THE NATURE OF MATERIALS

The nature of materials

1. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
THE NATURE OF MATERIALS
•Atomic Structure and the Elements
•Bonding between Atoms and Molecules
•Crystalline Structures
•Noncrystalline (Amorphous) Structures
•Engineering Materials

2. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Why Materials are Important 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

3. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
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

4. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
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
‑

5. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Figure 2.1 Periodic Table of Elements. The atomic number and
‑
symbol are listed for the 103 elements

6. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Bonding between Atoms and Molecules
•Atoms are held together in molecules by various
types of bonds
-Primary bonds - generally associated with
formation of molecules
-Secondary bonds - generally associated with
attraction between molecules
•Primary bonds are much stronger than secondary
bonds

7. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Primary Bonds
•Characterized by strong atom to atom attractions that
‑ ‑
involve exchange of valence electrons
•Following forms:
-Ionic
-Covalent
-Metallic

8. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
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
Figure 2.4 Three
‑
forms of primary
bonding: (a) ionic

9. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Covalent Bonding
•Electrons are shared (as opposed to transferred)
between atoms in their outermost shells to achieve a
stable set of eight
Figure 2.4 Primary
‑
bonding: (b) covalent

10. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Metallic Bonding
• Sharing of outer shell
electrons by all atoms to
form a general electron
cloud that permeates the
entire block
Figure 2.4 Primary bonding:
‑
(c) metallic

11. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Secondary Bonds
Whereas primary bonds involve atom to atom attractive
‑ ‑
forces, secondary bonds involve attraction forces
between molecules
•No transfer or sharing of electrons in secondary
bonding, and bonds are weaker than primary bonds
•Three forms:
1.Dipole forces
2.London forces
3.Hydrogen bonding

12. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
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
Figure 2.6 Types of secondary bonding: (a) dipole forces
‑

13. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
London Forces
•Attractive force between nonpolar 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
Figure 2.6 Secondary bonding: (b) London forces
‑

14. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Hydrogen Bonding
•Occurs in molecules containing hydrogen atoms
covalently bonded to another atom (e.g., H
2O)
•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
Figure 2.6
‑
Secondary bonding:
(c) hydrogen
bonding

15. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Macroscopic Structures of Matter
•Atoms and molecules are the building blocks of 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

16. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Crystalline Structure
Structure in which the 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

17. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Figure 2.7 Body centered cubic (BCC) crystal structure:
‑ ‑
(a)unit cell, with atoms indicated as point locations in a
three dimensional axis system
‑
(b) unit cell model showing closely packed atoms
(sometimes called the hard ball model)
‑
(c) repeated pattern of the BCC structure

18. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Figure 2.8 Three types of crystal structures in metals:
‑
(a)body centered cubic
‑
(b) face centered cubic
‑
(c)hexagonal close packed
‑

19. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Crystal Structures for Common Metals
(at Room Temperature)
•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

20. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
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

21. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Point Defects
Imperfections in crystal structure involving either a
single atom or a few number of atoms
Figure 2.9 Point defects: (a) vacancy, (b) ion pair vacancy, (c)
‑ ‑
interstitialcy, (d) displaced ion (Frenkel Defect)

22. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
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

23. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Edge Dislocation
Edge of an extra plane of atoms that exists in the lattice
Figure 2.10 Line
‑
defects: (a) edge
dislocation

24. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Screw Dislocation
Spiral within the lattice structure wrapped around an
imperfection line, like a screw is wrapped around its axis
Figure 2.10 Line
‑
defects: (b) screw
dislocation

25. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
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

26. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Elastic Strain
•When a crystal experiences a gradually increasing
stress, it first deforms elastically
•If force is removed lattice structure returns to its
original shape
Figure 2.11
‑
Deformation of a crystal
structure: (a) original
lattice: (b) elastic
deformation, with no
permanent change in
positions of atoms

27. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Plastic Strain
•If stress is higher than forces holding atoms in their
lattice positions, a permanent shape change occurs
•Atoms have permanently moved from their previous
locations, and a new equilibrium lattice is formed
Figure 2.11
‑
Deformation of a crystal
structure: (c) plastic
deformation (slip), in
which atoms in the lattice
are forced to move to
new "homes"

28. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Figure 2.12 Effect of dislocations in the lattice structure under stress
‑
In the series of diagrams, the movement of the dislocation allows
deformation to occur under a lower stress than in a perfect lattice

29. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
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

30. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Twinning
A second mechanism of plastic deformation in which
atoms on one side of a plane (called the twinning plane)
are shifted to form a mirror image of the other side
Figure 2.13 Twinning, involving the formation of an atomic
‑
mirror image (i.e., a "twin") on the opposite side of the
twinning plane: (a) before, and (b) after twinning

31. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
The 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

32. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Grains and Grain Boundaries in Metals
•How do polycrystalline structures form?
-As a block (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
-Grain boundaries are transition zones, perhaps
only a few atoms thick

33. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Noncrystalline (Amorphous) Structures
•Many materials are noncrystalline
-Water and air have noncrystalline structures
-A metal loses its crystalline structure when melted
•Important engineering materials have noncrystalline
forms in their solid state
-Glass
-Many plastics
-Rubber

34. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Features of
Noncrystalline (Amorphous) Structures
•Two features differentiate noncrystalline from
crystalline materials:
1.Absence of long range order in molecular
‑
structure
2.Differences in melting and thermal expansion
characteristics

35. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Figure 2.14 Illustration of difference in structure between:
‑
(a) crystalline and (b) noncrystalline materials. The
crystal structure is regular, repeating, and denser; while
the noncrystalline structure is more loosely packed and
random

36. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Figure 2.15 Characteristic change in volume for a pure metal (a
‑
crystalline structure), compared to the same volumetric changes
in glass (a noncrystalline structure)

37. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
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

38. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
Characteristics of Ceramics
•Most ceramics have crystal structure, while glass
(SiO
2
) is amorphous
•Molecules characterized by ionic or covalent bonding,
or both
•Properties: high hardness and stiffness, electrically
insulating, refractory, and chemically inert

39. ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”
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