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Transcript of THE NATURE OF MATERIALS Manufacturing Processes, 1311 Dr Simin Nasseri Southern Polytechnic State...
THE NATURE OF MATERIALS
Manufacturing Processes, 1311
Dr Simin Nasseri
Southern Polytechnic State University
Manufacturing ProcessesProf Simin Nasseri
THE NATURE OF MATERIALS
1. Atomic Structure and the Elements
2. Bonding between Atoms and Molecules
3. Crystalline Structures
4. Noncrystalline (Amorphous) Structures
Manufacturing ProcessesProf Simin Nasseri
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 Elements
Manufacturing ProcessesProf Simin Nasseri
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
Manufacturing ProcessesProf Simin Nasseri
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 tableNonmetals are on rightBetween them is a transition zone containing metalloids or semi‑metals
Metals Metalloids or Semimetals
NonMetals
Beryllium – Be Boron – B Helium – He
Lithium – Li Silicon – Si Neon – Ne
Magnesium – Mg Arsenic – As Argon – Ar
Cadmium – Cd Antimony – Sb Krypton – Kr
Copper- Cu Polonium - Po Xenon – Xe
Iron – Fe Tellurium - Te Radon – Rn
Zinc – Zn Germanium - Ge Fluorine – F
Titanium – Ti Chlorine – Cl
Gold – Au Oxygen – O
Manufacturing ProcessesProf Simin Nasseri
Figure 2.1 Periodic Table of Elements. Atomic number and symbol are listed for the 103 elements.
Periodic Table
Manufacturing ProcessesProf Simin Nasseri
Question?What are the noble metals?
CopperSilverGold
Noble metals (precious metals) are metals that are resistant to corrosion or oxidation, unlike most base metals.
Platinum (Pt), Palladium (Pd)
Bonding between Atoms and Molecules
Manufacturing ProcessesProf Simin Nasseri
Bonding between Atoms and Molecules
Atoms are held together in molecules by various types of bonds1. 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
Manufacturing ProcessesProf Simin Nasseri
Bonding between Atoms and Molecules
Primary Bonding
SecondaryBonding
IonicCovalentMetallic
Dipole forcesLondon forcesHydrogen bonding
Manufacturing ProcessesProf Simin Nasseri
Primary Bonds
Characterized by strong atom‑to‑atom attractions that involve exchange of valence electrons
Following forms: Ionic Covalent Metallic
The ones on the outer shell
Manufacturing ProcessesProf Simin Nasseri
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.
Properties:
Poor Ductility
Low Electrical Conductivity
Example: Sodium Chloride (NaCl)
Manufacturing ProcessesProf Simin Nasseri
Covalent Bonding
Figure 2.4 Second form of primary bonding: (b) covalent
Outer electrons are shared between two local atoms of different elements.
Properties:
High Hardness
Low Electrical Conductivity
Examples: Diamond, Graphite
Manufacturing ProcessesProf Simin Nasseri
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.
Properties:- Good Conductor (Heat and Electricity)- Good Ductility
Manufacturing ProcessesProf Simin Nasseri
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:
1. Dipole forces
2. London forces
3. Hydrogen bonding
Manufacturing ProcessesProf Simin Nasseri
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 Structures
Manufacturing ProcessesProf Simin Nasseri
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
Manufacturing ProcessesProf Simin Nasseri
CrystallinityWhen 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:
Manufacturing ProcessesProf Simin Nasseri
Question?
"What is glass... is it a liquid or a solid?"
What about glass?! Does glass
have a crystalline structure?!
• 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!!
Manufacturing ProcessesProf Simin Nasseri
Three Crystal Structures in Metals
1. Body-centered cubic (BCC)
2. Face centered cubic (FCC)
3. Hexagonal close-packed (HCP)
Figure 2.8 Three types of crystal structure in metals.
# of atoms in unit cell: 9 # of atoms: 14 # of atoms: 17
Manufacturing ProcessesProf Simin Nasseri
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
Manufacturing ProcessesProf Simin Nasseri
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).
Manufacturing ProcessesProf Simin Nasseri
Types of defects or imperfections
Point defects, Line defects, Surface defects.
Manufacturing ProcessesProf Simin Nasseri
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 (Schottky), (c) interstitialcy, (d) displaced ion (Frenkel Defect).
Extra atom present
Dislocation of an atom
Manufacturing ProcessesProf Simin Nasseri
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
Manufacturing ProcessesProf Simin Nasseri
Edge Dislocation
Figure 2.10 Line defects: (a) edge dislocation
Edge of an extra plane of atoms that exists in the lattice
Manufacturing ProcessesProf Simin Nasseri
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
Manufacturing ProcessesProf Simin Nasseri
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
Manufacturing ProcessesProf Simin Nasseri
Elastic Strain
Manufacturing ProcessesProf Simin Nasseri
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.
Manufacturing ProcessesProf Simin Nasseri
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“.
Manufacturing ProcessesProf Simin Nasseri
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.
Manufacturing ProcessesProf Simin Nasseri
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
Manufacturing ProcessesProf Simin Nasseri
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.
Manufacturing ProcessesProf Simin Nasseri
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
Manufacturing ProcessesProf Simin Nasseri
Crystalline Structure
Growth of crystals in metals
Grain
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
Grainboundary
Noncrystalline (Amorphous) Structures
Manufacturing ProcessesProf Simin Nasseri
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
Manufacturing ProcessesProf Simin Nasseri
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
What are the differences
between them?
Manufacturing ProcessesProf Simin Nasseri
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.
Manufacturing ProcessesProf Simin Nasseri
Solidification
Alloy Metal Pure Metal
Manufacturing ProcessesProf Simin Nasseri
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 temperatureTm=melting temperature
Manufacturing ProcessesProf Simin Nasseri
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
Manufacturing ProcessesProf Simin Nasseri
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.
?
Manufacturing ProcessesProf Simin Nasseri
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