Lecture 2

Report
THE NATURE OF MATERIALS
1. Atomic Structure and the Elements
2. Bonding between Atoms and Molecules
3. Crystalline Structures
4. Noncrystalline (Amorphous) Structures
5. Engineering Materials
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Importance of Materials in
Manufacturing
 Manufacturing is a transformation process
 It is the material that is transformed
 And it is the behavior of the material when
subjected to the forces, temperatures, and other
parameters of the process that determines the
success of the operation
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Element Groupings
 The elements can be grouped into families and
relationships established between and within the
families by means of the Periodic Table
 Metals occupy the left and center portions of the
table
 Nonmetals are on right
 Between them is a transition zone containing
metalloids or semi-metals
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Periodic Table
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Atomic Structure and the
Elements
 The basic structural unit of matter is the atom
 Each atom is composed of a positively charged
nucleus, surrounded by a sufficient number of
negatively charged electrons so the charges are
balanced
 More than 100 elements, and they are the
chemical building blocks of all matter
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Simple Model of Atomic
Structure for Several Atoms
 (a) Hydrogen, (b) helium, (c) fluorine, (d) neon, and
(e) sodium
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Bonding between Atoms and
Molecules

Atoms are held together in molecules by various
types of bonds
1. Primary bonds - generally associated with
formation of molecules
2. Secondary bonds - generally associated with
attraction between molecules

Primary bonds are much stronger than secondary
bonds
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Primary Bonds
 Characterized by strong atom-to-atom attractions that
involve exchange of valence electrons
 Following forms:
 Ionic
 Covalent
 Metallic
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Ionic Bonding
 Atoms of one element
give up their outer
electron(s), which are in
turn attracted to atoms of
some other element to
increase electron count
in the outermost shell to
eight
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Covalent Bonding
 Electrons are shared
(as opposed to
transferred) between
atoms in their outermost
shells to achieve a
stable set of eight
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Two Examples of
Covalent Bonding
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Metallic Bonding
 Sharing of outer shell
electrons by all atoms to
form a general electron
cloud that permeates the
entire block
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Secondary Bonds
Whereas primary bonds involve atom-to-atom attractive
forces, secondary bonds involve attraction forces
between molecules
 No transfer or sharing of electrons
 Bonds are weaker than primary bonds
 Three forms:
1. Dipole forces
2. London forces
3. Hydrogen bonding
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Dipole Forces
 Arise in a molecule comprised of two atoms with
equal and opposite electrical charges
 Each molecule therefore forms a dipole that attracts
other molecules
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
London Forces
 Attractive force between non-polar molecules, i.e.,
atoms in molecule do not form dipoles
 However, due to rapid motion of electrons in orbit,
temporary dipoles form when more electrons are on
one side
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Hydrogen Bonding
 Occurs in molecules containing hydrogen atoms
covalently bonded to another atom (e.g., H2O)
 Since electrons to complete shell of hydrogen atom
are aligned on one side of nucleus, opposite side has
a net positive charge that attracts electrons in other
molecules
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Macroscopic Structures of Matter
 Atoms and molecules are the building blocks of a
more macroscopic structure of matter
 When materials solidify from the molten state, they
tend to close ranks and pack tightly, arranging
themselves into one of two structures:
 Crystalline
 Noncrystalline
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Crystalline Structure
Structure in which atoms are located at regular and
recurring positions in three dimensions
 Unit cell - basic geometric grouping of atoms that is
repeated
 The pattern may be replicated millions of times within
a given crystal
 Characteristic structure of virtually all metals, as well
as many ceramics and some polymers
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Three Crystal Structures in
Metals
 Three types of crystal structure: (a) body-centered
cubic, (b) face-centered cubic, and (c) hexagonal
close-packed
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Crystal Structures for Common
Metals
 Room temperature crystal structures for some of the
common metals:
 Body-centered cubic (BCC)
 Chromium, Iron, Molybdenum, Tungsten
 Face-centered cubic (FCC)
 Aluminum, Copper, Gold, Lead, Silver, Nickel
 Hexagonal close-packed (HCP)
 Magnesium, Titanium, Zinc
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Imperfections (Defects) in
Crystals

Imperfections often arise due to inability of solidifying
material to continue replication of unit cell, e.g., grain
boundaries in metals

Imperfections can also be introduced purposely; e.g.,
addition of alloying ingredient in metal

Types of defects: (1) point defects, (2) line defects, (3)
surface defects
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Point Defects
Imperfections in crystal structure involving either a
single atom or a small number of atoms
Point defects: (a) vacancy, (b) ion-pair vacancy, (c) interstitialcy,
(d) displaced ion (Frenkel Defect).
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Line Defects
Connected group of point defects that forms a line in the
lattice structure
 Most important line defect is a dislocation, which can
take two forms:
 Edge dislocation
 Screw dislocation
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Edge Dislocation
Edge of an extra plane of atoms that exists in the
lattice
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Screw Dislocation
Spiral within the lattice
structure wrapped
around an
imperfection line,
like a screw is
wrapped around its
axis
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Surface Defects
Imperfections that extend in two directions to form a
boundary
 Examples:
 External: the surface of a crystalline object is
an interruption in the lattice structure
 Internal: grain boundaries are internal surface
interruptions
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Elastic Strain
 When a crystal experiences a gradually increasing
stress, it first deforms elastically
Deformation of a crystal structure: (a) original lattice: (b) elastic
deformation, no permanent change in positions of atoms
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Plastic Strain
 If the stress is higher
than forces holding
atoms in their lattice
positions, then a
permanent shape
change occurs
Plastic deformation (slip), in which atoms in the crystal lattice
structure are forced to move to new "homes“
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Effect of Dislocations on
Strain
 In the series of diagrams, the movement of the
dislocation allows deformation to occur under a lower
stress than in a perfect lattice
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Slip on a Macroscopic Scale
 Slip occurs many times over throughout the metal when
subjected to a deforming load, thus causing it to exhibit
its macroscopic behavior in the stress-strain relationship
 Dislocations are a good-news-bad-news situation
 Good news in manufacturing – the metal is easier to
form
 Bad news in design – the metal is not as strong as
the designer would like
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Twinning
 A second mechanism
of plastic deformation
in which atoms on one
side of a plane (the
twinning plane) are
shifted to form a mirror
image of the other side
 Before
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Twinning
 After
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Polycrystalline Nature of Metals
 A block of metal may contain millions of individual
crystals, called grains
 Such a structure is called polycrystalline
 Each grain has its own unique lattice orientation
 But collectively, the grains are randomly oriented in
the block
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Grains and Grain Boundaries in
Metals
 How do polycrystalline structures form?
 As a volume of metal cools from the molten state and
begins to solidify, individual crystals nucleate at
random positions and orientations throughout the
liquid
 These crystals grow and finally interfere with each
other, forming at their interface a surface defect - a
grain boundary, which are transition zones, perhaps
only a few atoms thick
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Noncrystalline (Amorphous)
Structures
 Water and air have noncrystalline structures
 A metal loses its crystalline structure when melted
 Some engineering materials have noncrystalline forms
in their solid state
 Glass
 Many plastics
 Rubber
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Features of Noncrystalline
Structures

Two features differentiate noncrystalline (amorphous)
from crystalline materials:
1. Absence of long-range order in molecular
structure
2. Differences in melting and thermal expansion
characteristics
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Crystalline versus Noncrystalline
Structures of Materials
 Difference in structure between: (a) crystalline and
(b) noncrystalline materials
 Crystal structure is regular, repeating; noncrystalline
structure is less tightly packed and random
(a)
(b)
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Volumetric Effects
 Characteristic change in
volume for a pure metal
(a crystalline structure),
compared to same
volumetric changes in
glass (a noncrystalline
structure)
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Summary: Characteristics of
Metals
 Crystalline structures in the solid state, almost
without exception
 BCC, FCC, or HCP unit cells
 Atoms held together by metallic bonding
 Properties: high strength and hardness, high
electrical and thermal conductivity
 FCC metals are generally ductile
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Summary: Characteristics of
Ceramics
 Most ceramics have crystalline structures, while glass
(SiO2) is amorphous
 Molecules characterized by ionic or covalent bonding,
or both
 Properties: high hardness and stiffness, electrically
insulating, refractory, and chemically inert
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Summary: Characteristics of
Polymers
 Many repeating mers in molecule held together by
covalent bonding
 Polymers usually carbon plus one or more other
elements: H, N, O, and Cl
 Amorphous (glassy) structure or mixture of
amorphous and crystalline
 Properties: low density, high electrical resistivity, and
low thermal conductivity, strength and stiffness vary
widely
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version

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