1. THE NATURE OF MATERIALS
Atomic Structure and the Elements
Bonding between Atoms and Molecules
Crystalline Structures
Noncrystalline (Amorphous) Structures
Engineering Materials
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

2. 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
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

3. 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
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4. Periodic Table
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5. 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
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

6. Simple Model of Atomic Structure for Several Atoms
(a) Hydrogen, (b) helium, (c) fluorine, (d) neon, and (e) sodium
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

7. 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
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

8. Primary Bonds Ionic Covalent Metallic
Characterized by strong atom‑to‑atom attractions that involve exchange of valence electrons
Types:
Ionic
Covalent
Metallic
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9. 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
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10. Covalent Bonding
Electrons are shared (as opposed to transferred) between atoms in their outermost shells to achieve a stable set of eight
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11. Two Examples of Covalent Bonding
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12. Metallic Bonding
Sharing of outer shell electrons by all atoms to form a general electron cloud that permeates the entire block
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13. 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:
Dipole forces
London forces
Hydrogen bonding
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

14. 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
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

15. 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
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

16. 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
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

17. 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
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

18. 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
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

19. Three Crystal Structures in Metals
(a) Body-centered cubic, (b) face-centered cubic, and (c) hexagonal close-packed
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20. 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
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21. 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
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22. 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).
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23. Line Defects Edge dislocation Screw dislocation
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
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24. Edge Dislocation
Edge of an extra plane of atoms that exists in the lattice
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25. Screw Dislocation
Spiral within the lattice structure wrapped around an imperfection line, like a screw is wrapped around its axis
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26. 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
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27. 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
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28. 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“
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29. 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
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30. 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
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31. 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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32. Twinning After plastic deformation
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33. 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
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

34. 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 is a transition zone, perhaps only a few atoms thick
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

35. 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
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

36. Features of Noncrystalline Structures
Two features differentiate noncrystalline (amorphous) from crystalline materials:
Absence of long‑range order in molecular structure
Differences in melting and thermal expansion characteristics
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

37. 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
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38. Volumetric Effects
Characteristic change in volume for a pure metal (a crystalline structure), compared to same volumetric changes in glass (a noncrystalline structure)
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39. 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
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e

40. 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
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41. 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, low thermal conductivity, strength and stiffness vary widely
©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e