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

changes – such as temperature and electric and magnetic

fields – are also covered.

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

REFERENCE

1.Abdou, O.A. (1996) Managing Construction Risks,

Journal of Architectural Engineering, 2(1), 3-10.

2.Ahmed, S.M., Ahmad, R. and Saram, D.D. (1999) Risk

Management Trends in the Hong Kong Construction

Indus try: a Comparison of Contractors and Owners

Perceptions, Engineering, Construction and Architectural

Management, 6(3), 225-234.

3.Akintoye, A.S. and MacLeod, M.J. (1997) Risk

Analysis and Management in Construction, International

Journal of Project Management, 15(1), 31-38.

4.AS/NZS 4360 (1999) Australian / New Zealand Standard on Risk

Management, Standards Australia and Standards New

Zealand.

5.G. N. Berman, A Problem Book in Mathematical

Analysis, English translation. Mir Publishers,

Moscow, 1977.

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Calculus of Variations. Mir Publishers, Moscow,

1970.

8.P. Hartman, Ordinary Differential Equations. Wiley,

New York, 1964.

M. L. Krasnov, A. I. Kiselyov, and G. I. Makarenko,

A Book of Problems in Ordinary Differential

Equations, English translation. Mir Publishers,

Moscow, 1981.

9.E. Kreyszig, Advanced Engineering Mathematics.

Wiley and Sons, New York, 1988.