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THE NATURE OF MATERIALS. Manufacturing Processes, 1311 Dr Simin Nasseri Southern Polytechnic State University. THE NATURE OF MATERIALS. Atomic Structure and the Elements Bonding between Atoms and Molecules Crystalline Structures Noncrystalline (Amorphous) Structures.

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THE NATURE OF MATERIALS


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the nature of materials

THE NATURE OF MATERIALS

Manufacturing Processes, 1311

Dr Simin Nasseri

Southern Polytechnic State University

the nature of materials2
THE NATURE OF MATERIALS
  • Atomic Structure and the Elements
  • Bonding between Atoms and Molecules
  • Crystalline Structures
  • Noncrystalline (Amorphous) Structures
importance of materials in manufacturing
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
atomic structure and the elements5
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
element groupings
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
slide7

Periodic Table

Figure 2.1 Periodic Table of Elements. Atomic number and symbol are listed for the 103 elements.

question

Copper

Silver

Gold

Platinum (Pt), Palladium (Pd)

Noble metals (precious metals) are metals that are resistant to corrosion or oxidation, unlike most base metals.

Question?

What are the noble metals?

bonding between atoms and molecules10
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
bonding between atoms and molecules11
Bonding between Atoms and Molecules

Primary

Bonding

Secondary

Bonding

Ionic

Covalent

Metallic

Dipole forces

London forces

Hydrogen bonding

primary bonds

The ones on the outer shell

Primary Bonds

Characterized by strong atom‑to‑atom attractions that involve exchange of valence electrons

  • Following forms:
    • Ionic
    • Covalent
    • Metallic
ionic bonding
Figure 2.4 First form of primary bonding: (a) Ionic

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.

Ionic Bonding
  • Properties:
    • Poor Ductility
    • Low Electrical Conductivity
  • Example: Sodium Chloride (NaCl)
covalent bonding
Figure 2.4 Second form of primary bonding: (b) covalent

Outer electrons are shared between two local atoms of different elements.

Covalent Bonding
  • Properties:
    • High Hardness
    • Low Electrical Conductivity
  • Examples: Diamond, Graphite
metallic bonding
Figure 2.4 Third form of primary bonding: (c) metallic

Outer shell electrons are shared by all atoms to form an electron cloud.

Metallic Bonding
  • Properties:
    • - Good Conductor (Heat and Electricity)
    • - Good Ductility
secondary bonds
Secondary Bonds

Secondary bonds involve attraction forces between molecules(whereas primary bonds involve atom‑to‑atom attractive forces),

  • No transfer or sharing of electrons in secondary bonding
  • Bonds are weaker than primary bonds
  • Three forms:
    • Dipole forces
    • London forces
    • Hydrogen bonding
macroscopic structures of matter
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
crystalline structure
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 somepolymers
crystallinity
Crystallinity

When the monomers are arranged in a neat orderly manner, the polymer is crystalline.Polymers are just like socks. Sometimes they are arranged in a neat orderly manner.

An amorphous solid is a solid in which the molecules have no order or arrangement.Some people will just throw their socks in the drawer in one big tangled mess. Their sock drawers look like this:

question21
Question?

What about glass?! Does glass have a crystalline structure?!

"What is glass... is it a liquid or a solid?"

  • Antique windowpanes are thicker at the bottom, because glass has flowed to the bottom over time!
  • Glass has no crystalline structure, hence it is NOT a solid.
  • Glass is a supercooled liquid.
  • Glass is a liquid that flows very slowly.
  • Glass is a highly viscous liquid!!
three crystal structures in metals

# of atoms: 14

# of atoms: 17

# of atoms in unit cell: 9

Three Crystal Structures in Metals
  • Body-centered cubic (BCC)
  • Face centered cubic (FCC)
  • Hexagonal close-packed (HCP)

Figure 2.8 Three types of crystal structure in metals.

crystal structures for common metals
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, (Iron at 1670oF)
  • Hexagonal close‑packed (HCP)
    • Magnesium, Titanium, Zinc
imperfections defects in crystals
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
  • It is in fact: Deviation in the regular pattern of the crystalline lattice structure.

Studying about imperfections is important:

Imperfection is bad: a perfect diamond (with no flaws) is more valuable than one containing imperfections.

Imperfection is good: the addition of an alloying ingredient in a metal to increase its strength (this is an imperfection which is introduced purposely).

types of defects or imperfections
Types of defects or imperfections
  • Point defects,
  • Line defects,
  • Surface defects.
point defects
Point Defects

Imperfections in crystal structure involving either a single atom or a few number of atoms

Dislocation of an atom

Extra atom present

Figure 2.9 Point defects: (a) vacancy, (b) ion‑pair vacancy (Schottky), (c) interstitialcy, (d) displaced ion (Frenkel Defect).

line defects
Line Defects

Defect happens along a line ( 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
edge dislocation
Figure 2.10 Line defects: (a) edge dislocation

Edge of an extra plane of atoms that exists in the lattice

Edge Dislocation
screw dislocation
Figure 2.10 Line defects: (b) screw dislocation

Spiral within the lattice structure wrapped around an imperfection line, like a screw is wrapped around its axis

Screw Dislocation
surface defects
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
elastic strain32
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.

plastic strain
Plastic Strain

If stress is higher than forces holding atoms in their lattice positions, a permanent shape change occurs

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“.

effect of dislocations on strain
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.
  • Slip involves the relative movement of atoms on the opposite sides of a plane in the lattice, called slip plane.

Figure 2.12 Effect of dislocations in the lattice structure under stress.

slip on a macroscopic scale
Slip on a Macroscopic Scale
  • When a lattice structure with an edge dislocation is subjected to a shear stress, the material deforms much more readily than in a perfect structure.
  • 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
twinning
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

Figure 2.13 Twinning, involving the formation of an atomic mirror image on the opposite side of the twinning plane: (a) before, and (b) after twinning.

polycrystalline nature of metals
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
crystalline structure38
Crystalline Structure
  • 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

Grain

Grain

boundary

Growth of crystals in metals

noncrystalline amorphous structures40
Noncrystalline (Amorphous) Structures
  • Many materials are noncrystalline
    • Water and air have noncrystalline structures
    • A metal loses its crystalline structure when melted
  • Some important engineering materials have noncrystalline forms in their solid state:
    • Glass
    • Many plastics
    • Rubber
features of noncrystalline structures
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

What are the differences between them?

crystalline versus noncrystalline
Crystalline versus Noncrystalline

Figure 2.14 Difference in structure between: (a) crystalline and (b) noncrystalline materials.

The crystal structure is regular, repeating, and denser

The noncrystalline structure is random and less tightly packed.

solidification
Solidification

Alloy Metal

Pure Metal

volumetric effects
Volumetric Effects

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).

Tg=glass temperature

Tm=melting temperature

summary characteristics of metals
Summary: Characteristics of Metals
  • Crystalline structuresin 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
summary characteristics of ceramics
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

?

Refractory materials retain their strength at high temperatures. They are used to make crucibles and linings for furnaces, kilns and incinerators.

summary characteristics of polymers
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