found to be continuous; but the thickness was not uniform. The

interfacial region physically separated the surfaces that were in contact with each other. Such an

interfacial layer could act as a self-formed barrier against the diffusion of the reacting species

towards each other; for example, the diffusion of Al towards the Fe2O3 substrate, and the diffusion

of Fe or O towards the Al film. The self-formation of the diffusion barrier between the reacting

components would be useful in structural applications and in electronic device applications. It was

necessary to understand the nucleation and growth of the interfacial compound that would be the

outcome of the interaction between the two surfaces in contact at elevated temperature.

Presence of magnesium enhances the diffusivity of Fe atom through the matrix by increasing

wettability.

So in case of our project the magnesium addition that we have preferred due to support the

diffusion and for the solid solution strengthening of Al matrix.

The formation process of the interfacial compound (FeAl) could be described in the following way:

Step I: Melted Al was in contact with the Fe2O3 substrate at elevated temperature.

Step II: Al diffused into the Fe2O3 substrates more quickly through the stacking faults in the Fe2O3

substrate.

Step III: At the stacking faults, Fe2O3 is reduced by Al to form Fe and O. Fe reacts with Al near the

stacking fault sites to form Fe3Al. These kind of stacking fault sites are accelerated formed due to

addition of a high amount of Mg.

Step IV: The interfacial region containing Fe3Al grows near the stacking fault sites into the Fe2O3

substrate.[60-62]

3.6 Metal-Matrix Composite Spectromicroscopy [63]

Composite materials appear everywhere in life, both man-made (such as fiberglass) and

biologically produced (like mammalian bones). The purpose of a composite man-made material is

to alter and improve the properties of the matrix material, by the addition of some secondmaterial with very different chemical and structural properties. Examples of an important area of

composite research are metal-matrix composites, in which a metallic host material is modified


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through the addition of a ceramic. The ceramic may be in the form of particles or fibers. The goal

is to supplement the desirable properties of the metal, such as ductility, by the addition of a

ceramic, which can improve the performance of the material in its final application. Common goals

in the creation of new metal-matrix composites are to achieve increases strength reduced friction

or prevent corrosion.There are many fundamental microscopic issues that are important in metal-matrix composites

that typically deal with the chemistry of the interface between the alloy matrix and the ceramic,

which may be in the form of a particle, fiber, or whisker. Typical concentrations of ceramic range

from 10-20%, with particle size ranging from a few microns to more than 100 microns in diameter.

We have begun some spectromicroscopy studies of SiC fiber reinforced Ti alloys, which illustrate

the kind of information one can get, by studying the changes in the chemistry of the matrix-

ceramic interface. Certain failure modes of the composites will be determined by the adhesion of

the metal matrix to the fiber surface. Also, the interaction of the metal with the fiber as a function

of temperature and time is a major factor in determining the life cycle of a composite, and its

suitability for processing.

A sequence of XPEEM images of SiC reinforced Ti alloy is shown in the figure above, from recent

work using the PRISM x-ray emission microscope at the Spectro Microscopy Facility. These images

are taken at certain photon energies, which create good chemical contrast between different

regions of the composite. The fiber itself is rather complex, consisting of a central core region of

pyrolitic carbon, a region of SiC, and an outer layer consisting mostly of carbon, with some small

SiC inclusions. All of these regions are easily distinguished in the x-ray micrographs, and the

elemental composition of the various regions can be used to preferentially enhance the emission

from one area or the other. This is done by selecting a photon of energy near the absorption edge

of Si, C or Ti respectively.We get chemical information from small areas of the sample using microXANES. XANES stands

for x-ray absorption near-edge structures, and is an x-ray absorption fine-structure spectroscopy

(XAFS). It tells us about the chemical state of the material. The microXANES spectra from each

region of the metal-matric composite contain sharp structures that aid in determining the

chemical composition. For example, the C K-edge spectra are very different for C in the central

core of the fiber, in the SiC region, and in Ti, reflecting the bond changes from graphitic, to carbidic

species. No Ti is found in the fiber region for this sample.

Interesting changes occur in these composites when they are heat-treated. The heat treatment

causes reactions to take place between the fiber and the Ti alloy matrix. Concentration profiles

develop indicating substantial migration of Si, C, and Ti, from one region of the sample to another.

The diffusion is very non-uniform, with different behavior for all three elements.

Metal-matrix composites like these have been studied by other techniques, including transmission

electron microscopy with energy loss spectroscopy (TEM-EELS), electron probe x-ray spectroscopy

and scanning Auger microscopy. The unique advantage we can offer is the enhanced ability to

detect the chemical state of reaction products at interfaces, directly, by XANES and XPS

spectroscopy. For example, one study of SiC-Al MMs found an increase of oxgen signals in

various parts of the sample, but could not directly say what was happening: Aluminium oxide

formation, hydroxide, silicon oxide, or organic oxygen? We can answer such questions easily with

the chemical-shift and fingerprinting of XANES and XPS.


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Preparation of Insitu Metal MatrixComposite by MeltCast Route

Characterizations of Metal MatrixComposite


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CHAPTER 4: EXPERIMENTAL PROCEDURE

The experimental procedure in this thesis has been classified and described under the following

heads.

1. Preparation of the insitu MMC by Melt-Cast route.2. Characterization of the insitu metal matrix composites.

4.1 Preparation of insitu Metal Matrix Composite by MeltCast Route

For preparing 10 Vol% Iron aluminide (Fe3Al) and Alumina reinforced Aluminium Metal Matrix

Composite following materials are taken:-

Commercially pure Aluminium Pure Aluminium (Al) powder Micron size Fe2O3 (Iron Oxide) Magnesium cake

First of all the Iron Oxide (Fe2O3) powder is taken into the ball mill as the required

amount (50 g) with respect to the amount of Aluminium (Al) taken for the melting to produce 10

vol% Al2O3 & Fe3Al reinforced Aluminium metal matrix composite. The ball mill (wet process) is run

for 35 hours to prepare nano size (30 to 40 nm) Fe2O3 powder. Toluene is used in this process for

wet medium. Hence after completion of ball mill operation toluene is evaporated by hot oven at

the temperature range of 75 150oC. Now after evaporation of Toluene the nano size Fe2O3

powder is taken out from the oven and intimate mixture of nano size iron oxide powder are

preheated in a sealed Aluminium tube at 550 for 1 hour and finall raised to 600 just beforeaddition.

50 g Mg is taken in the form of small cakes for the addition during melting to increase the

wettability of the reinforcement in the Al matrix and to prevent Al loss. It is considered here that

due to loss of ignition only 10-15 g Mg will go into the solution. That will lead a 2-3 vol% of Mg

addition.

On the other hand the commercially pure Aluminium (500 g) is taken in a graphite crucible and put

into a furnace at 750oC. When solid Aluminium is melt then Flux is added for preventing it from

oxidation. After 15 minutes holding when the preheated tubes containing the iron oxide are put

into the molten Aluminium then a splashing reaction takes place. The magnesium is added at thistime. Starring of the metal is done by a drill gun. As it is an exothermic reaction the furnace

temperature suddenly increases upto 1000-1100. Due to the high temperature, some amount

of Al gets oxidized; therefore additional amount of Al is taken into account whereas this additional

amount of Al is also used for the dilution of the reinforcement. Thus the molten composite is

produced and poured into the metal mould.

The following reactions are expected to be taken place during melting.

Fe2O3 + Mg FeO + MgO

Al + FeO Fe3Al + Al2O3

Al2O3 + MgMgO + AlNow this Fe3Al particles present there as the major reinforcement with very small amount of MgO

and Al2O3 particles.


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4.2 Characterizations of Metal Matrix Composite

The composite samples so prepared were characterized extensively by Microhardness

measurement, tensile test, Impact test, optical microscopy.

4.2.1 Tensile Test

For tensile testing the sample is given to some thermal as well as thermo mechanical treatment.

The sample is hot rolled twice giving 30% deformation in each case. Then specimens used in a

tensile test are prepared according to the standard specifications. The test pieces can be flat. Fig

4.1 shows the standard dimensions of a typical flat specimen. It is gripped into two ends and

pulled apart in a machine (INSTRON TENSILE TESTING MACHINE) by the application of a load. The

stress-strain curve obtained from the tensile of a typical ductile metal is shown in the figure 2.2.

On the axis, the engineering stress , defined as load P divided b the original cross -sectional

area A0 of the test piece is plotted. The engineering strain , defined as the change in length L

divided by the initial gauge length L0 is plotted on the x-axis. The % elongation is obtained by

multiplying the engineering strain by 100.

The stressstrain curve starts with elastic deformation. The stress is proportional to strain in this

region, as given b HOOKS LAW. At the end of the elastic region, plastic deformation starts. heengineering stress corresponding to this transition is known as the yield strength (YS), an

important design parameter. Many metals exhibit a continuous transition from the elastic region

to the plastic region. In such cases, the precise determination of the yield strength is difficult. A

parameter called proof strength (or offset yield strength) that corresponds to a specified

permanent set is used. After loading up to the proof stress level and unloading, the specimen

shows a permanent elongation of 0.1% or 0.2%. The stressstrain curve has a positive slope in the

plastic region, indicating that the stress required causing further deformation increases with

increasing strain, a phenomenon known as work hardening or strain hardening.

Fig 4.1: Flat tensile Specimen


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The engineering stress reaches a maximum and then decreases. The maximum value is known as

ultimate tensile strength (UTS) or simply the tensile strength. Up to the UTS, the strain is uniformly

distributed along the gauge length. Beyond UTS, somewhere near the middle of the specimen, a

localized decrease in cross-section known as necking develops. Once the neck forms, further

deformation is concentrated in the neck. The strain is no longer uniform along the gauge length.

The cross-sectional area of the neck continuously decreases, as the % elongation increases. Voids

nucleate in the necked region at the interface of hard second-phased particles in the material.

These voids grow and coalesce, as the strain increases. The true cross-section bearing the load

becomes very small, as compared to the apparent cross-section, due to the growth of these

internal voids. At this stage, the specimen may fracture.

4.2.2 Micro hardness test

Hardness is the property of materials by virtue of which it offers resistance to scratch, indentation

or rebound and accordingly they are called scratch hardness, indentation hardness and rebound

hardness respectively. The instrument used is Leica VMHT micro hardness tester.

The values calculated according to the following formula from the test load at the time when the

test surface is indented and the surface area obtained from the lengths of the diagonals of the

indentation by the use of Diamond indenter in the form of right pyramid with a square basehaving the angle between the opposite faces of 136

o.

Fig 4.2: Conventional Stress Strain Curve


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() ( )

Where

HV=Vickers hardness F=Test Load S=Surface area of indentation (mm2) d=mean value of the diagonal of the indentation (mm). =Angle between the opposite faces at the vortex of Diamond indenter.

In the case where the unit of test loads F is in kgf, Vickers hardness shall be calculated according to

the following formula:--

() ( ) ( )

()

Fig 4.3: LEICA VHMT Micro hardness Tester


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Samples were placed on the platform of the hardness tester and hold the sample vie a vice. Then

samples were focusing using 500X magnification. A place was fixed for taking Vickers hardness.

Hardness was taken at two different positions, one in the matrix and another in the particle rich

region. Two loads were used (i.e. 25 gf and 10 gf) in the two different micro hardness tester.

Photography of selected samples were done with impression on microstructure on rolling andcross-sectional surface at 500X magnification with NOVA BLACK and White Film(125 ASA).

4.2.3 Charpy Impact Testing

The ordinary Charpy test measures the total energy absorbed in fracturing the specimen.

Additional information can be obtained if the impact tester is instrumented to provide a load-time

history of the specimen during the test. With this kind of record it is possible to determine the

energy required for initiating fracture and the energy required for propagating fracture. It alsoyields information on the load foe general yielding, the maximum load and the fracture load.

Fig 4.4: Charpy Impact Testing Machine


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If the velocity of the impacting pendulum can be assumed constant throughout the test then

Where

V0 = initial pendulum velocity P = instantaneous load T=time

However the assumption of a constant pendulum velocit v is not valid, for v decreases in

proportion to the load on the specimen. It is usually assumed that

Et = E (1 - )

Where

Et = the total fracture energy =E / 4E0 where E0 is the initial energy of the pendulum

The impact testing is done with the sub sized samples having dimensions 55mm X 10mm X 5mm.

4.2.4 Microstructural study Optical Microscopy

As the metals are opaque in nature, they can be viewed under an optical microscope only in the

reflected light. In the optical microscope, Fig 4.5, light from a source is reflected by a semi-silvered

glass kept at 45o

tilt, passes through the objective on to the specimen surface. It is reflected back

by the metal surface and partly transmitted through the semi-silvered glass to the eyepiece. A

metallograph is an instrument in which the structure as seen under the microscope called themicrostructure is photographed.

Fig 4.5: Optical Microscopy


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Composite samples were cut to a reasonable size. Samples were ground in two different directions

i.e. one in the Rolling surface and another in the cross-sectional surface. To examine the metal

specimen properly, the surfaces were polished to a high degree of reflectivity. This is done by

polishing the surface on successively fine grades of emery papers. Final polishing is done on the

cloth impregnated with a very fine powder of alumina. Samples were polished in coarse polishingwheels followed by fine polishing wheel. After polishing, the specimens were etched with a

Kellers etchant *conc. HF (1ml) conc. Hl (1.5ml) conc. HNO3 (2.5ml) H2O (95ml)+. he grain

boundaries are selectively attacked by the etchant, as they possess a higher energy than the

interior of the crystal. Then the microstructural assessments were done under optical microscope.

Samples were seen under the magnification of 100X. Photography was done on both Rolling

surface and Cross-sectional surface at 100X magnification with Nova Black and White Film (125

ASA)

Samples are examined under optical Carl Zeiss (Fig 4.6) Microscope under different magnification

of 100X, 200X and 300X cutting after different treatment. Firstly as cast structure is observed and

then on the 30% hot rolled sample is examined under optical microscope.

Fig 4.6: Carl Zeiss Optical Microscope


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Prediction Through OpticalMicroscopy

2 Micro Hardness Measurementsand Analysis

Tensile Testing Charpy Impact testing


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Chapter 5: RESULTS AND DISCUSSIONS

The samples give some important and interesting results on characterization. The results are

thoroughly discussed below.

This section contains the analysis through mainly four types of characterization techniques, viz,

Prediction through optical microscopy, Micro hardness testing, Tensile testing, Impact testing.

As our main objective of the project is to produce a superior quality of composite the will always

compare the data with the subsequent data for the commercially pure Al, meanwhile there is

always a possibility of forming Al-Mg intermetallic matrix to strengthen it. But for the different

limitations we will compare it only with those of the values of commercially pure Aluminium.

5.1 Prediction through Optical Microscopy

Fig 5.1 - 5.7 shows the results of the optical microscopy done under different Magnifications at

different areas. Fig 5.1 5.3 show the conditions of as cast samples, and Fig 5.4 - 5.7 show thestructure of 30% hot rolled sample.

It is very prominent from the microstructural analysis that the composite mainly consists of two

major phases, the matrix which seemed to as the white phases and the black nodules which are

supposed to be Iron Aluminide reinforcements.

In 30% hot rolled samples the reinforcements are found to be more uniform. At 500X 30% hot

rolled sample contains some very large black patches. Those are probably because of coalescence

of the reinforcement particles due to long diffusion.

Now it should also be mentioned that as the samples are not etched the grain boundaries are not

so prominent.

Fig 5.1: Microstructure of Fe3Al Reinforced Al

matrix insitu composite

(As Cast Structure)

Magnification: 100X

Bright phase: MatrixDark phase: Reinforcements


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Fig 5.2: Microstructure of Fe3Al Reinforced

Al matrix insitu composite

(As Cast Structure)

Magnification: 200X

Bright phase: Matrix

Dark phase: Reinforcements



(As Cast Structure)

Magnification: 500X




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Page: 36



(30% hot rolled Structure)

Magnification: 100X






Magnification: 200X




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Page: 37




Magnification: 500X






Magnification: 500X

Bright phase: MatrixDark phase: Reinforcements


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Page: 38

As we have the limitation that we cannot do EDX analysis as well as image analysis, we cannot get

the confirmatory note about the matrix as well as the reinforcements. But it is clear from the

microstructure that the ceramic and intermetallic particle reinforcements being more or less

nodular in shape the composite must have superior mechanical properties.

5.2 Micro Hardness Measurements and Analysis

Table 5.1, 5.2 and 5.3 summarize the results of the micro-hardness value of composite. The first

two re for as cast sample and the third is for 30% hot rolled sample.

The measurement of micro hardness using 10 g load gives very small indentation and the hardness

values got from it very much localize hardness value not the bulk one. Hence the load is further

increased to 25 g. It gives the more or less bulk hardness as the size of indentation is

comparatively larger.

From the optical micrographs it can be seen that the depth of penetration of the indenter in the

matrix which is bigger as compared to the depth of penetration in the nodules. Therefore the

hardness value of the flakes is much higher than the matrix.

Table 5.3 also shows the micro-hardness values of pure Aluminium for comparison. The

comparative studies shows that the in-situ metal matrix composite containing the Aluminium

matrix and nodules is much harder than the pure Aluminium.

Obs.

No.

Sample

IDLoad

Time

of

Indentation

Hardness

(VHN)Remarks

1

Fe3Al

reinforced Al

matrix

composite

(As Cast)

10 Grams 15 Seconds

80

The HVN values around or

above 100 are got from the

indentation of which the major

parts are on reinforcement

phases. The points are bolded

in the table.

The values marked star are

eliminated from calculation as

they show abrupt low values of

HVN.

The indentations corresponding

to those points are on some

irregular shaped black patches.

They are most probably the

irregular surface dimple created

by the displacement of

reinforcement during grinding

from that point.

2 94.53 100.2

4 110

5 117.9

6 115.5

7 72.6

8 132.1

9 130.2

10 74.5

11 112.5

12 81.613 84.9

14 95.6

15 130.1

16 119.2

17 79.9

18 46.5*

19 126.2

20 38.7*

Table 5.1: HVN Data For As Cast Sample (I)


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Obs.

No.

Sample

IDLoad

Time

of

Indentation

Hardness

(VHN)Remarks

1

Fe3Al

reinforced Al

matrix

composite(As Cast)

25 Grams 15 Seconds

97.6

The points, bolded here show

the lowest values among all the

HVN values. This is probably

due to the highest part of the

indentation at the matrix.

Rest of all the data show theminimum amount of variation

due to extended indentation

size and get the bulk hardness

as particular.

2 94

3 77.4

4 86.2

5 82.6

6 99.1

7 72.4

8 110.4

9 101.3

10 114.3

11 97.5

12 92

13 97.1

14 92.2

15 92.5

16 96.7

17 118.4

18 103.3

19 110.4

20 94.2

Obs.

No.

Sample

IDLoad

Time

of

Indentation

Hardness

(VHN)Remarks

1

Fe3Al

reinforced Al

matrix

composite

(30% Hot

rolled at

250oC)

25 Grams 20 Seconds

64.7 The obvious trademark of these

observations is the lower

variation range of the hardness

values. The variation liesbetween minimum 64.7 HVN to

the maximum 87.2 HVN. The

indentations corresponding to

the points, bolded here are

taken surrounding the black

patches which are supposed to

be reinforced particles.

The rest are taken exclusively

on the white matrix.

Now the more or lessuniformity in hardness values

are pro

fabrication and characterization of iron aluminide reinforced aluminiun matrix composite

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