Solid materials have been conveniently grouped into three basic classifications:met-
als, ceramics, and polymers.This scheme is based primarily on chemical makeup and
atomic structure, and most materials fall into one distinct grouping or another,
although there are some intermediates. In addition, there are the composites, com-
binations of two or more of the above three basic material classes.A brief explana-
tion of these material types and representative characteristics is offered next.Another
classification is advanced materials—those used in high-technology applications—
viz. semiconductors, biomaterials, smart materials, and nanoengineered materials;

Metals
Materials in this group are composed of one or more metallic elements (such as iron,
aluminum, copper, titanium, gold, and nickel), and often also nonmetallic elements (for
example, carbon, nitrogen, and oxygen) in relatively small amounts. Atoms in metals
and their alloys are arranged in a very orderly manner and in comparison to the ceramics and polymers, are relatively dense.






regard to mechanical characteristics, these materials are relatively stiff (Figure 1.4)
and strong yet are ductile (i.e., capable of large amounts of deformation
without fracture), and are resistant to fracture , which accounts for their
widespread use in structural applications. Metallic materials have large numbers of
nonlocalized electrons; that is, these electrons are not bound to particular atoms.Many
properties of metals are directly attributable to these electrons. For example, metals
are extremely good conductors of electricity and heat, and are not trans-
parent to visible light; a polished metal surface has a lustrous appearance. In addi-
tion, some of the metals (viz., Fe, Co, and Ni) have desirable magnetic properties.
Figure 1.8 is a photograph that shows several common and familiar objects that
are made of metallic materials. Furthermore, the types and applications of metals


Ceramics
Ceramics are compounds between metallic and nonmetallic elements; they are most
frequently oxides, nitrides, and carbides. For example, some of the common ceramic







materials include aluminum oxide (or alumina,Al2O3), silicon dioxide (or silica, SiO2),
silicon carbide (SiC), silicon nitride (Si3N4), and, in addition, what some refer to as
the traditional ceramics—those composed of clay minerals (i.e., porcelain), as well as
cement, and glass. With regard to mechanical behavior, ceramic materials are rela-
tively stiff and strong—stiffnesses and strengths are comparable to those of the met-
als (Figures 1.4 and 1.5). In addition, ceramics are typically very hard. On the other
hand, they are extremely brittle (lack ductility), and are highly susceptible to fracture
(Figure 1.6). These materials are typically insulative to the passage of heat and elec-
tricity (i.e., have low electrical conductivities, Figure 1.7), and are more resistant to
high temperatures and harsh environments than metals and polymers.With regard to
optical characteristics, ceramics may be transparent, translucent, or opaque (Figure
1.2), and some of the oxide ceramics (e.g., Fe O ) exhibit magnetic behavior.






Polymers


Polymers include the familiar plastic and rubber materials.Many of them are organic
compounds that are chemically based on carbon, hydrogen, and other nonmetallic
elements (viz.O,N, and Si). Furthermore, they have very large molecular structures,
often chain-like in nature that have a backbone of carbon atoms. Some of the com-
mon and familiar polymers are polyethylene (PE), nylon, poly(vinyl chloride)
(PVC), polycarbonate (PC), polystyrene (PS), and silicone rubber. These materials
typically have low densities (Figure 1.3), whereas their mechanical characteristics
are generally dissimilar to the metallic and ceramic materials—they are not as stiff
nor as strong as these other material types (Figures 1.4 and 1.5). However, on the
basis of their low densities,many times their stiffnesses and strengths on a per mass



basis are comparable to the metals and ceramics. In addition,many of the polymers
are extremely ductile and pliable (i.e., plastic), which means they are easily formed
into complex shapes. In general, they are relatively inert chemically and unreactive
in a large number of environments. One major drawback to the polymers is their
tendency to soften and/or decompose at modest temperatures, which, in some in-
stances, limits their use. Furthermore, they have low electrical conductivities (Fig-
ure 1.7) and are nonmagnetic.
The photograph in Figure 1.10 shows several articles made of polymers that are
familiar to the reader. Chapters 14 and 15 are devoted to discussions of the struc-
tures, properties, applications, and processing of polymeric materials.



Fiberglass is sometimes also termed a “glass fiber-reinforced polymer” composite, abbrevi-
ated “GFRP.”


Composites


A composite is composed of two (or more) individual materials, which come from
the categories discussed above—viz.,metals, ceramics, and polymers.The design goal
of a composite is to achieve a combination of properties that is not displayed by
any single material, and also to incorporate the best characteristics of each of the
component materials.A large number of composite types exist that are represented
by different combinations of metals, ceramics, and polymers. Furthermore, some
naturally-occurring materials are also considered to be composites—for example,
wood and bone. However, most of those we consider in our discussions are syn-
thetic (or man-made) composites.
One of the most common and familiar composites is fiberglass, in which small
glass fibers are embedded within a polymeric material (normally an epoxy or
polyester).
The glass fibers are relatively strong and stiff (but also brittle), whereas
the polymer is ductile (but also weak and flexible). Thus, the resulting fiberglass is
relatively stiff, strong, (Figures 1.4 and 1.5) flexible, and ductile. In addition, it has
a low density (Figure 1.3).
Another of these technologically important materials is the “carbon fiber-
reinforced polymer” (or “CFRP”) composite—carbon fibers that are embedded within
a polymer. These materials are stiffer and stronger than the glass fiber-reinforced
materials (Figures 1.4 and 1.5), yet they are more expensive.The CFRP composites

are used in some aircraft and aerospace applications, as well as high-tech sporting
equipment (e.g., bicycles, golf clubs, tennis rackets, and skis/snowboards). Chapter 16
is devoted to a discussion of these interesting materials.


ADVANCED MATERIALS


Materials that are utilized in high-technology (or high-tech) applications are some-
times termed advanced materials. By high technology we mean a device or product
that operates or functions using relatively intricate and sophisticated principles; ex-
amples include electronic equipment (camcorders, CD/DVD players, etc.), com-
puters, fiber-optic systems, spacecraft, aircraft, and military rocketry.These advanced
materials are typically traditional materials whose properties have been enhanced,
and, also newly developed, high-performance materials. Furthermore, they may be
of all material types (e.g., metals, ceramics, polymers), and are normally expensive.
Advanced materials include semiconductors, biomaterials, and what we may term
“materials of the future” (that is, smart materials and nanoengineered materials),
which we discuss below. The properties and applications of a number of these
advanced materials—for example, materials that are used for lasers, integrated
circuits, magnetic information storage, liquid crystal displays (LCDs), and fiber
optics—are also discussed in subsequent chapters.



Semiconductors


Semiconductors have electrical properties that are intermediate between the elec-
trical conductors (viz. metals and metal alloys) and insulators (viz. ceramics and
polymers)—Figure 1.7. Furthermore, the electrical characteristics of these materi-
als are extremely sensitive to the presence of minute concentrations of impurity
atoms, for which the concentrations may be controlled over very small spatial re-
gions. Semiconductors have made possible the advent of integrated circuitry that
has totally revolutionized the electronics and computer industries (not to mention
our lives) over the past three decades.



Biomaterials


Biomaterials are employed in components implanted into the human body for
replacement of diseased or damaged body parts.These materials must not produce
toxic substances and must be compatible with body tissues (i.e., must not cause
adverse biological reactions). All of the above materials—metals, ceramics, poly-
mers, composites, and semiconductors—may be used as biomaterials. For example,
some of the biomaterials that are utilized in artificial hip replacements are dis-
cussed in Section 22.12.


Materials of the Future
Smart Materials


Smart (or intelligent) materials are a group of new and state-of-the-art materials
now being developed that will have a significant influence on many of our tech-
nologies.The adjective “smart” implies that these materials are able to sense changes
in their environments and then respond to these changes in predetermined manners—
traits that are also found in living organisms. In addition, this “smart” concept is be-
ing extended to rather sophisticated systems that consist of both smart and tra-
ditional materials.
function). Actuators may be called upon to change shape, position, natural
frequency, or mechanical characteristics in response to changes in temperature,
electric fields, and/or magnetic fields.
Four types of materials are commonly used for actuators: shape memory alloys,
piezoelectric ceramics, magnetostrictive materials, and electrorheological/magne-
torheological fluids.Shape memory alloys are metals that, after having been deformed,
revert back to their original shapes when temperature is changed (see the Materi-
als of Importance piece following Section 10.9). Piezoelectric ceramics expand and
contract in response to an applied electric field (or voltage); conversely, they also
generate an electric field when their dimensions are altered (see Section 18.25).The
behavior of magnetostrictive materials is analogous to that of the piezoelectrics, ex-
cept that they are responsive to magnetic fields.Also, electrorheological and mag-
netorheological fluids are liquids that experience dramatic changes in viscosity upon
the application of electric and magnetic fields, respectively.
Materials/devices employed as sensors include optical fibers (Section 21.14),
piezoelectric materials (including some polymers), and microelectromechanical
devices (MEMS, Section 13.8).
For example, one type of smart system is used in helicopters to reduce aero-
dynamic cockpit noise that is created by the rotating rotor blades. Piezoelectric
sensors inserted into the blades monitor blade stresses and deformations; feedback
signals from these sensors are fed into a computer-controlled adaptive device,which
generates noise-canceling antinoise.
Nanoengineered Materials
Until very recent times the general procedure utilized by scientists to understand
the chemistry and physics of materials has been to begin by studying large and com-
plex structures, and then to investigate the fundamental building blocks of these
structures that are smaller and simpler. This approach is sometimes termed “top-
down” science. However, with the advent of scanning probe microscopes (Sec-
tion 4.10), which permit observation of individual atoms and molecules, it has be-
come possible to manipulate and move atoms and molecules to form new structures
and, thus, design new materials that are built from simple atomic-level constituents
(i.e., “materials by design”). This ability to carefully arrange atoms provides op-
portunities to develop mechanical, electrical, magnetic, and other properties that
are not otherwise possible.We call this the “bottom-up” approach, and the study of
the properties of these materials is termed “nanotechnology”; the “nano” prefix de-
notes that the dimensions of these structural entities are on the order of a nanome-
ter (10 9
m)—as a rule, less than 100 nanometers (equivalent to approximately 500
atom diameters).
One example of a material of this type is the carbon nanotube,
discussed in Section 12.4. In the future we will undoubtedly find that increasingly
more of our technological advances will utilize these nanoengineered materials.

Materials are probably more deep-seated in our culture than most of us realize.Transportation, housing, clothing, communication, recreation, and food production—virtually every segment of our everyday lives is influenced to one degree or anotherby materials.Historically, the development and advancement of societies have been intimately tied to the members’ ability to produce and manipulate materials to fill their needs. In fact, early civilizations have been designated by the level of their materials development (Stone Age, Bronze Age, Iron Age).


The earliest humans had access to only a very limited number of materials, those that occur naturally: stone, wood, clay, skins, and so on.With time they dis- covered techniques for producing materials that had properties superior to those of the natural ones; these new materials included pottery and various metals. Fur- thermore, it was discovered that the properties of a material could be altered by heat treatments and by the addition of other substances.At this point,materials uti- lization was totally a selection process that involved deciding from a given, rather limited set of materials the one best suited for an application by virtue of its char- acteristics. It was not until relatively recent times that scientists came to understand the relationships between the structural elements of materials and their properties.
This knowledge, acquired over approximately the past 100 years, has empowered
them to fashion, to a large degree, the characteristics of materials.Thus, tens of thousands of different materials have evolved with rather specialized characteristics that meet the needs of our modern and complex society; these include metals, plastics, glasses, and fibers.


The development of many technologies that make our existence so comfortable has been intimately associated with the accessibility of suitable materials. An advancement in the understanding of a material type is often the forerunner to the stepwise progression of a technology. For example, automobiles would not have been possible without the availability of inexpensive steel or some other comparable substitute. In our contemporary era, sophisticated electronic devices rely on components that are made from what are called semicon-
ducting materials.



The approximate dates for the beginnings of Stone, Bronze, and Iron Ages were 2.5 million
BC, 3500 BC and 1000 BC, respectively.

The number of the section in which a symbol is introduced or explained is given
in parentheses.

A = area
Å = angstrom unit
Ai = atomic weight of element
APF = atomic packing factor
a = lattice parameter: unit cell
x=axial length
a = crack length of a surface crack
at% = atom percent
B = magnetic flux density
Br = magnetic remanence
BCC = body-centered cubic crystal structure
b = lattice parameter: unit cell
y-axial length
b = Burgers vector
C = capacitance
Ci = concentration (composition) of component i in wt%
C= concentration (composition) of component i in at%
Cv, Cp = heat capacity at constant volume, pressure
CPR = corrosion penetration rate
CVN= Charpy V-notch
%CW = percent cold work
c = lattice parameter: unit cell
z-axial length
c = velocity of electromagnetic radiation in a vacuum
D = diffusion coefficient
D = dielectric displacement
DP = degree of polymerization
d = diameter
d = average grain diameter
dhkl = interplanar spacing for planes of Miller indices h, k, and l
E = energy
E = modulus of elasticity or Young’s modulus
Ef = Fermi energy
Eg =band gap energy
Er(t)= relaxation modulus
%EL = ductility, in percent elongation
e = electric charge per electron
e= electron
erf =Gaussian error function
exp= e, the base for natural logarithms
F = force, interatomic or mechanical,Faraday constant
FCC = face-centered cubic crystal structure
G = shear modulus
H = magnetic field strength
Hc =magnetic coercivity
G =shear modulus
H = magnetic field strength
Hc = magnetic coercivity
HB = Brinell hardness (
HCP = hexagonal close-packed crystal structure
HK = Knoop hardness
HRF = Rockwell hardness: B and F scales
HR15N, HR45W = superficial Rockwell hardness: 15N and 45W
HV = Vickers hardness
h =’s constant
(hkl) = Miller indices for a crystallographic plane
I = electric current
I = intensity of electromagnetic radiation
i = current density
iC = corrosion current density
J = diffusion flux
J = electric current density
Kc = fracture toughness
KIc = plane strain fracture toughness for mode I crack surface displacement
k = Boltzmann’s constant
k = thermal conductivity
l =length
lc = critical fiber length
ln =natural logarithm
log = logarithm taken to base 10
M = magnetization
N = number of fatigue cycles
NA = Avogadro’s number
Nf = fatigue life
n = principal quantum number
n = number of atoms per unit cell
n = strain-hardening exponent
n = number of electrons in an electrochemical reaction
n =number of conducting electrons per cubic meter
n =index of refraction
n= ceramics, the number of formula units per unit cell
ni = intrinsic carrier (electron and hole) concentration
P = dielectric polarization
P–B ratio =Pilling–Bedworth ratio
p = number of holes per cubic meter
Q = activation energy
Q = magnitude of charge stored
R = atomic radius
R =gas constant
%RA = ductility, in percent reduction in area
r = interatomic distance
r = reaction rate
rA, rC = anion and cation ionic radii
S = fatigue stress amplitude
SEM = scanning electron microscopy or microscope
T = temperature
Tc = Curie temperature
TC = superconducting critical temperature
Tg = glass transition temperature
Tm = melting temperature
TEM = transmission electron microscopy or microscope
TS = tensile strength
t = time
tr = rupture lifetime
Ur = modulus of resilience
[uvw] = indices for a crystallographic direction
V = electrical potential difference
VC= unit cell volume
VC = corrosion potential
VH = Hall voltage
Vi = volume fraction of phase i
v = velocity
vol% = volume percent
Wi = mass fraction of phase i
wt% =weight percent
x = length
x = space coordinate
Y = dimensionless parameter or function in fracture toughness expression
y = space coordinate
z = space coordinate
lattice parameter: unit cell y–z interaxial angle
phase designations
l = linear coefficient of thermal expansion

WHY STUDY MATERIALS SCIENCE
AND ENGINEERING?


Why do we study materials? Many an applied scientist or engineer, whether me-
chanical, civil, chemical, or electrical, will at one time or another be exposed to a
design problem involving materials. Examples might include a transmission gear,
the superstructure for a building, an oil refinery component, or an integrated circuit
chip. Of course, materials scientists and engineers are specialists who are totally
involved in the investigation and design of materials.
Many times, a materials problem is one of selecting the right material from the
many thousands that are available. There are several criteria on which the final
decision is normally based. First of all, the in-service conditions must be character-
ized, for these will dictate the properties required of the material. On only rare
occasions does a material possess the maximum or ideal combination of properties.
Thus, it may be necessary to trade off one characteristic for another.The classic ex-
ample involves strength and ductility; normally, a material having a high strength
will have only a limited ductility. In such cases a reasonable compromise between
two or more properties may be necessary.
A second selection consideration is any deterioration of material properties that
may occur during service operation. For example, significant reductions in mechanical
strength may result from exposure to elevated temperatures or corrosive environments.
Finally, probably the overriding consideration is that of economics:What will
the finished product cost? A material may be found that has the ideal set of prop-
erties but is prohibitively expensive. Here again, some compromise is inevitable.
The cost of a finished piece also includes any expense incurred during fabrication
to produce the desired shape.
The more familiar an engineer or scientist is with the various characteristics
and structure–property relationships, as well as processing techniques of materials,
the more proficient and confident he or she will be to make judicious materials
choices based on these criteria

MATERIALS SCIENCE AND ENGINEERING

Sometimes it is useful to subdivide the discipline of materials science and engi-
neering into materials science and materials engineering subdisciplines. Strictly
speaking, “materials science” involves investigating the relationships that exist
between the structures and properties of materials. In contrast, “materials engi-
neering” is, on the basis of these structure–property correlations, designing or en-
gineering the structure of a material to produce a predetermined set of properties.

From a functional perspective, the role of a materials scientist is to develop or syn-
thesize new materials, whereas a materials engineer is called upon to create new
products or systems using existing materials, and/or to develop techniques for pro-
cessing materials. Most graduates in materials programs are trained to be both
materials scientists and materials engineers.
“Structure” is at this point a nebulous term that deserves some explanation. In
brief, the structure of a material usually relates to the arrangement of its internal
components. Subatomic structure involves electrons within the individual atoms and
interactions with their nuclei. On an atomic level, structure encompasses the or-
ganization of atoms or molecules relative to one another.The next larger structural
realm, which contains large groups of atoms that are normally agglomerated to-
gether, is termed “microscopic,”meaning that which is subject to direct observation
using some type of microscope. Finally, structural elements that may be viewed with
the naked eye are termed “macroscopic.”
The notion of “property” deserves elaboration.While in service use, all mate-
rials are exposed to external stimuli that evoke some type of response. For exam-
ple, a specimen subjected to forces will experience deformation, or a polished metal
surface will reflect light.A property is a material trait in terms of the kind and mag-
nitude of response to a specific imposed stimulus. Generally, definitions of proper-
ties are made independent of material shape and size.
Virtually all important properties of solid materials may be grouped into six dif-
ferent categories: mechanical, electrical, thermal, magnetic, optical, and deteriorative.
For each there is a characteristic type of stimulus capable of provoking different re-
sponses.Mechanical properties relate deformation to an applied load or force; exam-
ples include elastic modulus and strength. For electrical properties, such as electrical
conductivity and dielectric constant, the stimulus is an electric field. The thermal be-
havior of solids can be represented in terms of heat capacity and thermal conductiv-
ity. Magnetic properties demonstrate the response of a material to the application of
a magnetic field. For optical properties, the stimulus is electromagnetic or light radia-
tion; index of refraction and reflectivity are representative optical properties. Finally,
deteriorative characteristics relate to the chemical reactivity of materials.The chapters
that follow discuss properties that fall within each of these six classifications.
In addition to structure and properties, two other important components are
involved in the science and engineering of materials—namely, “processing” and
“performance.”With regard to the relationships of these four components, the struc-
ture of a material will depend on how it is processed. Furthermore, a material’s per-
formance will be a function of its properties. Thus, the interrelationship between
processing, structure, properties, and performance is as depicted in the schematic
illustration shown in Figure 1.1. Throughout this text we draw attention to the

relationships among these four components in terms of the design, production, and
utilization of materials.
We now present an example of these


processing -structure -properties -performance


principles with Figure 1.2, a photograph showing three thin disk specimens placed
over some printed matter. It is obvious that the optical properties (i.e., the light
transmittance) of each of the three materials are different; the one on the left is trans-
parent (i.e., virtually all of the reflected light passes through it), whereas the disks in
the center and on the right are, respectively, translucent and opaque.All of these spec-
imens are of the same material, aluminum oxide, but the leftmost one is what we call
a single crystal—that is, it is highly perfect—which gives rise to its transparency. The
center one is composed of numerous and very small single crystals that are all con-
nected; the boundaries between these small crystals scatter a portion of the light re-
flected from the printed page,which makes this material optically translucent. Finally,
the specimen on the right is composed not only of many small, interconnected crys-
tals, but also of a large number of very small pores or void spaces. These pores also
effectively scatter the reflected light and render this material opaque.
Processing Structure Properties Performance
Figure 1.1 The four components of the discipline of materials science and
engineering and their interrelationship.

Materials science is an interdisciplinary field involving the properties of matter and its applications to various areas of science and engineering. This science investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It includes elements of applied physics and chemistry. With significant media attention focused on nanoscience and nanotechnology in recent years, materials science has been propelled to the forefront at many universities. It is also an important part of forensic engineering and failure analysis. Materials science also deals with fundamental properties and characteristics of materials.