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SECTION ONE
1.0 INTRODUCTION
1.1 ENGINEERING MATERIALS AND THEIR PROPERTIES
In-depth knowledge of the structure and behaviour
of engineering materials is vital for anyone who is
expected to select or specify them for applications
within the engineering industry. This project will give
learners an understanding of the structures,
classifications and properties of materials used in
engineering and will enable them to select materials
for different applications. The project is appropriate
for learners engaged in manufacturing and mechanical
engineering, particularly where materials are sourced
in the form of stock to be used in a production
process. The project covers a range of materials, some
of which learners may not be familiar with initially.
This project will enable learners to identify and
describe the structures of metals, polymers, ceramics
and composites and classify them according to their
properties. Learners will also be able to describe the
effects of processing on the behaviour of given
materials. Smart materials whose properties can be
altered in a controlled fashion through external
changes – such as temperature and electric and magnetic
fields – are also covered. Learners will apply their
understanding of the physical and mechanical properties
of materials, design requirements, cost and
availability to specify materials for given
applications.
All materials have limits beyond which they will
fail to meet the demands placed on them. The common
modes of failure will be both demonstrated and
described to enable learners to recognise where an
informed choice can make the difference between the
success or failure of a product.
1.2 THE STRUCTURE OF ENGINEERING MATERIALS
A wealth of information can be obtained by looking
at the structure of a material. Though there are many
levels of structure (e.g., atomic vs. macroscopic),
many physical properties of a material can be related
directly to the arrangement and types of bonds that
make up that material. We will begin by reviewing some
general chemical principles that will aid us in our
description of material structure. Such topics as
periodic structure, types of bonding, and potential
energy diagrams will be reviewed.
We will then use this information to look at the
specific materials categories in more detail: metals,
ceramics, polymers, composites, and biological
materials (biologics). There will be topics that are
specific to each material class, and there will also be
some that are common to all types of materials. In
subsequent chapters, we will explore not only how the
building blocks of a material can significantly impact
the properties a material possesses, but also how the
material interacts with its environment and other
materials surrounding it.
By the end of this project you should be able to:
Identify trends in the periodic table for IE, EA,
electronegativity, and atomic/ionic radii.
Identify the type of bonding in a compound.
Utilize the concepts of molecular orbital and
hybridization theories to explain multiple bonds,
bond angle, diamagnetism, and paramagnetism.
Identify the seven crystal systems and 14 Bravais
lattices.
Calculate the volume of a unit cell from the
lattice translation vectors.
Calculate atomic density along directions, planes,
and volumes in a unit cell.
Calculate the density of a compound from its
crystal structure and atomic mass.
Locate and identify the interstitial sites in a
crystal structure.
Assign coordinates to a location, indices to a
direction, and Miller indices to a plane in a unit
cell.
Use Bragg’s Law to convert between diffraction
angle and interplanar spacing.
Read and interpret a simple X-ray diffraction
pattern.
Identify types of point and line defects in solids.
Calculate the concentration of point defects in
solids.
Draw a Burger’s circuit and identify the direction
of dislocation propagation.
Use Pauling’s rules to determine the stability of a
compound.
Identify the components in a composite material.
Approximate physical properties of a composite
material based on component properties.
1.2.1 THE ELEMENTS
Elements are materials, too. Oftentimes, this fact
is overlooked. Think about all the materials from our
daily lives that are elements: gold and silver for our
jewelry; aluminum for our soda cans; copper for our
plumbing; carbon, both as a luminescent diamond and as
a mundane pencil lead; mercury for our thermometers;
and tungsten for our light bulb filaments. Most of
these elements, however, are relatively scarce in the
grand scheme of things.
1.3 AIMS AND OBJECTIVES
This project gives learners the opportunity to
extend their knowledge of engineering materials, their
properties and applications.
1.4 LIMITATION OF THE STUDY
This project has gone through so many research and
the uses of engineering material is common and the risk
involue are large if not properly installed. They are
so many way of soving problems using engineering
materials.
1.5 SCOPE OF STUDY
The project covers a range of materials, some of
which learners may not be familiar with initially.
This project will enable learners to identify and
describe the structures of metals, polymers, ceramics
and composites and classify them according to their
properties. Learners will also be able to describe the
effects of processing on the behaviour of given
materials. Smart materials whose properties can be
altered in a controlled fashion through external
SECTION TWO
2.0 LITERTURE REVIEW
2.1 TYPES OF MATERIALS
Let us classify materials according to the way the
atoms are bound together.
Metals: valence electrons are detached from atoms, and
spread in an 'electron sea' that "glues" the ions
together. Strong, ductile, conduct electricity and heat
well, are shiny if polished.
Semiconductors: the bonding is covalent (electrons are
shared between atoms). Their electrical properties
depend strongly on minute proportions of contaminants.
Examples: Si, Ge, GaAs.
Ceramics: atoms behave like either positive or negative
ions, and are bound by Coulomb forces. They are usually
combinations of metals or semiconductors with oxygen,
nitrogen or carbon (oxides, nitrides, and carbides).
Hard, brittle, insulators. Examples: glass, porcelain.
Polymers: are bound by covalent forces and also by weak
van der Waals forces, and usually based on C and H.
They decompose at moderate temperatures (100 – 400 C),
and are lightweight. Examples: plastics rubber.
2.2 INTERMOLECULAR FORCES AND BONDING
We have described the different types of primary
bonds, but how do these bonds form in the first place?
What is it that causes a sodium ion and a chloride ion
to form a compound, and what is it that prevents the
nuclei from fusing together to form one element? These
questions all lead us to the topics of intermolecular
forces and bond formation.
We know that atoms approach each other only to a
certain distance, and then, if they form a compound,
they will maintain some equilibrium separation distance
known as the bond length. Hence, we expect that there
is some attractive energy that brings them together, as
well as some repulsive energy that keeps the atoms a
certain distance apart. Also known as chemical
affinity, the attractive energy between atoms is what
causes them to approach each other. This attraction is
due to the electrostatic force between the nucleus and
electron clouds of the separate atoms.
2.2.1 THE IONIC BOND
To form an ionic bond, we must account for the
complete transfer of electrons from one atom to the
other. The easiest approach is to first transfer the
electrons to form ions, then bring the two ions
together to form a bond.
Sodium chloride is a simple example that allows us
to obtain both the bond energy and equilibrium bond
distance using the potential energy approach.
2.2.2 THE COVALENT BOND
Recall that covalent bonding results when electrons
are “shared” by similar atoms. The simplest example is
that of a hydrogen molecule, H2. We begin by using
molecular orbital theory to represent the bonding. Two
atomic orbitals (1s) overlap to form two molecular
orbitals (MOs), represented by σ: one bonding orbital
(σ1s), and one antibonding orbital, (σ∗1s), where the
asterisk superscript indicates antibonding.
The antibonding orbitals are higher in energy than
corresponding bonding orbitals. The overlap of two s
orbitals results in one σ-bonding orbital and one σ-
antibonding orbital. When two p orbitals overlap in an
end-to-end fashion, they are interacting in a manner
similar to s –s overlap, so one σ-bonding orbital and
one σ-antibonding orbital once again are the result.
Note that all σ orbitals are symmetric about a plane
between the two atoms. Side-to-side overlap of p
orbitals results in one π-bonding orbital and one π-
antibonding orbital. There are a total of four π
orbitals: two for px and two for py .
Note that there is one more node (region of zero
electron density) in an antibonding orbital than in the
corresponding bonding orbital. This is what makes them
higher in energy. As in the case of ionic bonding, we
use a potential energy diagram to show how orbitals
form as atoms approach each other. The electrons from
the isolated atoms are then placed in the MOs from
bottom to top. As long as the number of bonding
electrons is greater than the number of antibonding
electrons, the molecule is stable. For atoms with p and
d orbitals, diagrams become more complex but the
principles are the same.
2.3 STRUCTURE OF METALS AND ALLOYS
Since the electrons in a metallic lattice are in a
“gas,” we must use the core electrons and nuclei to
determine the structure in metals. This will be true of
most solids we will describe, regardless of the type of
bonding, since the electrons occupy such a small volume
compared to the nucleus. For ease of visualization, we
consider the atomic cores to be hard spheres. Because
the electrons are delocalized, there is little in the
way of electronic hindrance to restrict the number of
neighbors a metallic atom may have.
As a result, the atoms tend to pack in a close-
packed arrangement, or one in which the maximum number
of nearest neighbors (atoms directly in contact) is
satisfied. The most hard spheres one can place in the
plane around a central sphere is six, regardless of the
size of the spheres (remember that all of the spheres
are the same size). You can then place three spheres in
contact with the central sphere both above and below
the plane containing the central sphere. This results
in a total of 12 nearest-neighbor spheres in contact
with the central sphere in the close-packed structure.
They can be directly aligned with the layer below
in an ABA type of structure, or they can be rotated so
that the top layer does not align core centers with the
bottom layer, resulting in an ABC structure. This leads
to two different types of close-packed structures.
2.4 STRUCTURE OF CERAMICS AND GLASSES
Inorganic materials constitute the largest class of
solids in the world. We have already described metals;
and while they are not organic (they contain no
biological carbon), they are also not inorganic in the
strict sense of the word—they are metals due to the
unique characteristics of their valence electronic
structure. Inorganic materials are typically compounds,
such as metal oxides, carbides, or nitrides.
They possess many interesting properties that we
will only begin to describe at this point. They can
also differ structurally from other types of materials
like metals and polymers. Let us begin by describing
the structure of inorganic materials.
2.4.1 PAULING’S RULES
Recall that the structure of a crystal is
determined mostly by how the atoms pack together. The
same is true of binary compounds such as alloys, and of
binary compounds that contain noncovalent bonds, such
as ionic compounds. In addition to the concept of
electronegativity, Linus Pauling also produced a set of
generalizations that are used to describe the majority
of ionic crystal structures. Pauling’s first rule
states that coordination polyhedra are formed.
Coordination polyhedra are three-dimensional
geometric constructions such as tetrahedra and
octahedra. Which polyhedron will form is related to the
radii of the anions and cations in the compound.
2.4.2 CERAMIC CRYSTAL STRUCTURES
Most crystalline inorganic compounds are based on
nearly close-packing of the anions (generically
referred to as O or X, though oxygen is the most common
anion) with metal atom cations (generically called M or
A) placed interstitially within the anion lattice.
2.4.3 SILICATE STRUCTURES
The silicates, made up of base units of silicon and
oxygen, are an important class of ceramic compounds
that can take on many structures, including some of
those we have already described. They are complex
structures that can contain several additional atoms
such as Mg, Na, K.
What makes the silicates so important is that they
can be either crystalline or amorphous (glassy) and
provide an excellent opportunity to compare these two
disparate types of structure. Let us first examine the
crystalline state, which will lead us into the
amorphous state.
2.4.4 GLASS CERAMICS
There are materials that are hybrids between
glasses and ceramics. Glass ceramics are a family of
fine-grained crystalline materials achieved through
controlled crystallization of glasses. They are
nonporous and are either opaque or transparent.
Their optical transparency arises from the fact
that the crystals are smaller than the wavelength of
visible light such that the light waves are transmitted
without significant scattering.
SECTION THREE
3.0 STRUCTURE OF OTHER ENGINEERING MATERIALS
3.1 STRUCTURE OF POLYMERS
The term polymer comes from “poly,” meaning many,
and “mer,” meaning units. Hence, polymers are composed
of many units—in this case, structural units called
monomers. A monomer is any unit that can be converted
into a polymer. Similarly, a dimer is a combination of
two monomers, a trimer is a combination of three
monomers, and so on. Before describing the chemical
composition of typical monomers and how they are put
together to form polymers, it is useful to have a brief
organic chemistry review.
You may wish to refer to an organic chemistry text
for more detailed information. We will reserve
discussion of how these organic molecules are brought
together to form polymers until when the kinetics of
polymerization are described. For the remainder of the
description of polymer structure, it is sufficient to
know that polymer chains are formed from the reaction
of many monomers to form longchain hydrocarbons,
sometimes called macromolecules, but more commonly
referred to as polymers.
3.2 STRUCTURE OF COMPOSITES
The first three sections of this chapter have
described the three traditional primary classifications
of materials: metals, ceramics, and polymers. There is
an increasing emphasis on combining materials from
these different categories, or even different materials
within each category, in such a way as to achieve
properties and performance that are unique. Such
materials are called composites. A composite can be
different things, depending on the level of definition
we use.
In the most basic sense, all materials except
elements are composites. For example, a binary mixture
of two elements, like an alloy, can be considered a
composite structure on an atomic scale. In terms of
microstructure, which is a larger scale than the atomic
level definition, composites are composed of crystals,
phases and compounds. With this definition, steel,
which is a suspension of carbon in iron, is a
composite, but brass, a single-phase alloy, is not a
composite. If we move up one more level on the size
scale, we find that there are macrostructural
composites: materials composed of fibers, matrices, and
particulates—they are materials systems.
This highest level of structural classification is
the one we will use, so our definition of a composite
is this: a material brought about by combining
materials differing in composition or form on a
macroscale for the purpose of obtaining specific
characteristics and properties.
3.2.1 COMPOSITE CONSTITUENTS
The constituents in a composite retain their
identity such that they can be physically identified
and they exhibit an interface between one another. The
body constituent gives the composite its bulk form, and
it is called the matrix. The other component is a
structural constituent, sometimes called the
reinforcement, which determines the internal structure
of the composite.
The region between the body and structural
constituents is called the interphase. It is quite
common (even in the technical literature), but
incorrect, to use the term interface to describe this
region.
3.3 STRUCTURE OF BIOLOGICS
The boundary between the disciplines of biology and
materials engineering is becoming increasingly blurred.
Molecules that were once thought of as being biological
“macromolecules” such as DNA are now being used as
templates to produce molecules with desired gene
sequences, and live tissue cultures are being used to
reconstruct human body parts much as an auto body is
repaired with fiberglass and epoxy.
More importantly, however, the scientific and
engineering principles behind the application and
development of biological materials are becoming more
well understood, so it is useful to provide some
information on the structure of these materials in the
context of their use as materials of (human)
construction. We begin with some simple biochemistry,
then use this to describe how nature uses these
building blocks to create complex composite structures
of tissue, both hard and soft.
3.4 DETERMINATION OF MECHANICAL PROPERTIES
3.4.1 THE TENSILE TEST
The tensile test is widely used for measuring the
stiffness, strength and ductility of a material. The
testing machine subjects the test-piece to an axial
elongation and the resultant load on the specimen is
measured. Depending on the nature of the product being
tested, the specimen may be round or rectangular in
cross-section, with the region between the grips
usually being of reduced cross–section. The gauge
length is marked in this region.
We will consider the response of a ductile metal as
an illustration. The load–elongation data are normally
converted to stress and strain:
Stress = Load/Cross-sectional area
Strain = Extension of gauge length/Original gauge
length. The linear part of the curve may correspond to
easily measured elongations in some polymeric
3.4.2 THE BAUSCHINGER EFFECT
If a metallic specimen is deformed plastically in
tension up to a tensile stress of +t and is then
subjected to a compressive strain, it will first
contract elastically and then, instead of yielding
plastically in compression at a stress of –t as might
have been expected, it is found that plastic
compression starts at a lower stress (–c) – a
phenomenon known as the Bauschinger Effect (BE).
The BE arises because, during the initial tensile
plastic straining, internal stresses accumulate in the
test-piece and oppose the applied strain. When the
direction of straining is reversed these internal
stresses now assist the applied strain, so that plastic
yielding commences at a lower stress than that
operating in tension.
3.5 HARDNESS TESTING
Hardness is not a well-defined property of
materials and the tests employed assess differing
combinations of the elastic, yielding and work-
hardening characteristics. All the tests are
essentially simple and rapid to carry out and are
virtually non-destructive, so they are well-suited as a
means of quality control.
The hardness of materials has been assessed by a
wide variety of tests, but we will confine ourselves to
discussing two types of measurement – the resistance to
indentation and the height of rebound of a ball or
hammer dropped from a given distance.
3.5.1 INDENTATION HARDNESS TESTS
There are two types of indentation hardness test.
The first type (Brinell and Vickers) measures the size
of the impression left by an indenter of prescribed
geometry under a known load whereas the second type
(Rockwell) measures the depth of penetration of an
indenter under specified conditions.
3.5.2 THE BRINELL TEST
The surface of the material is indented by a
hardened steel ball (whose diameter D is usually 10 mm)
under a known load (L) (e.g. 3000 kg for steel) and the
average diameter of the impression measured with a low-
power microscope.
The Brinell number (HB) is the ratio of the load to
the contact surface area of the indentation. Most
machines have a set of tables for each loading force,
from which the hardness may be read in units of kgf mm–
2. If other sizes of indenter are used, the load is
varied according to the relation: L/D2 = constant in
order to obtain consistent results. The constant is 30
for steel, 10 for copper and 5 for aluminium.
3.5.3 THE VICKERS TEST
A diamond square-based pyramid of 136angle is
used as the indenter, which gives geometrically similar
impressions under differing loads (which may range from
5 to 120 kg). A square indent is thus produced, and the
user measures the average diagonal length and again
reads the hardness number (HV) from the tables. The
Brinell and Vickers hardness values are identical up to
a hardness of about 300 kgf mm–2, but distortion of the
steel ball occurs in Brinell tests on hard materials,
so that the test is not reliable above values of 600
kgf mm–2.
3.5.4 THE ROCKWELL TEST
Either a steel ball (Scale B) or a diamond cone
(Scale C) is used and the indenter is first loaded with
a minor load of 10 kg f, while the indicator for
measuring the depth of the impression is set to zero.
The appropriate major load is then applied and, after
its removal, the dial gauge records the depth of the
impression in terms of Rockwell numbers.
It should be pointed out that, in the case of
materials which exhibit timedependence of elastic
modulus or yield stress (for example, most polymers),
the size of the indentation will increase with time, so
the hardness value will depend on the duration of
application of the load.
3.5.5 THE KNOOP TEST
The Knoop test uses a diamond pyramidal indenter of
apex angles 130and 172.5, thus giving a rhombohedral
impression with one diagonal (L) being 7 longer than
the other and with a depth which is one thirtieth of L.
It is particularly useful for measuring the relative
hardnesses of brittle materials, such as glasses and
ceramics, when lower loads (P) are employed than in the
Vickers test.
3.5.6 THE SHORE SCLEROSCOPE
A small diamond-pointed hammer weighing 2.5 g falls
freely from a standard height down a graduated glass
tube. The height of the first rebound is taken as the
index of hardness and this simple apparatus may be
readily transported for testing rolls, gears, etc. in
situ. The Shore test may also be employed to measure
the elastic response of elastomers, as a check of the
degree of cross-linking.
3.6 FRACTURE TOUGHNESS TESTING
The toughness of a material must be distinguished
from its ductility. It is true that ductile materials
are frequently tough, but toughness combines both
strength and ductility, so that some soft metals like
lead are too weak to be tough whereas glass-reinforced
plastics are very tough although they exhibit little
plastic strain. One approach to toughness measurement
is to measure the work done in breaking a specimen of
the material, such as in the Charpy-type of impact
test. Here a bar of material is broken by a swinging
pendulum and the energy lost by the pendulum in
breaking the sample is obtained from the height of the
swing after the sample is broken.
A serious disadvantage of such tests is the
difficulty of reproducibility of the experimental
conditions by different investigators, so that impact
tests can rarely be scaled up from laboratory to
service conditions and the data obtained cannot be
considered to be true material parameters.
SECTION FOUR
4.0 PROCESSING OF MATERIALS
4.1 INTRODUCTION
With an understanding of the structure and
properties of engineering materials now firmly in
place, we can discuss how these materials can be formed
or fabricated into useful products and components. Most
of the important processing methods are described here,
with little or no distinction made between microscale
and macroscale processes—for example, processes that
form both integrated circuits and components for
highway bridges are described here.
4.2 PROCESSING OF METALS AND ALLOYS
Despite the relatively recent development of
polymers and composites as structural materials, metals
continue to be the dominant group of engineering
materials for many applications, including the
automotive, aerospace, and construction industries.
This is due, in part, to their ease in component
fabrication, or formability.
In this section, we describe three of the most
common and widely utilized metal-processing techniques:
casting, wrought processing, and powder metallurgy. The
topic of metal joining (e.g., welding), albeit an
important one, is beyond the scope of this text.
4.2.1 CASTING
In the context of metals processing, casting is the
process of melting a metal and solidifying it in a
cavity, called a mold, to produce an object whose shape
is determined by mold configuration. Almost all metals
are cast during some stage of the fabrication process.
They can be cast either
a.directly into the shape of the component or
b.as ingots that can be subsequently shaped into a
desired form using processes that are described
later in this section.
Casting offers several advantages over other
methods of metal forming: It is adaptable to intricate
shapes, to extremely large pieces, and to mass
production; and it can provide parts with uniform
physical and mechanical properties throughout. From an
economic standpoint, it would be desirable to form most
metal components directly from casting, since
subsequent operations such as forming, extruding,
annealing and joining add additional expense.
4.2.1.1 MELTING.
The initial step in the casting operation is
melting. Melting can be achieved through any number of
well-established techniques, including resistance
heating elements or open flame, but there are several
alternative methods that offer distinct advantages in
the melting of certain metals.
For example, metals such as zirconium, titanium,
and molybdenum are melted in water-cooled copper
crucibles by arc melting, in which a direct or
alternating current is applied across an electrode and
the material in the crucible, called the charge.
4.2.1.2 FORMING OPERATIONS.
In principle, the molten metal can then be poured
into a mold of the desired shape and become solidified
to form a component. As mentioned above, and as
described further in the next section, solidification
from casting operations usually leads to undesirable
crystal structures, such that the most common operation
after melting is the formation of a large metal “blank”
called an ingot. Thus, there are two main categories of
metal casting processes: ingot casting and casting to
shape. Ingot casting makes up the majority of all metal
castings and can be separated into three categories:
static casting, semicontinuous or direct-chill casting,
and continuous casting.
4.2.1.3 SOLIDIFICATION.
When the ingot or casting solidifies, there are
three main possible microstructures that form.
4.2.2 WROUGHT METALS AND ALLOYS
The ingots produced by casting can either be used
as cast, or further processed into usable geometries
such as bars, rods and sheets. Those metals and alloys
that are manufactured by plastic deformation in the
solid state are referred to as wrought metals and
alloys. ∗ The mechanical working of metals during
wrought processing improves the properties of the
wrought metals over that of the cast products. Plastic
deformation, when carried out below the
recrystallization temperature, results in work
hardening, which improves the mechanical properties of
the metal.
In all wrought processes, the flow of metal is
caused by application of an external force or pressure
that pushes or pulls a piece of metal or alloy through
a metal die.
4.2.2.1 FORGING.
Forging is the process of applying large forces,
mainly compressive forces, to cause plastic deformation
to occur in the metal. The use of compressive forces
means that tensile necking and fracture are generally
avoided. Forging can be used to produce objects of
irregular shape. There are two main categories of
forging: pressing and hammering. Pressing is the
relatively slow deformation of metal to the required
shape between mating dies.
This method is preferred for homogenizing large
cast ingots because the deformation zone extends
throughout the cross section. All presses are composed
of some basic components. These include a ram for
applying the compressive force, a drive to move the ram
up and down, power to the drive, a bed to rest the
workpiece upon, and a frame to hold the components.
Most press forging is done on upright hydraulic presses
or mechanical presses. There are a variety of frame
geometries and types of ram drives that vary depending
upon the pressing application. A press is rated on the
basis of the force (typically in tons) it can deliver
near the bottom of a stroke.
Pressures (in tons/in.2 of projected area) have
been found to be 5–20 for brass, 19–20 for aluminum,
15–30 for steel, and 20–40 for titanium. The force the
press must deliver is equal to the unit pressure times
the projected area. Presses in excess of 50,000 tons
have been built for forging large and complex parts.
4.2.2.2 ROLLING
In the process of rolling, the metal is
continuously drawn between rotating rollers by the
friction forces between the surfaces of the rolls and
the metal, where hf < h0. The process can be run
either hot or cold, and it is much more economical than
forging since it is faster, consumes less power, and
produces items of a uniform cross section in a
continuous fashion. In cold rolling, it is possible to
attain production speeds of over 1500 m/min for thin
strips of metal. Temperatures in the hot rolling
process are similar to those in forging, namely 400–
450◦C for aluminum alloys, 820◦C for copper alloys,
930–1260◦C for alloys steels, 760–980◦C for titanium
alloys, and 980–1650◦C for refractory metals.
4.2.2.3 EXTRUSION.
Extrusion involves the pressing of a small piece of
metal, called a billet or slug, through a die by the
application of force. A billet or slug is
differentiated from an ingot primarily by its smaller
size. The die in extrusion processing is sometimes
called an orifice or nozzle. The forces necessary to
push the metal through the orifice are generated by a
ram that pushes on the billet, generating both
compressive and shear forces. Because no tensile forces
are involved, fracture is avoided.
4.2.3 POWDER METALLURGY
The process of converting metallic powders into
ingots or finished components via compaction and
sintering is called powder metallurgy. Powder
metallurgy is used whenever porous parts are needed,
whenever the parts have intricate shapes, whenever the
alloy or mixture of metals cannot be achieved in any
other manner, or whenever the metals have very high
melting points. The technique is suitable to virtually
any metal, however.
An organic binder is frequently added to the
metallic powders during the mixing state to facilitate
compaction, but volatilizes at low temperatures during
the sintering process. The advantages of powder
metallurgy included improved microstructures and
improved production economies. Although the cost of the
powders can be greater than that of ingots and mill
products obtained by casting or wrought processing, the
relative ease of forming the powder into a final
product more than offsets the labor costs and metal
losses associated with the other two processes.
The powder metallurgy process consists of three
basics steps: powder formation; powder compaction; and
sintering. Each of the steps in powder metallurgy will
be described in more detail.
4.2.3.1 POWDER FORMATION.
Metallic powders can be formed by any number of
techniques, including the reduction of corresponding
oxides and salts, the thermal dissociation of metal
compounds, electrolysis, atomization, gas-phase
synthesis or decomposition, or mechanical attrition.
The atomization method is the one most commonly used,
because it can produce powders from alloys as well as
from pure metals. In the atomization process, a molten
metal is forced through an orifice and the stream is
broken up with a jet of water or gas. The molten metal
forms droplets to minimize the surface area, which
solidify very rapidly.
Currently, iron–nickel–molybdenum alloys, stainless
steels, tool steels, nickel alloys, titanium alloys,
and aluminum alloys, as well as many pure metals, are
manufactured by atomization processes. Chemical powder
formation methods include the reduction of oxides,
reduction of ions in solution by gases, electrolytic
reduction of ions in solution, and thermal
decomposition of gaseous molecules containing metal
atoms. Sponge iron powder, for example, is produced by
reacting a mixture of magnetite (Fe3O4) ore, coke and
limestone. The product is crushed to control the iron
particle size, and the iron is magnetically separated.
Iron powder can also be produced by hydrogen reduction
of ground mill scale and the reaction of atomized high-
carbon steel particles with iron oxide particles.
Copper, tungsten and cobalt powders can be produced by
hydrogen reduction of oxide powders. Nickel and copper
powders can be produced by hydrogen reduction of their
corresponding amine sulfates, and iron and nickel
powders of high purity and fine particle size are
produced by thermal decomposition of their gaseous
carbonyls, Fe(CO)5 and Ni(CO)4, respectively.
4.2.3.2 COMPACTION AND HOT PRESSING.
The shaping of components from metallic powders is
accomplished by compacting the particles under
pressure, and sometimes at elevated temperatures.
Typically, compaction temperatures are in excess of 75%
of the absolute melting temperature, and compaction
times are on the order of 103 to 104s. The compaction
operation is performed primarily to provide form,
structure, and strength, known as green strength,
necessary to allow for handling before sintering.
Compaction at room temperature involves pressures of
70–700 MPa, depending on the powder, and green
densities (density prior to sintering) of 65–95% of
theoretical (powder material) densities are achievable.
Pressures for hot pressing are usually lower due to die
or pressure chamber limitations.
4.3 PROCESSING OF CERAMICS AND GLASSES
There are perhaps a wider variety of techniques
that can be used to form and shape ceramics and glasses
than for any of the other materials classes. Much of
the variation in processing is due to the end use of
the product. Certain techniques are used for structural
ceramics, while others are used for semiconductor,
superconductors, and magnetic materials. In general,
there are three types of processes we will consider:
powder-forming, melt processing, and chemical
processing. The latter category, which includes vapor
phase and sol-gel processing, is considered optional in
this chapter. Two topics related to powder processing,
firing and sintering, will be described here.
The powder-forming processes are similar in many
ways to those used for powder metallurgy described in
the previous section. For example, pressing is a common
method for processing ceramics; however, ceramic
powders can be pressed in either dry or wet form. In
wet form, they can also be extruded, just like metals,
and cast in a variety of process variations.
4.3.1 PRESSING
In much the same way that metal powders can be
compacted and sintered to form a densified product, so,
too, can many ceramic powders. Recall that pressing is
the simultaneous compaction and shaping of a powder
within a die or flexible mold. Just as we did for
metals processing, we will look at each of the separate
steps involved in the pressing operation: powder
formation (called granulation for ceramics), die
filling, and compaction.
4.3.1.1 GRANULATION.
Powders of glass and ceramic materials may be
formed in many of the same ways as those used to form
metals particles. However, wet milling is much more
common with ceramic materials, particularly when
finer particle sizes are required. The combination of
dry powders with a dispersant such as water is called a
slurry. Unlike a solution or suspension, the solids
contents of slurries are very high, typically 50 wt%
solids and above. Depending on the type of milling
equipment being used, the solids content can have an
effect on the efficiency of the milling operation. In
general, a high solids content is desirable, in order
to develop a coating of adequate viscosity on the
milling media (balls and vial) to prevent escape of the
particles from the grinding zone (cf. Figure 7.15), but
not completely dissipate the grinding stress. The
effect of slurry solids content on milling efficiency,
in terms of the percentage fines (smaller particles)
produced, for the milling of an alumina slurry.
4.3.1.2 COMPACTION.
Compaction is the process of granule densification
via applied pressure to produce a cohesive part having
a particular shape and microstructure.
SECTION FIVE
5.0 RECOMMENDATION AND CONCLUSION
5.1 CONCLUSION
While most research has focused on some aspects of
construction risk management, this research endeavoured
to identify key risks associated with the achievement
of all project objectives in terms of cost, time,
quality, environment and safety.
5.2 RECOMMENDATION
1.Know the structure of and classify engineering
materials
Atomic structure: element; atom eg nucleus, electron;
compound; molecule; mixture; bonding mechanisms eg
covalent, ionic, metallic Structure of metals: lattice
structure; grain structure; crystals; crystal growth;
alloying eg interstitial, substitutional; phase
equilibrium diagrams eg eutectic, solid solution,
combination; intermetallic compounds Structure of polymeric
materials: monomer; polymer; polymer chains eg linear,
branched, cross-linked; crystallinity; glass transition
temperature
Structure of ceramics: amorphous; crystalline; bonded Structure
of composites: particulate; fibrous; laminated
Structure of smart materials: crystalline; amorphous; metallic
Classification of metals: ferrous eg plain carbon steel, cast
iron (grey, white, malleable, wrought iron), stainless
and heat-resisting steels (austenitic, martensitic,
ferritic); non-ferrous eg aluminium, copper, gold,
lead, silver, titanium, zinc; non-ferrous alloys eg
aluminium-copper heat treatable – wrought and cast,
non-heat-treatable – wrought and cast, copper-zinc
(brass), copper-tin (bronze), nickel-titanium alloy
Classification of non-metals (synthetic): thermoplastic polymeric
materials eg acrylic, polytetrafluoroethylene (PTFE),
polythene, polyvinyl chloride (PVC), nylon,
polystyrene; thermosetting polymeric materials eg
phenol-formaldehyde, melamine-formaldehyde, urea-
formaldehyde; elastomers; ceramics eg glass, porcelain,
cemented carbides; composites eg laminated, fibre
reinforced (carbon fibre, glass reinforced plastic
(GRP)), concrete, particle reinforced, sintered; smart
materials eg electro-rheostatic (ER) fluids, magneto-
rheostatic (MR) fluids, piezoelectric crystals
Classification of non-metals (natural): eg wood, rubber, diamond
2.Know material properties and the effects of
processing on the structure and behaviour of
engineering materials
Mechanical properties: strength (tensile, shear,
compressive); hardness; toughness; ductility;
malleability; elasticity; brittleness
Physical properties: density; melting temperature
Thermal properties: expansivity; conductivity
Electrical and magnetic properties: conductivity; resistivity;
permeability; permittivity
Effects of processing metals: recrystallisation temperature;
grain structure eg hot working, cold working, grain
growth; alloying elements in steel eg manganese,
phosphorous, silicon, sulphur, chromium, nickel.
Effects of processing thermoplastic polymers: polymer processing
temperature; process parameters eg mould temperature,
injection pressure, injection speed, mould clamping
force, mould open and closed time
Effects of processing thermosetting polymers: process parameters
eg moulding pressure and time, mould temperature,
curing
Effects of processing ceramics: eg water content of clay,
sintering pressing force, firing temperature
Effects of processing composites: fibres eg alignment to the
direction of stress, ply direction; de-lamination;
matrix/reinforcement ratio on tensile strength;
particle reinforcement on cermets
Effects of post-production use: smart materials eg impact
(piezoelectric), electric field (electro-rheostatic),
magnetic field (magneto-rheostatic), temperature (shape
memory alloys), colour change (temperature or
viscosity)
3.Be able to use information sources to select
materials for engineering uses
Information sources: relevant standard specifications eg
British Standards (BS), European Standards (EN),
International Standards (ISO); material manufacturers’
and stockholders’ information eg data sheets,
catalogues, websites, CD ROMs
Design criteria: properties eg mechanical, physical,
thermal, electrical and magnetic; surface finish;
durability eg corrosion resistance, solvent resistance,
impact resistance, wear resistance
Cost criteria: initial cost eg raw material, processing,
environmental impact, energy requirements; processing
eg forming, machining, casting, joining (thermal,
adhesive, mechanical); quantity; mode of delivery eg
bulk, just-in-time (JIT); recycling
Availability criteria: standard forms e.g sheet and plate,
bar-stock, pipe and tube, sectional, extrusions,
ingots, castings, forgings, pressings, granular,
powder, liquid
4.Know about the modes of failure of engineering
materials
Principles of ductile and brittle fracture: effects of gradual and
impact loading eg tensile, compressive, shear; effects
of grain size; transition temperature; appearance of
fracture surfaces
Principles of fatigue: cyclic loading; effects of stress
concentrations eg internal, external; effects of
surface finish; appearance of fracture surfaces
Principles of creep: primary; secondary; tertiary; effects of
temperature; strain versus time curve; creep limit;
effect of grain size; effect of variations in the
applied stress
Tests: destructive eg tensile, hardness, impact,
ductility, fatigue, creep; non-destructive eg dye
penetrant, ultrasonic, radiographic (x-ray, gamma ray),
magnetic powder, visual
Degradation processes: on metals eg oxidation, erosion,
stress corrosion; on polymers eg solvent attack,
radiation and ageing; on ceramics eg thermal shock,
sustained high temperature
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