History of Metal-Matrix Composites (MMCs)

• Focused efforts to develop MMCs originated in the 1950s and early 1960s.
• The principal motivation was to extend the structural efficiency of metallic materials while
retaining their advantages, including high chemical inertness, high shear strength, and
good property retention at high temperatures.
• Early work on sintered aluminum powder was a precursor to discontinuously reinforced
metal matrix composites (MMCs). The development of high-strength monofilaments—first
boron and then silicon carbide (SiC)—enabled significant efforts on fibre-reinforced metal
matrix composites (MMCs) throughout the 1960s and early 1970s.
• Recession in the early 1970s produced significant research and development funding cuts,
leading to the end of this phase of MMC discovery and development.
• In the late 1970s, efforts were renewed on discontinuously reinforced MMCs using SiC
whisker reinforcements. The high cost of the whiskers and difficulty in avoiding whisker
damage during consolidation led to the concept of particulate reinforcements.
• The full impact of MMC technology was not widely appreciated. In the early 1990s, a U.S.
Air Force Title III program provided a significant investment to establish an MMC
technology base for the aerospace industry in the United States. This program produced
several landmark military and commercial aerospace applications of Discontinuously
Reinforced Aluminum (DRA),
• The MMC market for thermal management and electronic packaging alone was 5 times
larger than the aerospace market in 1999, and this gap is expected to increase in the
coming years, due to aggressive growth in the ground transportation and thermal
management markets.
 Metal Matrix Composites
• Metal matrix composites (MMCs), like all composites, consist of at least two chemically
and physically distinct phases, suitably distributed to provide properties not obtainable
with either of the individual phases.
Why metal matrix composites?
Compared to metals, MMCs offer the following advantages:

1. Major weight savings due to higher strength-to-weight ratio

2. Exceptional dimensional stability (compare, for example, SiC,/Al to Al)
3. Higher elevated temperature stability, i.e., creep resistance
4. Significantly improved cyclic fatigue characteristics
With respect to other composite types like PMCs and CMCs, MMCs offer these distinct
advantages:
1. Higher strength and stiffness
2. Higher service temperatures
3. Higher electrical conductivity (grounding, space charging)
4. Higher thermal conductivity
5. Better transverse properties
6. Improved joining characteristics
7. Radiation survivability (laser, UV, nuclear, etc.)
8. Little or no contamination (no out-gassing or moisture absorption problems) 2
 Types of MMCs
• All metal matrix composites have a metal or a metallic
alloy as the matrix. The reinforcement can be metallic or
ceramic.

• There are three kinds of metal matrix composites

(MMCs):
(i) Particle reinforced MMCs
(ii) Short fibre or whisker reinforced MMCs
(iii) Continuous fibre or sheet reinforced MMCs

• Continuous fibre reinforced, short fibre or whisker

reinforced, particle reinforced, and laminated or layered
composites.
Table 1 provides examples of some important
reinforcements used in metal matrix composites as well as
their aspect ratios (length/diameter) and diameters.

Figure 1: Different types of metal matrix

composites. 3
 Table 1: Typical Reinforcements Used in Metal Matrix Composites

Particle or discontinuously reinforced MMCs have become very important for the
following reasons.
1. Particle reinforced composites are inexpensive compared to continuous fibre reinforced
composites. Cost is an important and essential parameter, particularly in applications
where large volumes are required (e.g., automotive applications).
2. Conventional metallurgical processing techniques, such as casting or powder metallurgy,
followed by the conventional secondary processing methods like rolling, forging, and
extrusion, can be used.
3. Higher use temperatures than the unreinforced metal.
4. Enhanced modulus and strength.
5. Increased thermal stability.
6. Better wear resistance.
7. Relatively isotropic properties compared to fibre reinforced composites.
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 There are two types of cast metal matrix composites:
1. Cast composites having local reinforcement.
2. Cast composites in the form of a billet having uniform reinforcement with a wrought alloy
matrix. Such composite billets are forged and/or extruded, followed by rolling or other
forming operations.
Characteristics of MMCs
• One of the driving forces for metal matrix composites is, of course, enhanced stiffness and
Strength
• The ability to control thermal expansion in applications involving electronic packaging.
• Wear resistance
Reinforcements in MMCs
i. Fibrous Materials
• Any material (metal, ceramics, polymer) can be transformed into a fibre. A fibre can be
defined as an elongated material having a more or less uniform diameter or thickness of
less than 250 µm and an aspect ratio of more than 100
Unique features of fibres
• High degree of flexibility.
• Higher strength than the bulk material of the same composition.
ii. Fibre Flexibility
• Flexibility is an important attribute of fine fibres. A high degree of flexibility is an intrinsic
characteristic of a material having a small diameter and a low modulus 5
 Matrix Materials
Bonding and Crystalline Structure in Metals
• Metals are characterized by metallic bonding, i.e., valence electrons are not bound to a
particular ion in the solid. A major result of this electron cloud surrounding the atomic
nuclei is that the electronic bonding in metals is non-directional.
• This non-directionality of bonding is very important since it contributes to isotropy in many
properties.
• Because of non-directional bonding, we can model
the arrangement of atoms in the form of hard
spheres.
• There are two arrangements of packing of hard,
identical spheres that result in close packed
structures: Face centered cubic (FCC) and hexagonal
close packed (HCP). There is a third arrangement that
is observed in a number of metals, namely, body
centered cubic (BCC).
• It turns out that the ABABAB…. (or ACACAC...) results
in HCP while the ABCABCABC. Stacking sequence
results in the FCC structure.
Figure 3: (a) A layer of close packed atoms (b) Positions of layers B and C on top of layer A. Close packed
layers in FCC has ABCABC stacking, while HCP has ABAB . . . stacking sequence. 6
 Common Matrix Materials
Aluminum and Aluminum Alloys
• Aluminum alloys, because of their low density and excellent strength, toughness, and
corrosion resistance, have been used extensively in the automotive and aerospace fields.
• Al-Cu-Mg and Al-Zn-Mg-Cu alloys, which are very important precipitation hardenable alloys.
• Aluminum alloys can be classified as cast, wrought, or age-hardenable alloys. Some of the
common age-hardening or precipitation-hardenable treatments for Al alloys are the
following:
1. T4: Solutionizing and quenching, followed by aging at room temperature, or "natural aging."
2. T6: Solutionizing and quenching, followed by aging at a temperature above room
temperature (120-190°C), or "peak-aging."
3. T7x: Solutionizing, quenching, and overaging.
4. T8xx: Solutionizing, quenching, cold working, and peak-aging

Figure 4: (a) Schematic of nucleation, growth, and

coalescence of micro-voids at precipitate particles in a
ductile metallic alloy, (b) Characteristic dimples on the
fracture surface in an aluminum alloy. Note the presence
of precipitate particles that served as the nucleating sites
for micro-voids (Chawla et al., 2002). 7
 Titanium Alloys
• Titanium is one of the most important aerospace materials. Pure titanium has a density of
4.5 gcm-3 and a Young's modulus of 115 GPa.
• Titanium has a relatively high melting point (1672°C) and retains strength to high
temperatures with good oxidation and corrosion resistance. All these factors make it an
ideal material for aerospace applications.
• Titanium alloys are used in jet engine (turbine and compressor blades), fuselage parts, etc.
• At extremely high speeds, such as in a supersonic military aircraft, the skin of an airplane
heats up so much that aluminum alloys are no longer an option. Titanium alloys must be
used at such high temperatures. In a supersonic plane, flying at speeds over Mach 2, the
temperatures will be even higher than what titanium alloys can withstand. Titanium
aluminides are one of the candidate materials in this case.
• Titanium has two polymorphs; alpha (α) titanium has an HCP structure and is stable below
885°C and beta (β) titanium which has a BCC structure and is stable above 885°C.
• Aluminum raises the α→β transformation temperature, while most other alloying elements
(Fe, Mn, Cr, Mo, V, No, Ta) lower the α→β transformation temperature,
• Three general alloy types can be produced, viz., α, α+β , and β titanium alloys. The Ti-6%Al-
4% V, belongs to the α+β group
• Titanium has a great affinity for oxygen, nitrogen, and hydrogen and so welding of titanium
by any technique requires protection from the atmosphere
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Magnesium and its alloys
• Magnesium and its alloys form another group of light metals. Magnesium is one of the
lightest metals, with a density of 1.74 g/cm³. Magnesium alloys, especially those used in
castings, are employed in automotive and aircraft gearbox housings, chainsaw housings,
laptop casings, and various electronic equipment.
• Magnesium has an HCP structure, which makes it challenging to deform plastically by slip
at room temperature.

Cobalt
• Cobalt is a very common metal matrix that is used in WC/Co composites, also known as
cemented carbides, which are used as inserts for cutting tools and in oil drilling.
• Pure Co is stable below 417°C in the HCP crystal structure. Above this temperature, the
high-temperature face-centred cubic (FCC) structure is stable.

• It turns out that the FCC cobalt becomes stable at room temperature because of the
dissolution of carbon from WC during the processing of WC/Co composites. The FCC
structure has more slip systems available, which results in higher ductility.

• In WC/Co composites, the small amount of Co matrix holds the WC particles in place and
provides toughness, which stems from its ability to deform plastically.
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 Copper
• Copper has an FCC structure and is used extensively as an electrical conductor due to its
high electrical conductivity (only silver and gold are better). Their thermal conductivity
values are also high, which enables their use in thermal management applications.
• One of the major applications of copper in a composite is as a matrix material in niobium-
based superconductors.
• Copper-Zinc alloys (brass) and Copper-Tin alloys (bronze) are solid-solution strengthened
and among the earliest alloy metals used.
Silver
• Silver has the FCC structure with good electrical and thermal conductivity values, is highly
ductile, and has good corrosion resistance.
• It is a matrix material for high-temperature oxide superconductors
Nickel
• Nickel also has the FCC structure with good ductility, and alloys based on Nickel show an
excellent combination of properties
• Nickel-based super-alloys, primarily nickel-iron-cobalt alloys, have superior high-
temperature creep resistance, making them suitable for turbine blades.

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 Intermetallics
• Intermetallics are formed when two dissimilar metals are combined following the rules of
chemical valence.
• Generally, the bonding in intermetallics is not metallic, but ionic and covalent in nature.
Intermetallics generally have a stoichiometric composition and appear as a line compound
in the phase diagram. Some intermetallics have a range of compositions.
• In general, intermetallics have a complex crystal structure and are brittle due to their ionic
and covalent bonding.
• When they are used as matrix materials for making composites, they have the potential to
increase the operating temperatures over conventional materials.
• Intermetallics can have either a disordered or an ordered structure.

• Ordered intermetallic alloys possess structures characterized by long-range ordering, which

makes dislocations much more restricted, leading to retention of their strength at elevated
temperatures, e.g., nickel aluminide shows a marked increase in strength up to 800°C.
• Molybdenum disilicide (MoSi2) is a disordered intermetallic with a high melting point and
good stability at temperatures greater than 1200°C, in an oxidizing atmosphere

• Usually used as a heating element in furnaces.

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 Superconductivity
• One of the major applications of metal matrix composites is as filamentary
superconducting composites
• Certain metals and alloys lose all resistance to the flow of electricity when cooled to
within a few degrees of absolute zero. This phenomenon is known as superconductivity,
and materials exhibiting this phenomenon are referred to as superconductors.
• Superconductors can potentially carry as much as 100 times the amount of electricity of
ordinary copper or aluminum wires of the same size
• Superconductors can generate very high magnetic fields that are common in high-energy
physics and fusion energy programs. Other fields of application include magnetic
resonance imaging, magneto-hydrodynamic generators, rotating machines, magnetic
levitation vehicles, and magnets in general.

Figure 5: Variation of electrical resistivity with

temperature for a normal metal and that of a
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superconducting material.
 Types of Superconductors - There are two types of superconductors:

Type I: These are characterized by low T values, and they lose their superconductivity abruptly
at H.
Type II: These behave as diamagnetic materials up to a field Hc2. Above this field, the
magnetic field penetrates gradually into the material, and concomitantly, the
superconductivity is gradually lost, until at the critical magnetic field Hc2, the material reverts
to its normal state. All major applications of superconductivity utilize type II superconductors.
Interface
• This is a bounding surface between the reinforcement and matrix across which there is a
discontinuity in chemical composition, elastic modulus, coefficient of thermal expansion,
and/or thermodynamic properties such as chemical potential
• The interface (fiber/matrix or particle/matrix) is critical in all kinds of composites because
in most composites, the interfacial area per unit volume is very large
• In most metal matrix composite systems, the reinforcement and the matrix will not be in
thermodynamic equilibrium, i.e., a thermodynamic driving force will be present for an
interfacial reaction that will reduce the energy of the system.
• An ideal interface in a metal matrix composite should promote wetting and bond the
reinforcement and the matrix to a desirable degree.
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• The interface should protect the ceramic reinforcement and allow load transfer from the
soft metallic matrix to the strong reinforcement.
Crystallographic Nature of the Interface
• An interface between two crystalline phases can be described as coherent, semi-coherent,
or incoherent.
• A coherent interface implies one-to-one correspondence between lattice planes on the
two sides of the interface.

• An incoherent interface, on the other hand, consists of such severe atomic disorder that
no matching of lattice planes occurs across the boundary, i.e., no continuity of lattice
planes is maintained across the interface.

• Crystallographically, most of the interfaces that one encounters in fibre, whisker, or particle
reinforced metal matrix composites are incoherent and high-energy interfaces.

Wettability
• Wettability is defined as the ability of a liquid to spread on a solid surface.

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 • There are three specific energy (energy per unit
area) terms: ysv, the energy of the solid-vapour
interface; yLs, the energy of the liquid-solid
interface; and γLV, the energy of the liquid-vapour
interface.
• When we put a liquid drop on a solid substrate, we
replace a portion of the solid-vapour interface by a
liquid-solid and a liquid-vapour interface.
Thermodynamically, spreading of the liquid will
occur if this results in a decrease in the free energy
of the system, i.e.:

• An important parameter with respect to wettability

Figure 6: Contact angle, θ, a measure of is the contact angle, θ, which is a measure of
wettability for a system is defined by wettability for a system.
interaction among three surface energies:
Solid-liquid surface energy, γSL, solid
vapour surface energy, γsv, and liquid-
vapour surface energy, γLV
• We see that for θ = 0°, we have perfect wetting, while for θ = 180°, we have no wetting. For
0o < θ < 180°, there will be partial wetting. It should be pointed out that the contact angle
for a given system can vary with temperature, stoichiometry, hold time, interfacial
reactions, presence of any adsorbed gases, roughness and geometry of the substrate,15etc.
• It is worth emphasizing that wettability only describes the extent of intimate contact
between a liquid and a solid. It does not necessarily mean a strong bond at the interface
• One can have excellent wettability but only a weak physical, low energy bond. A low
contact angle, implying good wettability, is a necessary but not sufficient condition for
strong bonding.
Types of Bonding
There are two important types of bonding at an interface in a metal matrix composite:
• Mechanical bonding and Chemical bonding
• Most fibres have a characteristic surface roughness or texture resulting from the
fabrication process. This in turn imparts a roughness to the interface when the fibres are
incorporated in a matrix to make a composite.
• Interface roughness-induced mechanical bonding is quite important in all kinds of
composites.
• Surface roughness can contribute to bonding only if the liquid matrix wets the
reinforcement surface.
Chemical Bonding
• Ceramic/metal interfaces in metal matrix composites are generally formed at high
temperatures. Diffusion and chemical reaction kinetics are faster at elevated temperatures.
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• Chemical bonding in MMCs involves atomic transport by diffusion. Thus, chemical bonding
includes solid solution and/or chemical compound formation at the interface. It may lead
to the formation of an interfacial zone containing a solid solution and/or a
reinforcement/matrix interfacial reaction zone.
Measurement of Interfacial Bond Strength
• Once the matrix and the reinforcement of a composite are chosen, it is the set of
characteristics of the interface region that determines the final properties of the
composite.
Techniques to measure interfacial bond strength.
Bend Tests
Fibre Pullout and Pushout Tests
Applications of MMCs
Metal matrix composites are used in a lot of applications.
Increasingly MMCs have been used in several areas
• Aerospace
• Transportation (automotive and railway)
• Electronics and thermal management
• Filamentary superconducting magnets
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 • Power conduction
• Recreational Products and Sporting Goods
• Wear-resistant materials
Aerospace: In aerospace applications, low density, tailored thermal expansion and
conductivity, high stiffness and strength, are the primary drivers.
Aircraft Structures
• Materials with increased specific stiffness and strength can significantly enhance the
performance of the aircraft.

Figure 7: Application of a SiC particle reinforced

Al MMC in the fan-exit guide vane of a Pratt &
Whitney engine on a Boeing 777 (courtesy of
D. Miracle). The MMC replaced a carbon/epoxy
composite that had problems with foreign
object damage POD) and at a lower cost. 18
 Transportation

Figure 8: Models of sinter-forged MMC connecting

rods: (a) two-dimensional view and (c) three-
dimensional view (courtesy of F. Liu). Figure 9: Particulate MMCs for use in brake
drums and brake rotors, as a replacement for
cast iron (courtesy of D. Miracle). The high
wear resistance and thermal conductivity
coupled with 50-60% weight savings, make
MMCs quite attractive for this application.

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 Transportation

Figure 10: High speedway railroad coach bogey

with four brake disks produced by SAB Wabco Figure 11: Thermal cracking behaviour in (a) Al/SiC, and
(courtesy of H. Ruppert). (b) steel after wear testing (courtesy of H. Ruppert).
The steel brake shows significant amount of cracking
(white lines) while the MMC is in relatively good
condition.
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 Recreational Products and Sporting Goods
Wear-resistant materials

Figure 13: Roller cone bit used for oil well

Figure 12: (a) Particle reinforced MMCs used in drilling. The rock cutting inserts are made of
track shoe spikes and (b) track shoe with MMC WC/Co metal matrix composites. (Courtesy of
spikes (courtesy of T. Wang, Omni-Lite Corp.). A. Griffo, Smith International, Inc.)
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