A COMPOSITE MATERIAL is a macroscopic combination of two or more distinct materials, having a recognizable interface between them. Composites are used not only for their structural properties, but also for electrical, thermal, tribological, and environmental applications. Modern composite materials are usually optimized to achieve a particular balance of properties for a given range of applications. Given the vast range of materials that may be considered as composites and the broad range of uses for which composite materials may be designed, it is difficult to agree upon a single, simple, and useful definition. However, as a common practical definition, composite materials may be restricted to emphasize those materials that contain a continuous matrix constituent that binds together and

tickbitewryΜηχανική

30 Οκτ 2013 (πριν από 3 χρόνια και 10 μήνες)

536 εμφανίσεις

A COMPOSITE MATERIAL is a macroscopic combination of two or more distinct materials, having a
recognizable interface between them. Composites are used not only for their structural properties, but
also for electrical, thermal, tribological, and
environmental applications. Modern composite materials
are usually optimized to achieve a particular balance of properties for a given range of applications.
Given the vast range of materials that may be considered as composites and the broad range of uses

for
which composite materials may be designed, it is difficult to agree upon a single, simple, and useful
definition. However, as a common practical definition, composite materials may be restricted to
emphasize those materials that contain a continuous m
atrix constituent that binds together and
provides form to an array of a stronger, stiffer reinforcement constituent.


The resulting composite material has a balance of structural properties that is superior to either
constituent material alone. The improv
ed structural properties generally result from a load
-
sharing
mechanism. Although composites optimized for other functional properties (besides high structural
efficiency) could be produced from completely different constituent combinations than fit this s
tructural
definition, it has been found that composites developed for structural applications also provide
attractive performance in these other functional areas as well. As a result, this simple definition for
structural composites provides a useful defin
ition for most current functional composites.


in general there are two phases in a composite material the outer material which keeps the other
material stiff and un damaged form the external force is called matrix. the main function of matrix is to
provid
e a best cover for the secondary phase particles.based on the type of secondary phase the
composite materials are classified in to


1) Fiber reinforced composite material

2) Flake rein forced composite materials

3) particle reinforced composite materials


some of the desirable properties of the matrix material are ductile and soft where as the properties of
secondary phase are quite opposite. they should posses high hardness and also highly brittle in nature
due to high hardness.


why are composite materia
ls needed when there are more than 100 variety of materials available and
numerous number of alloys and ceramics available? this may be the question which had raised in many
minds? here is the solution for it. when you take the life of a air crafts whose b
ody is developed by
duralumin, its life time is a maximum of 10 years after then the air crafts becomes useless. but due to
introduction of sintered aluminum the life time of air crafts hiked by 80 years. this development is
observed in latest air crafts.
this indicates the importance of the composite materials.


Fiber reinforced composites:
-

in these composite materials the secondary phase is made up of fibers.
these composite materials are again sub classified in to sub categories based on the length of i
t they are
namely

1) continuous fiber reinforced composite materials

2) discontinuous fiber reinforced composite materials


continuous fiber reinforced composite materials have a continuous fibers and do not have any breaks
through out the material. where
as it is not the same in case of dis continuous materials. they are very
discontinuous and again classified based on the orientation of the fibers. aligned and random are the
two types.


flake reinforced composites contain secondary particles in the form
of flakes. a flake is a piece of
material having no particular shape or orientation.


last but not least of this classification is particle reinforced composite materials. these have particles
sprinkled in the primary phase. the classifications in this ar
e large particle reinforced composite
materials and dispersion strengthened materials.


finally i would like to thank in advance for all the readers of my artilce and ISC for giving me a great
opportunity for expressing my learnt knowledge in a great platf
orm like this

Introduction to Materials and Processes



Introduction

Introduction

General Material Classifications

Metals

Ceramics

Polymers

Composites

Structure of Materials

Atomic Bonds

Solid State Structure

Primary M
etallic Crystalline Structures

Solidification

Anisotropy and Isotropy

Crystal Defects

Elastic/Plastic Deformation

Fatigue Crack Initiation

Diffusion

Property Modification

Ceramic Structures

Polymer Structure

Composite Structures

Physical and Chemical
Properties

Section Introduction

Phase Transformation Temperature

Density

Specific Gravity


Thermal Conductivity

Thermal Expansion


Electrical Conductivity

Magnetic Properties

Oxidation and Corrosion

Mechanical Properties

Section
Introduction

-

Loading

-

Stress & Str
ain

Tensile


Compression, Bearing, & S
hear


Hardness

Creep & Stress Rupture


Toughness



-

Impact Toughness



-

Notch Toughness



-

Fracture Toughness

Fatigue



-

S
-
N Fati
gue



-

Fatigue Crack Growth Rate


Selection of Materials

Specific Metals


Metal Ores


Iron and Steel


Decarburization


Aluminum and Al
uminum Alloys


Nickel and Nickel Alloys


Titanium and Titanium Alloys

Polymers

Composites

General Manufacturing
Processes

Metallic



Wrought



Castings



Welding



Brazing and Soldering



Forming



Machining



Powdered Metal Processes



Heat Treatment



Surface Treatment

Ceramic and Glass



Basic Materials Processing

Polymers/Plastic



Basic Materials Processing



Adhesive Joining

Composites



Basic Materials Processing



Adhesive Joining

Origins of Discontinuities

Inherent Discontinuities

Process
-
Induc
ed Discontinuities

Service
-
Induced Discontinuities

Material Specifications

Component Design,
Performance and NDE

Strength

Durability

Fracture Mechanics

Nondestructive Evaluation

Introduction to Materials

This section will provide a basic introduction to materials and material fabrication processing.


It
is important that NDT personnel have some background in material science for a couple of
reasons.


First, nondestructive testing almost always involves the

interaction of energy of some
type (mechanics, sound, electricity, magnetism or radiation) with a material.


To understand how
energy interacts with a material, it is necessary to know a little about the material.


Secondly,
NDT often involves detecting
manufacturing defects and service induced damage and, therefore,
it is necessary to understand how defects and damage occur.



This section will begin with an introduction to the four common types of engineering materials.
The structure of materials at the

atomic level will then be considered, along with some atomic
level features that give materials their characteristic properties. Some of the properties that are
important for the structural performance of a material and methods for modifying these
propert
ies will also be covered.

In the second half of this text, methods used to shape and form materials into useful shapes will
be discussed. Some of the defects that can occur during the manufacturing process, as well as
service induced damage will be highli
ghted. This section will conclude with a summary of the
role that NDT plays in ensuring the structural integrity of a component.

General Material Classifications

There are thousands of materials available for use in
engineering applications. Most
materials fall into one of three
classes that are based on the atomic bonding forces of a
particular material. These three classifications are metallic,
ceramic and polymeric. Additionally, different materials can
be combined to create a composite material
. Within each of
these classifications, materials are often further organized
into groups based on their chemical composition or certain
physical or mechanical properties. Composite materials are
often grouped by the types of materials combined or the way
the materials are arranged together. Below is a list of some of the commonly classification of
materials within these four general groups of materials.

Metals



Ferrous metals and alloys (irons,
carbon steels, alloy steels,
stainless steels, tool and die
ste
els)



Nonferrous metals and alloys
(aluminum, copper, magnesium,
nickel, titanium, precious metals,
refractory metals, superalloys)

Polymeric



Thermoplastics plastics



Thermoset plastics



Elastomers





Ceramics



Glasses

Composites



Reinforced plastics



Glass ceramics



Graphite



Diamond






Metal
-
matrix composites



Ceramic
-
matrix composites



Sandwich structures



Concrete

Each of these general groups will be discussed in more detail in the following pages.

Metals

Metals account for about two thirds of all the elements and about 24% of the mass of the planet.
Metals have useful properties including strength, ductility, high melting points, thermal and
electrical conductivity, and toughness. From the periodic table,
it can be seen that a large number
of the elements are classified as being a metal. A few of the common metals and their typical
uses are presented below.

Common Metallic Materials



Iron/Steel
-

Steel alloys are used for strength critical applications



Aluminum
-

Aluminum and its alloys are used because they are easy to form, readily
available, inexpensive, and recyclable.



Copper
-

Copper and copper alloys have a number of properties that make them useful,
including high electrical and thermal conductivi
ty, high ductility, and good corrosion
resistance.



Titanium
-

Titanium alloys are used for strength in higher temperature (~1000° F)
application, when component weight is a concern, or when good corrosion resistance is
required



Nickel
-

Nickel alloys are
used for still higher temperatures (~1500
-
2000° F) applications
or when good corrosion resistance is required.



Refractory materials are used for the highest temperature (> 2000° F) applications.


The key feature that distinguishes metals from non
-
metals i
s their bonding. Metallic materials
have free electrons that are free to move easily from one atom to the next. The existence of these
free electrons has a number of profound consequences for the properties of metallic materials.
For example, metallic mate
rials tend to be good electrical conductors because the free electrons
can move around within the metal so freely. More on the structure of metals will be discussed
later.

Ceramics

A ceramic has traditionally been defined as “an inorganic, nonmetallic sol
id that is prepared
from powdered materials, is fabricated into products through the application of heat, and displays
such characteristic properties as hardness, strength, low electrical conductivity, and brittleness."
The word ceramic comes the from Gree
k word "keramikos", which means "pottery." They are
typically crystalline in nature and are compounds formed between metallic and nonmetallic
elements such as aluminum and oxygen (alumina
-
Al
2
O
3
), calcium and oxygen (calcia
-

CaO),
and silicon and nitrogen
(silicon nitride
-
Si
3
N
4
).

Depending on their method of formation, ceramics can be
dense or lightweight. Typically, they will demonstrate
excellent strength and hardness properties; however, they
are often brittle in nature. Ceramics can also be formed to
s
erve as electrically conductive materials or insulators.
Some ceramics, like superconductors, also display
magnetic properties. They are also more resistant to high
temperatures and harsh environments than metals and
polymers. Due to ceramic materials wide

range of
properties, they are used for a multitude of applications.

The broad categories or segments that make up the ceramic industry can be classified as:



Structural clay products (brick, sewer pipe, roofing and wall tile, flue linings, etc.)



Whiteware
s (dinnerware, floor and wall tile, electrical porcelain, etc.)



Refractories (brick and monolithic products used in metal, glass, cements, ceramics,
energy conversion, petroleum, and chemicals industries)



Glasses (flat glass (windows), container glass (bot
tles), pressed and blown glass
(dinnerware), glass fibers (home insulation), and advanced/specialty glass (optical
fibers))



Abrasives (natural (garnet, diamond, etc.) and synthetic (silicon carbide, diamond, fused
alumina, etc.) abrasives are used for grin
ding, cutting, polishing, lapping, or pressure
blasting of materials)



Cements (for roads, bridges, buildings, dams, and etc.)



Advanced ceramics

o

Structural (wear parts, bioceramics, cutting tools, and engine components)

o

Electrical (capacitors, insulators,
substrates, integrated circuit packages,
piezoelectrics, magnets and superconductors)

o

Coatings (engine components, cutting tools, and industrial wear parts)

o

Chemical and environmental (filters, membranes, catalysts, and catalyst supports)

The atoms in cera
mic materials are held together by a chemical bond which will be discussed a
bit later. Briefly though, the two most common chemical bonds for ceramic materials are
covalent and ionic. Covalent and ionic bonds are much stronger than in metallic bonds and,
generally speaking, this is why ceramics are brittle and metals are ductile.

Polymers

A polymeric solid can be thought of as a material that contains many chemically bonded parts or
units which themselves are bonded together to form a solid. The word polym
er literally means
"many parts." Two industrially important polymeric materials are plastics and elastomers.
Plastics are a large and varied group of synthetic materials which are processed by forming or
molding into shape. Just as there are many types of
metals such as aluminum and copper, there
are many types of plastics, such as polyethylene and nylon. Elastomers or rubbers can be
elastically deformed a large amount when a force is applied to them and can return to their
original shape (or almost) when t
he force is released.

Polymers have many properties that make them attractive to use in certain conditions. Many
polymers:



are less dense than metals or ceramics,



resist atmospheric and other forms of corrosion,



offer good compatibility with human tissue,
or



exhibit excellent resistance to the conduction of electrical current.

The polymer plastics can be divided into two classes,
thermoplastics and thermosetting plastics, depending on
how they are structurally and chemically bonded.
Thermoplastic polymers
comprise the four most
important commodity materials


polyethylene,
polypropylene, polystyrene and polyvinyl chloride. There
are also a number of specialized engineering polymers.
The term ‘thermoplastic’ indicates that these materials
melt on heating and

may be processed by a variety of
molding and extrusion techniques. Alternately,
‘thermosetting’ polymers can not be melted or remelted.
Thermosetting polymers include alkyds, amino and
phenolic resins, epoxies, polyurethanes, and unsaturated
polyesters.

Rubber is a natural occurring polymer. However, most
polymers are created by engineering the combination of hydrogen and carbon atoms and the
arrangement of the chains they form. The polymer molecule is a long chain of covalent
-
bonded
atoms and secondary b
onds then hold groups of polymer chains together to form the polymeric
material. Polymers are primarily produced from petroleum or natural gas raw products but the
use of organic substances is growing. The super
-
material known as Kevlar is a man
-
made
polym
er. Kevlar is used in bullet
-
proof vests, strong/lightweight frames, and underwater cables
that are 20 times stronger than steel.

Composites

A composite is commonly defined as a combination of two or more distinct materials, each of
which retains its own d
istinctive properties, to create a new material with properties that cannot
be achieved by any of the components acting alone. Using this definition, it can be determined
that a wide range of engineering materials fall into this category. For example, conc
rete is a
composite because it is a mixture of Portland cement and aggregate. Fiberglass sheet is a
composite since it is made of glass fibers imbedded in a polymer.

Composite materials are said to have two phases. The reinforcing phase is the fibers, she
ets, or
particles that are embedded in the matrix phase. The reinforcing material and the matrix material
can be metal, ceramic, or polymer. Typically,
reinforcing materials are strong with low densities
while the matrix is usually a ductile, or tough,
mat
erial.

Some of the common classifications of
composites are:



Reinforced plastics



Metal
-
matrix composites



Ceramic
-
matrix composites



Sandwich structures



Concrete

Composite materials can take many forms but they can be separated into three categories based
on the strengthening mechanism. These categories are dispersion strengthened, particle
reinforced and fiber reinforced. Dispersion strengthened composites have a
fine distribution of
secondary particles in the matrix of the material. These particles impede the mechanisms that
allow a material to deform. (These mechanisms include dislocation movement and slip, which
will be discussed later). Many metal
-
matrix compos
ites would fall into the dispersion
strengthened composite category. Particle reinforced composites have a large volume fraction of
particle dispersed in the matrix and the load is shared by the particles and the matrix. Most
commercial ceramics and many f
illed polymers are particle
-
reinforced composites. In fiber
-
reinforced composites, the fiber is the primary load
-
bearing component. Fiberglass and carbon
fiber composites are examples of fiber
-
reinforced composites.

If the composite is designed and fabric
ated correctly, it combines the strength of the
reinforcement with the toughness of the matrix to achieve a combination of desirable properties
not available in any single conventional material. Some composites also offer the advantage of
being tailorable
so that properties, such as strength and stiffness, can easily be changed by
changing amount or orientation of the reinforcement material. The downside is that such
composites are often more expensive than conventional materials.

Structure of Materials

It should be clear that all matter is made of atoms. From the periodic table, it can be seen that
there are only about 100 different kinds of atoms in the entire Universe. These same 100 atoms
form thousands of different substances ranging from the air we
breathe to the metal used to
support tall buildings. Metals behave differently than ceramics, and ceramics behave differently
than polymers. The properties of matter depend on which atoms are used and how they are
bonded together.


The structure of mater
ials can be classified by the general magnitude of various features being
considered. The three most common major classification of structural, listed generally in
increasing size, are:

Atomic structure, which includes features that cannot be seen, such as

the types of bonding
between the atoms, and the way the atoms are arranged.

Microstructure, which includes features that can be seen using a microscope, but seldom with the
naked eye.

Macrostructure, which includes features that can be seen with the nake
d eye)

The atomic structure primarily affects the chemical, physical, thermal, electrical, magnetic, and
optical properties. The microstructure and macrostructure can also affect these properties but
they generally have a larger effect on mechanical proper
ties and on the rate of chemical reaction.
The properties of a material offer clues as to the structure of the material. The strength of metals
suggests that these atoms are held together by strong bonds. However, these bonds must also
allow atoms to move
since metals are also usually formable. To understand the structure of a
material, the type of atoms present, and how the atoms are arranged and bonded must be known.
Let’s first look at atomic bonding.

Atomic Bonding

(Metallic, Ionic, Covalent, and van de
r Waals Bonds)

From elementary chemistry it is known that the atomic
structure of any element is made up of a positively charged
nucleus surrounded by electrons revolving around it. An
element’s atomic number indicates the number of
positively charged pro
tons in the nucleus. The atomic
weight of an atom indicates how many protons and
neutrons in the nucleus. To determine the number of
neutrons in an atom, the atomic number is simply
subtracted from the atomic weight.

Atoms like to have a balanced electrica
l charge. Therefore,
they usually have negatively charged electrons surrounding
the nucleus in numbers equal to the number of protons. It is also known that electrons are present
with different energies and it is convenient to consider these electrons surr
ounding the nucleus in
energy “shells.” For example, magnesium, with an atomic number of 12, has two electrons in the
inner shell, eight in the second shell and two in the outer shell.

All chemical bonds involve electrons. Atoms will stay close together i
f they have a shared
interest in one or more electrons. Atoms are at their most stable when they have no partially
-
filled electron shells. If an atom has only a few electrons in a shell, it will tend to lose them to
empty the shell. These elements are meta
ls. When metal atoms bond, a metallic bond occurs.
When an atom has a nearly full electron shell, it will try to find electrons from another atom so
that it can fill its outer shell. These elements are usually described as nonmetals. The bond
between two n
onmetal atoms is usually a covalent bond. Where metal and nonmetal atom come
together an ionic bond occurs. There are also other, less common, types of bond but the details
are beyond the scope of this material. On the next few pages, the Metallic, Covalen
t and Ionic
bonds will be covered in more detail.




Ionic Bonds

Ionic bonding occurs between charged particles. These may be atoms or groups of atoms, but
this discuss will be conducted in terms of single atoms. Ionic bonding occurs between metal
atoms a
nd nonmetal atoms. Metals usually have 1, 2, or 3 electrons in their outermost shell.
Nonmetals have 5, 6, or 7 electrons in their outer shell. Atoms with outer shells that are only
partially filled are unstable. To become stable, the metal atom wants to g
et rid of one or more
electrons in its outer shell. Losing electrons will either result in an empty outer shell or get it
closer to having an empty outer shell. It would like to have an empty outer shell because the next
lower energy shell is a stable shel
l with eight electrons.



Since electrons have a negative charge, the atom that gains electrons becomes a negatively
charged ions (aka anion) because it now has more electrons than protons. Alternately, an atom
that loses electrons becomes a positively
charged ion (aka cations). The particles in an ionic
compound are held together because there are oppositely charged particles that are attracted to
one another.

The images above schematically show the process that takes place during the formation of an
io
nic bond between sodium and chlorine atoms. Note that sodium has one valence electron that it
would like to give up so that it would become stable with a full outer shell of eight. Also note
that chlorine has seven valence electrons and it would like to ga
in an electron in order to have a
full shell of eight. The transfer of the electron causes the previously neutral sodium atom to
become a positively charged ion (cation), and the previously neutral chlorine atom to become a
negatively charged ion (anion).
The attraction for the cation and the anion is called the ionic
bond.

Generally,
solid materials

with ionic bonds:



are hard because particles cannot easily slide past one another.



are good insulators because there are no free electrons or ions (unless dis
solved or
melted).



are transparent because their electrons are not moving from atom to atom and less likely
to interact with light photons.



are brittle and tend to cleave rather than deform because bonds are strong.



have high melting point because ionic bo
nds are relatively strong.

Covalent Bonding

Where a compound only contains nonmetal atoms, a covalent bond is formed by atoms sharing
two or more electrons. Nonmetals have 4 or more electrons in their outer shells (except boron).
With this many electrons
in the outer shell, it would require more energy to remove the electrons
than would be gained by making new bonds. Therefore, both the atoms involved share a pair of
electrons. Each atom gives one of its outer electrons to the electron pair, which then spe
nds some
time with each atom. Consequently, both atoms are held near each other since both atoms have a
share in the electrons.



More than one electron pair can be formed with half of the electrons coming from one atom and
the rest from the other atom.

An important feature of this bond is that the electrons are tightly
held and equally shared by the participating atoms. The atoms can be of the same element or
different elements. In each molecule, the bonds between the atoms are strong but the bonds
betw
een molecules are usually weak. This makes many solid materials with covalent bonds
brittle. Many ceramic materials have covalent bonds.

Compounds with covalent bonds may be solid, liquid or gas at room temperature depending on
the number of atoms in the
compound. The more atoms in each molecule, the higher a
compound’s melting and boiling temperature will be. Since most covalent compounds contain
only a few atoms and the forces between molecules are weak, most covalent compounds have
low melting and boili
ng points. However, some, like carbon compounds, can be very large. An
example is the diamond in which carbon atoms each share four electrons to form giant lattices.

Some Common Features of Materials with Covalent Bonds:



Hard



Good insulators



Transparent



B
rittle or cleave rather than deform

Metallic Bonding

A common characteristic of metallic elements is they contain only one to
three electrons in the outer shell. When an element has only one, two or
three valence electrons (i.e. electrons in the outer sh
ell), the bond
between these electrons and the nucleus is relatively weak. So, for
example, when aluminum atoms are grouped together in a block of
metal, the outer electrons leave individual atoms to become part of
common “electron cloud.” In this arrangem
ent, the valence electrons
have considerable mobility and are able to conduct heat and electricity
easily. Also, the delocalized nature of the bonds, make it possible for the
atoms to slide past each other when the metal is deformed instead of fracturing l
ike glass or other
brittle material.


Since the aluminum atoms lose two electrons, they end up having a positive charge and are
designated Al
3+

ions (cations). These ions repel each other but are held together in the block
because the negative electrons are attracted to the positively charged ions. A result of the sharing
of electrons is the cations arrange themselves in a regular pattern. This r
egular pattern of atoms is
the crystalline structure of metals. In the crystal lattice, atoms are packed closely together to
maximize the strength of the bonds. An actual piece of metal consists of many tiny crystals
called grains that touch at grain bound
aries.

Some Common Features of Materials with Metallic Bonds:



Good electrical and thermal conductors due to their free valence electrons



Opaque



Relatively ductile




Solid State Structure

In the previous pages, some of the mechanisms that bond together th
e multitude of individual
atoms or molecules of a solid material were discussed. These forces may be primary chemical
bonds, as in metals and ionic solids, or they may be secondary van der Waals’ forces of solids,
such as in ice, paraffin wax and most poly
mers. In solids, the way the atoms or molecules
arrange themselves contributes to the appearance and the properties of the materials.

Atoms can be gathered together as an aggregate through a number of different processes,
including condensation, pressuriza
tion, chemical reaction, electrodeposition, and melting. The
process usually determines, at least initially, whether the collection of atoms will take to form of
a gas, liquid or solid. The state usually changes as its temperature or pressure is changed.
M
elting is the process most often used to form an aggregate of atoms. When the temperature of a
melt is lowered to a certain point, the liquid will form either a crystalline solid or and amorphous
solid.

Amorphous Solids

A solid substance with its atoms he
ld apart at equilibrium spacing, but with no long
-
range
periodicity in atom location in its structure is an amorphous solid. Examples of amorphous solids
are glass and some types of plastic. They are sometimes described as supercooled liquids
because their

molecules are arranged in a random manner some what as in the liquid state. For
example, glass is commonly made from silicon dioxide or quartz sand, which has a crystalline
structure. When the sand is melted and the liquid is cooled rapidly enough to avoi
d
crystallization, an amorphous solid called a glass is formed. Amorphous solids do not show a
sharp phase change from solid to liquid at a definite melting point, but rather soften gradually
when they are heated. The physical properties of amorphous solid
s are identical in all directions
along any axis so they are said to have isotropic properties, which will be discussed in more
detail later

.

Crystalline Solids

More than 90% of naturally occurring and artificially prepared solids are crystalline. Minera
ls,
sand, clay, limestone, metals, carbon (diamond and graphite), salts ( NaCl, KCl etc.), all have
crystalline structures. A crystal is a regular, repeating arrangement of atoms or molecules. The
majority of solids, including all metals, adopt a crystalli
ne arrangement because the amount of
stabilization achieved by anchoring interactions between neighboring particles is at its greatest
when the particles adopt regular (rather than random) arrangements. In the crystalline
arrangement, the particles pack ef
ficiently together to minimize the total intermolecular energy.

The regular repeating pattern that the atoms arrange in is called the crystalline lattice. The
scanning tunneling microscope (STM) makes it possible to image the electron cloud associated
ind
ividual atoms at the surface of a material. Below is an STM image of a platinum surface
showing the regular alignment of atoms.


Courtesy: IBM Research, Almaden Research Center.

Crystal Structure

Crystal structures may be conveniently specified by descri
bing the arrangement within the solid
of a small representative group of atoms or molecules, called the ‘unit cell.’ By multiplying
identical unit cells in three directions, the location of all the particles in the crystal is determined.
In nature, 14 diff
erent types of crystal structures or lattices are found. The simplest crystalline
unit cell to picture is the cubic, where the atoms are lined up in a square, 3D grid. The unit cell is
simply a box with an atom at each corner. Simple cubic crystals are rel
atively rare, mostly
because they tend to easily distort. However, many crystals form body
-
centered
-
cubic (bcc) or
face
-
centered
-
cubic (fcc) structures, which are cubic with either an extra atom centered in the
cube or centered in each face of the cube. Mo
st metals form bcc, fcc or Hexagonal Close Packed
(hpc) structures; however, the structure can change depending on temperature. These three
structures will be discussed in more detail on the following page.

Crystalline structure is important because it co
ntributes to the properties of a material. For
example, it is easier for planes of atoms to slide by each other if those planes are closely packed.
Therefore, lattice structures with closely packed planes allow more plastic deformation than
those that are
not closely packed. Additionally, cubic lattice structures allow slippage to occur
more easily than non
-
cubic lattices. This is because their symmetry provides closely packed
planes in several directions. A face
-
centered cubic crystal structure will exhibi
t more ductility
(deform more readily under load before breaking) than a body
-
centered cubic structure. The bcc
lattice, although cubic, is not closely packed and forms strong metals. Alpha
-
iron and tungsten
have the bcc form. The fcc lattice is both cubic

and closely packed and forms more ductile
materials. Gamma
-
iron, silver, gold, and lead have fcc structures. Finally, HCP lattices are
closely packed, but not cubic. HCP metals like cobalt and zinc are not as ductile as the fcc
metals.

Primary Metallic Cr
ystalline Structures

(BCC, FCC, HCP)

As pointed out on the previous page, there are 14 different types of crystal
unit cell structures or lattices are found in nature. However most metals
and many other solids have unit cell structures described as body
center
cubic (bcc), face centered cubic (fcc) or Hexagonal Close Packed (hcp).
Since these structures are most common, they will be discussed in more
detail.

Body
-
Centered Cubic (BCC) Structure

The body
-
centered cubic unit cell has atoms at each of the ei
ght corners of a cube (like the cubic
unit cell) plus one atom in the center of the cube (left image below). Each of the corner atoms is
the corner of another cube so the corner atoms are shared among eight unit cells. It is said to
have a coordination num
ber of 8. The bcc unit cell consists of a net total of two atoms; one in the
center and eight eighths from corners atoms as shown in the middle image below (middle image
below). The image below highlights a unit cell in a larger section of the lattice.


The bcc arrangement does not allow the atoms to pack together as closely as the fcc or hcp
arrangements. The bcc structure is often the high temperature form of metals that are close
-
packed at lower temperatures. The volume of atoms in a cell per the total

volume of a cell is
called the
packing factor
. The bcc unit cell has a packing factor of 0.68.

Some of the materials that have a bcc structure include lithium, sodium, potassium, chromium,
barium, vanadium, alpha
-
iron and tungsten. Metals which have a bc
c structure are usually harder
and less malleable than close
-
packed metals such as gold. When the metal is deformed, the
planes of atoms must slip over each other, and this is more difficult in the bcc structure. It should
be noted that there are other imp
ortant mechanisms for hardening materials, such as introducing
impurities or defects which make slipping more difficult. These hardening mechanisms will be
discussed latter.

Face Centered Cubic (FCC) Structure

The face centered cubic structure has atoms located at each of the corners and the centers of all
the cubic faces (left image below). Each of the corner atoms is the corner of another cube so the
corner atoms are shared among eight unit cells. Additionally
, each of its six face centered atoms
is shared with an adjacent atom. Since 12 of its atoms are shared, it is said to have a coordination
number of 12. The fcc unit cell consists of a net total of four atoms; eight eighths from corners
atoms and six halve
s of the face atoms as shown in the middle image above. The image below
highlights a unit cell in a larger section of the lattice.



In the fcc structure (and the hcp structure) the atoms can pack closer together than they can in the
bcc structure. The a
toms from one layer nest themselves in the empty space between the atoms
of the adjacent layer. To picture packing arrangement, imagine a box filled with a layer of balls
that are aligned in columns and rows. When a few additional balls are tossed in the b
ox, they will
not balance directly on top of the balls in the first layer but instead will come to rest in the pocket
created between four balls of the bottom layer. As more balls are added they will pack together
to fill up all the pockets. The packing fa
ctor (the volume of atoms in a cell per the total volume
of a cell) is 0.74 for fcc crystals. Some of the metals that have the fcc structure include
aluminum, copper, gold, iridium, lead, nickel, platinum and silver.

Hexagonal Close Packed (HPC) Structure

Another common close packed structure is the hexagonal close pack. The hexagonal structure of
alternating layers is shifted so its atoms are aligned to the gaps of the preceding layer. The atoms
from one layer nest themselves in the empty space between the

atoms of the adjacent layer just
like in the fcc structure. However, instead of being a cubic structure, the pattern is hexagonal.
(See image below.) The difference between the HPC and FCC structure is discussed later in this
section.


The hcp structure

has three layers of atoms. In each the top and bottom layer, there are six atoms
that arrange themselves in the shape of a hexagon and a seventh atom that sits in the middle of
the hexagon. The middle layer has three atoms nestle in the triangular "groove
s" of the top and
bottom plane. Note that there are six of these "grooves" surrounding each atom in the hexagonal
plane, but only three of them can be filled by atoms.

As shown in the middle image above, there are six atoms in the hcp unit cell. Each of th
e 12
atoms in the corners of the top and bottom layers contribute 1/6 atom to the unit cell, the two
atoms in the center of the hexagon of both the top and bottom layers each contribute ½ atom and
each of the three atom in the middle layer contribute 1 ato
m. The image on the right above
attempts to show several hcp unit cells in a larger lattice.

The coordination number of the atoms in this structure is 12. There are six nearest neighbors in
the same close packed layer, three in the layer above and three i
n the layer below. The packing
factor is 0.74, which is the same as the fcc unit cell. The hcp structure is very common for
elemental metals and some examples include beryllium, cadmium, magnesium, titanium, zinc
and zirconium.




Similarities and Diffe
rence

B
etween the

FCC and HCP Structure

The face centered cubic and hexagonal close packed structures both have a packing factor of
0.74, consist of closely packed planes of atoms, and have a coordination number of 12. The
difference between the fcc and h
cp is the stacking sequence. The hcp layers cycle among the two
equivalent shifted positions whereas the fcc layers cycle between three positions. As can be seen
in the image, the hcp structure contains only two types of planes with an alternating ABAB
arr
angement. Notice how the atoms of the third plane are in exactly the same position as the
atoms in the first plane. However, the fcc structure contains three types of planes with a
ABCABC arrangement. Notice how the atoms in rows A and C are no longer alig
ned.
Remember that cubic lattice structures allow slippage to occur more easily than non
-
cubic
lattices, so hcp metals are not as ductile as the fcc metals.


The table below shows the stable room temperature crystal structures for several elemental
metals
.

Metal

Crystal Structure

Atomic Radius (nm)

Aluminum

FCC

0.1431

Cadmium

HCP

0.1490

Chromium

BCC

0.1249

Cobalt

HCP

0.1253

Copper

FCC

0.1278

Gold

FCC

0.1442

Iron (Alpha)

BCC

0.1241

Lead

FCC

0.1750

Magnesium

HCP

0.1599

Molybdenum

BCC

0.1363

Nickel

FCC

0.1246

Platinum

FCC

0.1387

Silver

FCC

0.1445

Tantalum

BCC

0.1430

Titanium (Alpha)

HCP

0.1445

Tungsten

BCC

0.1371

Zinc

HCP

0.1332

A nanometer (nm) equals 10
-
9

meter or 10 Angstrom units.

Solidification

The crystallization of a large amount of material from a single point of nucleation results in a
single crystal. In engineering materials, single crystals are produced only under carefully
controlled conditions. The expense of producing single crystal mate
rials is only justified for
special applications, such as turbine engine blades, solar cells, and piezoelectric materials.
Normally when a material begins to solidify, multiple crystals begin to grow in the liquid and a
polycrystalline (more than one cryst
al) solid forms.

The moment a crystal begins to grow is know as nucleation and the point where it occurs is the
nucleation point. At the solidification temperature, atoms of a liquid, such as melted metal, begin
to bond together at the nucleation points a
nd start to form crystals. The final sizes of the
individual crystals depend on the number of nucleation points. The crystals increase in size by
the progressive addition of atoms and grow until they impinge upon adjacent growing crystal.


a)
Nucleation
of crystals,
b)

crystal growth,
c)

irregular grains form as crystals grow together,
d)

grain boundaries as seen in a microscope.

In engineering materials, a crystal is usually referred to as a grain. A grain is merely a crystal
without smooth faces because its growth was impeded by contact with another grain or a
boundary surface. The interface formed between grains is called a grai
n boundary. The atoms
between the grains (at the grain boundaries) have no crystalline structure and are said to be
disordered.

Grains are sometimes large enough to be visible under an ordinary light microscope or even to
the unaided eye. The spangles tha
t are seen on newly galvanized metals are grains. Rapid cooling
generally results in more nucleation points and smaller grains (a fine grain structure). Slow
cooling generally results in larger grains which will have lower strength, hardness and ductility.

Dendrites

In metals, the crystals that form in the liquid during
freezing generally follow a pattern consisting of a main
branch with many appendages. A crystal with this
morphology slightly resembles a pine tree and is called a
dendrite, which means bra
nching. The formation of
dendrites occurs because crystals grow in defined planes
due to the crystal lattice they create. The figure to the right
shows how a cubic crystal can grow in a melt in three
dimensions, which correspond to the six faces of the cub
e.
For clarity of illustration, the adding of unit cells with
continued solidification from the six faces is shown simply
as lines. Secondary dendrite arms branch off the primary
arm, and tertiary arms off the secondary arms and etcetera.

During freezing
of a polycrystalline material, many
dendritic crystals form and grow until they eventually
become large enough to impinge upon each other.
Eventually, the interdendriticspaces between the dendrite
arms crystallize to yield a more regular crystal. The
origi
nal dendritic pattern may not be apparent when
examining the microstructure of a material. However,
dendrites can often be seen in solidification voids that
sometimes occur in castings or welds, as shown to the
right..

Shrinkage

Most materials contract or
shrink during solidification and
cooling. Shrinkage is the result of:



Contraction of the liquid as it cools prior to its
solidification



Contraction during phase change from a liquid to solid



Contraction of the solid as it continues to cool to ambient temp
erature.

Shrinkage can sometimes cause cracking to occur in component as it solidifies. Since the coolest
area of a volume of liquid is where it contacts a mold or die, solidification usually begins first at
this surface. As the crystals grow inward, the m
aterial continues to shrink. If the solid surface is
too rigid and will not deform to accommodate the internal shrinkage, the stresses can become
high enough to exceed the tensile strength of the material and cause a crack to form. Shrinkage
cavitation som
etimes occurs because as a material solidifies inward, shrinkage occurred to such
an extent that there is not enough atoms present to fill the available space and a void is left.

Anisotropy and Isotropy

In a single crystal, the physical and mechanical prop
erties often differ with orientation. It can be
seen from looking at our models of crystalline structure that atoms should be able to slip over
one another or distort in relation to one another easier in some directions than others. When the
properties of
a material vary with different crystallographic orientations, the material is said to be
anisotropic
.

Alternately, when the properties of a material are the same in all directions, the material is said to
be

isotropic
. For many polycrystalline materials t
he grain orientations are random before any
working (deformation) of the material is done. Therefore, even if the individual grains are
anisotropic, the property differences tend to average out and, overall, the material is isotropic.
When a material is fo
rmed, the grains are usually distorted and elongated in one or more
directions which makes the material anisotropic. Material forming will be discussed later but
let’s continue discussing crystalline structure at the atomic level.




Crystal Defects

A per
fect crystal, with every atom of the same type in the correct position, does not exist. All
crystals have some defects. Defects contribute to the mechanical properties of metals. In fact,
using the term “defect” is sort of a misnomer since these features a
re commonly intentionally
used to manipulate the mechanical properties of a material. Adding alloying elements to a metal
is one way of introducing a crystal defect. Nevertheless, the term “defect” will be used, just keep
in mind that crystalline defects a
re not always bad. There are basic classes of crystal defects:



point defects, which are places where an atom is missing or irregularly placed in the
lattice structure. Point defects include lattice vacancies, self
-
interstitial atoms,
substitution impurity
atoms, and interstitial impurity atoms



linear defects, which are groups of atoms in irregular positions. Linear defects are
commonly called dislocations.



planar defects, which are interfaces between homogeneous regions of the material. Planar
defects inclu
de grain boundaries, stacking faults and external surfaces.

It is important to note at this point that plastic deformation in a material occurs due to the
movement of dislocations (linear defects). Millions of dislocations result for plastic forming
opera
tions such as rolling and extruding. It is also important to note that any defect in the regular
lattice structure disrupts the motion of dislocation, which makes slip or plastic deformation more
difficult. These defects not only include the point and plan
er defects mentioned above, and also
other dislocations. Dislocation movement produces additional dislocations, and when
dislocations run into each other it often impedes movement of the dislocations. This drives up the
force needed to move the dislocation

or, in other words, strengthens the material. Each of the
crystal defects will be discussed in more detail in the following pages.

Point Defects

Point defects are where an atom is missing or is in an
irregular place in the lattice structure. Point defects
include self interstitial atoms, interstitial impurity
atoms, substitutional atoms and vacancies. A self
interstitial atom is an extra atom that
has crowded its
way into an interstitial void in the crystal structure.
Self interstitial atoms occur only in low
concentrations in metals because they distort and
highly stress the tightly packed lattice structure.

A substitutional impurity atom is an at
om of a
different type than the bulk atoms, which has
replaced one of the bulk atoms in the lattice.
Substitutional impurity atoms are usually close in
size (within approximately 15%) to the bulk atom.
An example of substitutional impurity atoms is the
zin
c atoms in brass. In brass, zinc atoms with a
radius of 0.133 nm have replaced some of the copper
atoms, which have a radius of 0.128 nm.


Interstitial impurity atoms are much smaller than the
atoms in the bulk matrix. Interstitial impurity atoms fit into

the open space between the bulk
atoms of the lattice structure. An example of interstitial impurity atoms is the carbon atoms that
are added to iron to make steel. Carbon atoms, with a radius of 0.071 nm, fit nicely in the open
spaces between the larger (
0.124 nm) iron atoms.

Vacancies are empty spaces where an atom should be, but is missing. They are common,
especially at high temperatures when atoms are frequently and randomly change their positions
leaving behind empty lattice sites. In most cases diffu
sion (mass transport by atomic motion) can
only occur because of vacancies.

Linear Defects
-

Dislocations

Dislocations are another type of defect in crystals. Dislocations are areas were the atoms are out
of position in the crystal structure. Dislocations
are generated and move when a stress is applied.
The motion of dislocations allows slip


plastic deformation to occur.

Before the discovery of the dislocation by Taylor, Orowan and Polyani in 1934, no one could
figure out how the plastic deformation prop
erties of a metal could be greatly changed by solely
by forming (without changing the chemical composition). This became even bigger mystery
when in the early 1900’s scientists estimated that metals undergo plastic deformation at forces
much smaller than t
he theoretical strength of the forces that are holding the metal atoms together.
Many metallurgists remained skeptical of the dislocation theory until the development of the
transmission electron microscope in the late 1950’s. The TEM allowed experimental
evidence to
be collected that showed that the strength and ductility of metals are controlled by dislocations.


There are two basic types of dislocations, the edge dislocation and the screw dislocation.
Actually, edge and screw dislocations are just extre
me forms of the possible dislocation
structures that can occur. Most dislocations are probably a hybrid of the edge and screw forms
but this discussion will be limited to these two types.

Edge Dislocations

The edge defect can be easily visualized as an ex
tra half
-
plane of atoms in a lattice. The
dislocation is called a line defect because the locus of defective points produced in the lattice by
the dislocation lie along a line. This line runs along the top of the extra half
-
plane. The inter
-
atomic bonds ar
e significantly distorted only in the immediate vicinity of the dislocation line.


Understanding the movement of a dislocation is key to understanding why dislocations allow
deformation to occur at much lower stress than in a perfect crystal. Dislocation
motion is
analogous to movement of a caterpillar. The caterpillar would have to exert a large force to move
its entire body at once. Instead it moves the rear portion of its body forward a small amount and
creates a hump. The hump then moves forward and ev
entual moves all of the body forward by a
small amount.


As shown in the set of images above, the dislocation moves similarly moves a small amount at a
time. The dislocation in the top half of the crystal is slipping one plane at a time as it moves to
th
e right from its position in image (a) to its position in image (b) and finally image (c). In the
process of slipping one plane at a time the dislocation propagates across the crystal. The
movement of the dislocation across the plane eventually causes the
top half of the crystal to
move with respect to the bottom half. However, only a small fraction of the bonds are broken at
any given time. Movement in this manner requires a much smaller force than breaking all the
bonds across the middle plane simultaneou
sly.

Screw Dislocations

There is a second basic type of dislocation,
called screw dislocation. The screw dislocation
is slightly more difficult to visualize. The motion
of a screw dislocation is also a result of shear
stress, but the defect line movement is
perpendicular to direc
tion of the stress and the
atom displacement, rather than parallel. To
visualize a screw dislocation, imagine a block of
metal with a shear stress applied across one end
so that the metal begins to rip. This is shown in
the upper right image. The lower rig
ht image
shows the plane of atoms just above the rip. The
atoms represented by the blue circles have not
yet moved from their original position. The
atoms represented by the red circles have moved
to their new position in the lattice and have
reestablished

metallic bonds. The atoms
represented by the green circles are in the
process of moving. It can be seen that only a
portion of the bonds are broke at any given time.
As was the case with the edge dislocation,
movement in this manner requires a much
smalle
r force than breaking all the bonds across the middle plane simultaneously.

If the shear force is increased, the atoms will continue to slip to the right. A row of the green
atoms will find there way back into a proper spot in the lattice (and become red)

and a row of the
blue atoms will slip out of position (and become green). In this way, the screw dislocation will
move upward in the image, which is perpendicular to direction of the stress. Recall that the edge
dislocation moves parallel to the direction

of stress. As shown in the image below, the net plastic
deformation of both edge and screw dislocations is the same, however.


The dislocations move along the densest planes of atoms in a material, because the stress needed
to move the dislocation increa
ses with the spacing between the planes. FCC and BCC metals
have many dense planes, so dislocations move relatively easy and these materials have high
ductility. Metals are strengthened by making it more difficult for dislocations to move. This may
involve

the introduction of obstacles, such as interstitial atoms or grain boundaries, to “pin” the
dislocations. Also, as a material plastically deforms, more dislocations are produced and they
will get into each others way and impede movement. This is why strai
n or work hardening
occurs.

In ionically bonded materials, the ion must move past an area with a repulsive charge in order to
get to the next location of the same charge. Therefore, slip is difficult and the materials are
brittle. Likewise, the low densit
y packing of covalent materials makes them generally more
brittle than metals.




Planar Defects

Stacking Faults and Twin Boundaries

A disruption of the long
-
range stacking sequence can produce two other common types of crystal
defects: 1) a stacking fault and 2) a twin region. A change in the stacking sequence over a few
atomic spacings produces a stacking fault whereas a change over m
any atomic spacings produces
a twin region.


A stacking fault is a one or two layer interruption in the stacking sequence of atom planes.
Stacking faults occur in a number of crystal structures, but it is easiest to see how they occur in
close packed stru
ctures. For example, it is know from a previous discussion that face centered
cubic (fcc) structures differ from hexagonal close packed (hcp) structures only in their stacking
order. For hcp and fcc structures, the first two layers arrange themselves ident
ically, and are said
to have an AB arrangement. If the third layer is placed so that its atoms are directly above those
of the first (A) layer, the stacking will be ABA. This is the hcp structure, and it continues
ABABABAB. However it is possible for the t
hird layer atoms to arrange themselves so that they
are in line with the first layer to produce an ABC arrangement which is that of the fcc structure.
So, if the hcp structure is going along as ABABAB and suddenly switches to ABABABCABAB,
there is a stacki
ng fault present.

Alternately, in the fcc arrangement the pattern is ABCABCABC. A stacking fault in an fcc
structure would appear as one of the C planes missing. In other words the pattern would become
ABCABCAB_ABCABC.

If a stacking fault does not correc
ts itself immediately but continues over some number of atomic
spacings, it will produce a second stacking fault that is the twin of the first one. For example if
the stacking pattern is ABABABAB but switches to ABCABCABC for a period of time before
switch
ing back to ABABABAB, a pair of twin stacking faults is produced. The red region in the
stacking sequence that goes ABCABC
ACBACB
ABCABC is the twin plane and the twin
boundaries are the A planes on each end of the highlighted region.

Grain Boundaries in Po
lycrystals

Another type of planer defect is the grain boundary. Up to this point, the discussion has focused
on defects of single crystals. However, solids generally consist of a number of crystallites or
grains. Grains can range in size from nanometers to

millimeters across and their orientations are
usually rotated with respect to neighboring grains. Where one grain stops and another begins is
know as a grain boundary. Grain boundaries limit the lengths and motions of dislocations.
Therefore, having small
er grains (more grain boundary surface area) strengthens a material. The
size of the grains can be controlled by the cooling rate when the material cast or heat treated.
Generally, rapid cooling produces smaller grains whereas slow cooling result in larger

grains.
For more information, refer to the discussion on solidification.

Elastic/Plastic Deformation

When a sufficient load is applied to a metal or other structural material, it will cause the material
to change shape. This change in shape is called defo
rmation. A temporary shape change that is
self
-
reversing after the force is removed, so that the object returns to its original shape, is called
elastic deformation. In other words, elastic deformation is a change in shape of a material at low
stress that
is recoverable after the stress is removed. This type of deformation involves stretching
of the bonds, but the atoms do not slip past each other.

When the stress is sufficient to
permanently deform the metal, it is called
plastic deformation. As discusse
d in the
section on crystal defects, plastic
deformation involves the breaking of a
limited number of atomic bonds by the
movement of dislocations. Recall that the
force needed to break the bonds of all the
atoms in a crystal plane all at once is very
grea
t. However, the movement of
dislocations allows atoms in crystal planes
to slip past one another at a much lower
stress levels. Since the energy required to
move is lowest along the densest planes of
atoms, dislocations have a preferred
direction of travel

within a grain of the
material. This results in slip that occurs
along parallel planes within the grain.
These parallel slip planes group together to
form slip bands, which can be seen with an
optical microscope. A slip band appears as
a single line under

the microscope, but it is
in fact made up of closely spaced parallel
slip planes as shown in the image.

Fatigue Crack Initiation

While on the subject of dislocations, it is appropriate to briefly
discuss fatigue. Fatigue is one of the primary reasons fo
r the
failure of structural components. The life of a fatigue crack has
two parts, initiation and propagation. Dislocations play a major
role in the fatigue crack initiation phase. It has been observed
in laboratory testing that after a large number of loa
ding cycles
dislocations pile up and form structures called persistent slip
bands (PSB). An example
of a PSB is shown in the
micrograph image to the
right.

PSBs are areas that rise
above (extrusion) or fall below
(intrusion) the surface of the
component
due to movement of
material along slip planes.
This leaves tiny steps in the
surface that serve as stress
risers where fatigue cracks can
initiate. A crack at the edge of a PSB is shown in the image below taken with a scanning electron
microscope (SEM).




Diffusion

Diffusion is the migration of atoms from a region of high concentration to
a region of low concentration. In a homogeneous material, atoms are
routinely moving around but the movement is random (i.e. there is
always an equal number of atoms moving in all d
irections). In an
inhomogeneous material, all the atoms are moving near randomly, but
there is a migration of atoms to areas where their concentrations are
lower. In other words, there is a net diffusion.

Atom diffusion can occur by the motion of host or
substitutional atoms to
vacancies (vacancy diffusion), or interstitial impurities atoms to different
interstitial positions (interstitial diffusion). In order to move, an atom
must overcome the bond energy due to nearby atoms. This is more easily
achieved
at high temperatures when the atoms are vibrating strongly.
Carburizing, which will be discussed later, is an example of diffusion is
used.

Strengthening/Hardening Mechanisms

As discussed in the previous section, the ability of a crystalline material to
plastically deform
largely depends on the ability for dislocation to move within a material. Therefore, impeding the
movement of dislocations will result in the strengthening of the material. There are a number of
ways to impede dislocation movement, which

include:



controlling the grain size (reducing continuity of atomic planes)



strain hardening (creating and tangling dislocations)



alloying (introducing point defects and more grains to pin dislocation)

Control of Grain Size

The size of the grains within

a material also has an effect
on the strength of the material. The boundary between
grains acts as a barrier to dislocation movement and the
resulting slip because adjacent grains have different
orientations. Since the atom alignment is different and slip

planes are discontinuous between grains. The smaller the
grains, the shorter the distance atoms can move along a
particular slip plane. Therefore, smaller grains improve the
strength of a material. The size and number of grains within a material is contro
lled by the rate of
solidification from the liquid phase.

Strain Hardening

Strain hardening (also called work
-
hardening or cold
-
working) is the process of making a metal
harder and stronger through plastic deformation. When a metal is plastically deformed
,
dislocations move and additional dislocations are generated. The more dislocations within a
material, the more they will interact and become pinned or tangled. This will result in a decrease
in the mobility of the dislocations and a strengthening of the
material. This type of strengthening
is commonly called cold
-
working. It is called cold
-
working because the plastic deformation must
occurs at a temperature low enough that atoms cannot rearrange themselves. When a metal is
worked at higher temperatures (h
ot
-
working) the dislocations can rearrange and little
strengthening is achieved.

Strain hardening can be easily demonstrated with piece of wire or a paper clip. Bend a straight
section back and forth several times. Notice that it is more difficult to bend

the metal at the same
place. In the strain hardened area dislocations have formed and become tangled, increasing the
strength of the material. Continued bending will eventually cause the wire to break at the bend
due to fatigue cracking. (After a large nu
mber of bending cycles, dislocations form structures
called Persistent Slip Bands (PSB). PSBs are basically tiny areas where the dislocations have
piled up and moved the material surface out leave steps in the surface that act as stress risers or
crack ini
tiation points.)


It should be understood, however, that increasing the
strength by cold
-
working will also result in a reduction in
ductility. The graph to the right shows the yield strength
and the percent elongation as a function of percent cold
-
work fo
r a few example materials. Notice that for each
material, a small amount of cold
-
working results in a
significant reduction in ductility.



Effects of Elevated Temperature on Strain Hardened Materials

When strain hardened materials are exposed to elevated

temperatures, the strengthening that
resulted from the plastic deformation can be lost. This can be a bad thing if the strengthening is
needed to support a load. However, strengthening due to strain hardening is not always desirable,
especially if the mat
erial is being heavily formed since ductility will be lowered.

Heat treatment can be used to remove the effects of strain hardening. Three things can occur
during heat treatment:

1.

Recovery

2.

Recrystallization

3.

Grain growth

Recovery

When a stain hardened
material is held at
an elevated temperature an increase in
atomic diffusion occurs that relieves
some of the internal strain energy.
Remember that atoms are not fixed in
position but can move around when they
have enough energy to break their bonds.
Diffus
ion increases rapidly with rising
temperature and this allows atoms in
severely strained regions to move to
unstrained positions. In other words,
atoms are freer to move around and
recover a normal position in the lattice
structure. This is known as the re
covery
phase and it results in an adjustment of
strain on a microscopic scale. Internal residual stresses are lowered due to a reduction in the
dislocation density and a movement of dislocation to lower
-
energy positions. The tangles of
dislocations condens
e into sharp two
-
dimensional boundaries and the dislocation density within
these areas decrease. These areas are called subgrains. There is no appreciable reduction in the
strength and hardness of the material but corrosion resistance often improves.


Recr
ystallization

At a higher temperature, new, strain
-
free grains nucleate and grow inside the old distorted grains
and at the grain boundaries. These new grains grow to replace the deformed grains produced by
the strain hardening. With recrystallization, the

mechanical properties return to their original
weaker and more ductile states. Recrystallization depends on the temperature, the amount of time
at this temperature and also the amount of strain hardening that the material experienced. The
more strain hard
ening, the lower the temperature will be at which recrystallization occurs. Also,
a minimum amount (typically 2
-
20%) of cold work is necessary for any amount of
recrystallization to occur. The size the new grains is also partially dependant on the amount o
f
strain hardening. The greater the stain hardening, the more nuclei for the new grains, and the
resulting grain size will be smaller (at least initially).


Grain Growth

If a specimen is left at the high temperature beyond the time needed for complete
rec
rystallization, the grains begin to grow in size. This occurs because diffusion occurs across the
grain boundaries and larger grains have less grain boundary surface area per unit of volume.
Therefore, the larger grains lose fewer atoms and grow at the exp
ense of the smaller grains.
Larger grains will reduce the strength and toughness of the material.




Alloying

Only a few elements are widely used commercially in their pure form. Generally, other elements
are present to produce greater strength, to impro
ve corrosion resistance, or simply as impurities
left over from the refining process. The addition of other elements into a metal is called alloying
and the resulting metal is called an alloy. Even if the added elements are nonmetals, alloys may
still have

metallic properties.

Copper alloys were produced very early in our history. Bronze, an alloy of copper and tin, was
the first alloy known. It was easy to produce by simply adding tin to molten copper. Tools and
weapons made of this alloy were stronger th
an pure copper ones. The typical alloying elements
in some common metals are presented in the table below.

Alloy

Composition

Brass

Copper, Zinc

Bronze

Copper, Zinc, Tin

Pewter

Tin, Copper, Bismuth, Antimony

Cast Iron

Iron, Carbon, Manganese, Silicon

Steel

Iron, Carbon (plus small amounts of other elements)

Stainless Steel

Iron, Chromium, Nickel

The properties of alloys can be manipulated by varying composition. For example steel formed
from iron and carbon can vary substantially in hardness dependin
g on the amount of carbon
added and the way in which it was processed.

When a second element is added, two basically different structural changes are possible:

1.

Solid solution strengthening occurs when the atoms of the new element form a solid
solution with the original element, but there is still only one phase. Recall that the term
‘phase’ refers to that region of space occupied by a physically homogeneous mater
ial.

2.

The atoms of the new elements form a new second phase. The entire microstructure may
change to this new phase or two phases may be present.

Solid Solution Strengthening

Solid solution strengthening involves the addition of other metallic elements th
at will dissolve in
the parent lattice and cause distortions because of the difference in atom size between the parent
metal and the solute metal. Recall from the section on crystal point defects that it is possible to
have substitutional impurity atoms, a
nd interstitial impurity atoms. A substitutional impurity
atom is an atom of a different type than the bulk atoms, which has replaced one of the bulk atoms
in the lattice. Substitutional impurity atoms are usually close in size (within approximately 15%)
t
o the bulk atom. Interstitial impurity atoms are much smaller than the atoms in the bulk matrix.
Interstitial impurity atoms fit into the open space between the bulk atoms of the lattice structure.

Since the impurity atoms are smaller or larger than the s
urrounding atoms they introduce tensile
or compressive lattice strains. They disrupt the regular arrangement of ions and make it more
difficult for the layers to slide over each other. This makes the alloy stronger and less ductile
than the pure metal. For

example, an alloy of 30% nickel raises the cast tensile strength of copper
from 25,000 PSI to 55,000 PSI.

Multiphase Metals

Still another method of strengthening the metal is adding elements that have no or partial
solubility in the parent metal. This wi
ll result in the appearance of a second phase distributed
throughout the crystal or between crystals. These secondary phases can raise or reduce the
strength of an alloy. For example, the addition of tin, zinc, or aluminum to copper will result in
an alloy

with increased strength, but alloying with lead or bismuth with result in a lower strength
alloy. The properties of a polyphase (two of more phase) material depend on the nature, amount,
size, shape, distribution, and orientation of the phases. Greek lett
ers are commonly used to
distinguish the different solid phases in a given alloy.


Phases can be seen on a microscopic scale with an optical microscope after the surface has been
properly polished and etched. Below is a micrograph take at 125x of lead
-
tin

alloy composed of
two phases. The light colored regions are a tin
-
rich phase and the dark colored regions are a lead
-
rich phase.


Alloying (continued)

Phase Diagrams

As previously stated, the phase diagram is simply a map showing the structure of phase
s present
as the temperature and overall composition of the alloy are varied. It is a very useful tool for
understanding and controlling the structures of polyphase materials. A binary phase diagram
shows the phases formed in differing mixtures of two elem
ents over a range of temperatures.
When an alloy exhibits more than two phases, a different type of phase diagram must be used,
such as a ternary diagram for three phase alloys. This discussion will focus on the binary phase
diagram.

On the binary phase
diagram, compositions run
from 100% Element A on the left, through all
possible mixtures, to 100% Element B on the
right. The composition of an alloy is given in the
form A
-

x%B. For example, Cu
-

20%Al is 80%
copper and 20% aluminum. Weight percentages
a
re often used to specify the proportions of the
alloying elements, but atomic percent are
sometimes used. Weight percentages will be used
throughout this text.

Alloys generally do not have a single melting
point, but instead melt (or alternately solidify)

over a range of temperatures. At each end of the
phase diagram only one of the elements is present
(100% A or 100% B) so a specific melting point
does exists. Additionally, there is sometimes a
mixture of the constituent elements which
produces melting at

a single temperature like a
pure element. This is called the eutectic point.

At compositions other than at the pure A, pure
B and the eutectic points, when the alloy is
cooled from a high temperature it will begin to
solidify at a certain temperature bu
t will
remain in a mushy (liquid plus solid) condition
over a range of temperatures. If experiments
are conducted over a range of compositions to
determine the temperature at which the alloys
start to solidify, this data can be potted on the phase diagram
to produce a curve. This “start of
solidification curve” will join the three single solidification points and is called the liquidus line.

Up to a few percent of composition, it is possible
for one element to remain dissolve in another
while both are in t
he solid state. This is called
solid solubility and the solubility limit normally
changes with temperature. The extent of the solid
solubility region can be plotted onto the phase
diagram. In this example, the alpha phase is the
region of solid solution wh
ere some of B atoms
have dissolved in a matrix of A atoms. The beta
phase is the region where a small percentage of A
atoms have dissolved in a matrix of B atoms. It is
important to note that some elements have zero solid solubility in other elements. An e
xample is
aluminum/silicon alloys, where aluminum has zero solid solubility in silicon.

If an alloy's composition does not place it within
the alpha or beta solid solution regions, the alloy
will become fully solid at the

eutectic
temperature
. The eutecti
c line on the phase
diagram indicates where this transformation will
occur over the range of compositions. At alloy
compositions and temperatures between the
liquidus temperature and the eutectic temperature,
a mushy mix of either alpha or beta phase will
exist as solid masses within a liquid mixture of A
and B. These are the alpha plus liquid and the
beta plus liquid areas on the phase diagram. The region below the eutectic line, and outside the
solid solution region, will be a solid mixture of alpha and b
eta.

Alloying (continued)

Tie and Lever Rules

Simply by looking at a phase diagram it is possible to tell what phase or phases an alloy will
have at a given temperature. But, it is also possible to get quantitative information from the
diagram. Consider the alloy at the temperature
shown on the phase
diagram. It is easy to see that
at this temperature, it is a mixture of alpha and
liquid phases. Using a tie line it is also
possible to
determine the composition of the phases at this
temperature. A tie line is an isothermal (constant
temperature) line d
rawn through the alloy's
position on the phase diagram when it is in a two
phase field. The points where the ends of the tie
line intersect the two adjacent solubility curves
indicate the compositions of the two phases that
exist in equilibrium at this tem
perature. In this
example, the tie line shows that the alpha phase is
5.2%B

and the liquid phase is
34.5%B

at this
temperature. It is important to keep in mind that the tie rule addresses the determination of the
compositions of the constituent phases with
in the sample and it does not address the overall
chemical composition of the sample, which remains unchanged.

It is also possible to determine how much of each phase exists at the given temperature using the
lever rule. It is important to know the amount
s of each phase present because the properties of
the alloy depend on the amount of each phase present. The lever rule uses the tie line and the
basic scientific principle of the conservation of mass to determine the ratio of the two phases
present. The ti
e
-
line gives the chemical compositions of each of the two phases, and the
combined amounts of these two compositions must add up to the alloy's overall composition
(C
o
), which is known. In other words, C
o

must be composed of the appropriate amount of α at
composition C
α

and of liquid at C
liq
. So basically, the proportions of the phases present are given
by the relative lengths of the two sections of the tie line.

The fraction of alpha phase present is the given by the ratio of the C
o

to C
liq

portion of the

tie line
and the total length of the tie line (C
liq

to C
α
). Mathematically the relationships can be written as
f
α

<< (C
liq



C
o
)/(C
liq

-

C
α
). The fraction of liquid phase present is given by the ratio of the C
o

to
C
α

portion of the tie line and the total
length of the tie line (C
liq

to C
α
). Mathematically this
relationships can be written as f
liq

<< (C
o

-

C
α
)/(C
liq

-

C
α
). Of course, the two values must total to
equal one.

Note that the right side of the tie line gives the proportion of the phase on the le
ft (α phase in this
example) and left side of the tie line gives the proportion of the phase to the right (liquid phase in
this example). It is easy to keep this relationship straight by simply considering what the ratio
would be near one of the tie line i
ntersect points. For example, if C
o

were near the liquidus line
the ratio of the liquid section of the line to the total length of the line will be nearly one.

Alloying (continued)

Composition, Microstructure, and the Phase Diagram


Let’s finish this discussion on phase diagrams by briefly looking at three different compositions
of elements A and B, and how their microstructures will differ because of their positions on the
phase diagram. First a eutectic alloy, which is an alloy with

composition right at the eutectic
point, will be considered. Then compositions on both sides of the eutectic point will be
discussed. An alloy with a composition that lies to the left of the eutectic point on the phase
diagram is called a hypoeutectic all
oy, and an alloy with a composition that lies to the right of
the eutectic point is called hypereutectic alloy. At this point, only the condition of slow cooling,
which will allow the alloy to solidify into it equilibrium condition, will be considered. The

microstructure can be controlled by manipulating the speed of cooling the alloy, but this will be
covered in the section on heat treatments.

Eutectic Alloys

First, consider the eutectic alloy of elements A
and B as it is cooled from a temperature at
loc
ation 1 to location 4 on the phase diagram. At
location 1, the alloy is at a high enough
temperature to make the mixture fully liquid. The circles below show a representation of the
alloy's microstructure at each of the locations numbered on the phase diag
ram.

At location 1, there is nothing of interest as the alloy is completely liquid. As the alloy is slow
cooled, it remains liquid until it reaches the eutectic temperature (location 2) where it starts to
solidify at any favorable nucleation sites. From th
e microstructure image 2, it can be see that as
the alloy solidifies it forms into alternate layers of alpha and beta phase. This layered
microstructure is known as lamellar microstructure and the layers are often only of the order of 1
micron across. The
reason that a eutectic alloy forms in this way has to do with the diffusion
times required to form the solid.


The grains grow by adding alpha to alpha and beta to beta until they encounter another grain
(location 3). Further nucleation sites will also
continue to form within the liquid parts of the
mixture. This solidification happens very rapidly as any given volume of liquid in the melt
reaches the eutectic temperature. Remember that a eutectic composition solidifies at a single
temperature like a pur
e element and not over a temperature range.

As the now sold alloy cools to location 4, the composition of the layers of alpha and beta
continue to change as it cools. Atoms of A and B will diffuse between the two phases to produce
the equilibrium compositi
ons of alpha and beta phase at a given temperature. By drawing tie
lines at various temperatures the eutectic point on the phase diagram, it can be seen that the
solubility of A in the beta phase and B in the alpha phase decreases as the temperature decrea
ses.
Since this phase composition change is due to diffusion, which is a relatively a slow process), it
is important that eutectic alloys be allowed to cool slowly to produce the correct microstructure.

Hypoeutectic Alloys

Next, consider an alloy of A and

B that has an
overall composition that places it to the left of the
eutectic point. When an alloy falls to the left of
the eutectic point it is called a hypoeutectic alloy.
At location 1, the alloy is at a temperature that is
high enough to put it in a fu
lly liquid phase.

When the alloy is cooled, it remains in the liquid
state until it reaches the temperature where it
crosses the liquidus line (location 2). At this
temperature, the alpha phase starts to solidify at
any favorable nucleation sites. The alp
ha solidifies as dendrites which grow to become grains of
alpha. The first solid phase to form is called the primary phase so, in this case, primary alpha is