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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 thermo- dynamically 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 stress- strain 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
Fig 5.3: Microstructure of Fe3Al Reinforced
Al matrix insitu composite
(As Cast Structure)
Magnification: 500X
Bright phase: Matrix
Dark phase: Reinforcements
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Fig 5.4: Microstructure of Fe3Al Reinforced
Al matrix insitu composite
(30% hot rolled Structure)
Magnification: 100X
Bright phase: Matrix
Dark phase: Reinforcements
Fig 5.5: Microstructure of Fe3Al Reinforced
Al matrix insitu composite
(30% hot rolled Structure)
Magnification: 200X
Bright phase: Matrix
Dark phase: Reinforcements
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Fig 5.4: Microstructure of Fe3Al Reinforced
Al matrix insitu composite
(30% hot rolled Structure)
Magnification: 500X
Bright phase: Matrix
Dark phase: Reinforcements
Fig 5.4: Microstructure of Fe3Al Reinforced
Al matrix insitu composite
(30% hot rolled Structure)
Magnification: 500X
Bright phase: MatrixDark phase: Reinforcements
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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