Chapter 1 Manufacturing and Engineering Technology

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The Structure of Metals

• Chapter 1 Manufacturing and Engineering Technology

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Atomic structure-arrangement of atoms within the
metals

• All matter is made up of atoms containing a
  nucleus of protons and neutrons and surrounding
  clouds, or orbits, of electrons.
• Atoms can transfer or share electrons in doing
  so, multiple atoms combine to form molecules.
  Molecules are held together by attractive forces
  called bonds

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FIGURE 1.1 An outline of the topics described
in Chapter 1.
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Types of Atomic Bonds

• Ionic bonds-one or more electrons from an outer
  orbit are transferred from one material to
  another (example Na and Cl- form salt)
• Covalent bonds- electrons in outer orbits are
  shared by atoms to form molecules (H20 water).
  Typically low conductivity and high hardness
• Metallic bonds-available electrons are shared by
  all atoms in contact. The resultant electron
  cloud provides attractive forces to hold the
  atoms together and results in generally high
  thermal and electrical conductivity.
• Van Der Waals forces are weak attractions
  occurring between molecules.

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CRYSTAL STRUCTURE

• The crystal structure of metals- when metals
  solidify from a molten state, the atoms arrange
  themselves into various orderly configurations
  called CRYSTALS.
• Body-centered cubic (BCC) least dense
• Face-centered cubic (FCC) more dense
• Hexagonal close-packet (HCP) most dense

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FIGURE 1.2 The body-centered cubic (bcc)
crystal structure (a) hard-ball model (b) unit
cell and (c) single crystal with many unit cells.
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FIGURE 1.3 The face-centered cubic (fcc)
crystal structure (a) hard-ball model (b) unit
cell and (c) single crystal with many unit cells.
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FIGURE 1.4 The hexagonal close-packed (hcp)
crystal structure (a) unit cell and (b) single
crystal with many unit cells.
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The reason that metals form different crystal
structures is to minimize the energy required to
fill space

• At different temperatures the same metal may form
  different structures

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Allotropism or polymorphism (MEANING MANY
SHAPES)

• - the appearance of more than one type of crystal
  structure

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Deformation Strength of Single Crystals

• Elastic deformation- a single crystal is subject
  to an external force, but returns to its original
  shape when the force is removed
• Plastic deformation-a permanent deformation when
  the crystal does not return to its original shape

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Two Basic Mechanisms for Plastic Deformations

• Slipping of one plane of atoms over another
  adjacent plane (slip plane) under shear stress
• Twinning- the second and less common mechanism of
  plastic deformation where a portion of the
  crystal forms a mirror image of itself across the
  plane of twinning
• Definition Anisotropy-a single crystal exhibits
  different properties when tested in different
  directions (ex. Woven cloth, plywood)

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FIGURE 1.5 Permanent deformation of a single
crystal under a tensile load. The highlighted
grid of atoms emphasizes the motion that occurs
within the lattice. (a) Deformation by slip. The
b/a ratio influences the magnitude of the shear
stress required to cause slip. (b) Deformation by
twinning, involving the generation of a twin
around a line of symmetry subjected to shear.
Note that the tensile load results in a shear
stress in the plane illustrated.
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Imperfections in the crystal structure of metals
explains why actual strength levels are one or
two orders of magnitude lower than the
theoretical calculations

• Point defects-vacancy, missing atoms,
  interstitial atom extra atom in the lattice or
  impurity foreign atom that has replaced the atom
  of pure metal
• Linear defections called dislocations
• Planar imperfections such as grain boundaries and
  phase boundaries
• Volume or bulk imperfections-voids, inclusions,
  other phases, cracks

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FIGURE 1.7 Schematic illustration of types of
defects in a single-crystal lattice
selfinterstitial, vacancy, interstitial, and
substitutional.
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Dislocations-defects in the orderly arrangement
of a metals atomic structure. Because a slip
plane containing a dislocation requires less
shear stress to allow slip than does a plane in a
perfect lattice, dislocations are the most
significant defects that explain the discrepancy
between the actual and theoretical strengths of
metals.
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FIGURE 1.8 Types of dislocations in a single
crystal (a) edge dislocation and (b) screw
dislocation.
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FIGURE 1.9 Movement of an edge dislocation
across the crystal lattice under a shear stress.
Dislocations help explain why the actual strength
of metals is much lower than that predicted by
theory.
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Work Hardening (Strain Hardening)

• Dislocations can become entangled and interfere
  with each other and be impeded by barriers such
  as grain boundaries, impurities, and inclusions
  in the material. The increased shear stress
  required to overcome entanglements and
  impediments results in an increase in overall
  strength and hardness of the metal and is known
  as work hardening or strain hardening. (Ex. Cold
  rolling, forging, drawing)

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Grains and Grain Boundaries

• When molten metal solidifies, crystals begin for
  form independently of each other. They have
  random and unrelated orientations. Each of these
  crystals grows into a crystalline structure or
  GRAIN.
• The number and size of the grains developed in a
  unit volume of the metal depends on the rate at
  which NUCLEATION (the initial stage of crystal
  formation) takes place

Is this what I mean by grain?
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FIGURE 1.10 Schematic illustration of the
stages during the solidification of molten metal
each small square represents a unit cell. (a)
Nucleation of crystals at random sites in the
molten metal note that the crystallographic
orientation of each site is different. (b) and
(c) Growth of crystals as solidification
continues. (d) Solidified metal, showing
individual grains and grain boundaries note the
different angles at which neighboring grains meet
each other.
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• Rapid cooling smaller grains
• Slow cooling larger grains
• Grain boundaries the surfaces that separate
  individual grains
• Grain size- at room temperature a large grain
  size is generally associated with low strength,
  low hardness, and low ductility (ductility is a
  solid material's ability to deform under tensile
  stress)
• Grain size is measured by counting the number of
  grains in a given area or by counting the number
  of grains that intersect a length of line
  randomly drawn on an enlarged photograph of the
  grains

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TABLE 1.1 Grain Sizes
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Plastic deformation of polycrystalline metals

• Cold working a polycrystalline metal with
  uniform equiaxed grains is subject to plastic
  deformation at room temperature.
• The grains become deformed and elongated.
• The deformed metal exhibits higher strength
  because of the entanglement of dislocations with
  grain boundaries and with each other.
• The higher the deformation, the stronger the
  metal becomes.
• Strength is higher for metals with small grains
  because they have larger grain-boundary surface
  area per unit volume of metal hence more
  entanglements of dislocations

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FIGURE 1.11 Plastic deformation of idealized
(equiaxed) grains in a specimen subjected to
compression (such as occurs in the forging or
rolling of metals) (a) before deformation and
(b) after deformation. Note the alignment of
grain boundaries along a horizontal direction
this effect is known as preferred orientation.
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ANISOTROPY (texture)

• Metal properties are different in the vertical
  direction from those in the horizontal direction
• It influences both mechanical and physical
  properties of metals

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FIGURE 1.12 (a) Schematic illustration of a
crack in sheet metal that has been subjected to
bulging (caused, for example, by pushing a steel
ball against the sheet). Note the orientation of
the crack with respect to the rolling direction
of the sheet this sheet is anisotropic. (b)
Aluminum sheet with a crack (vertical dark line
at the center) developed in a bulge test the
rolling direction of the sheet was vertical.
Courtesy J.S. Kallend, Illinois Institute of
Technology.
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Recovery- stresses in the highly deformed regions
of the metal piece are relieved. Subgrain
boundaries begin to form

• Annealing heating metal to a specific
  temperature range for a given period of time

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Recrystallization

• New equiaxed and strain-free grains are formed
  replacing the older grains. Between .3Tm and
  .5Tm where Tm is melting point of the metal on
  the absolute scale. Recrystallization
  temperature is defined as the temperature at
  which complete recrystallization occurs in
  approximately one hour.
• Decrease density of dislocations
• Lowers strength
• Raises ductility

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Grain growth

• temperature of metal increases further, the grain
  size grows and the size may exceed the original
  grain size

We grow lots of grain in Indiana, but this is not
what is meant by grain growth
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FIGURE 1.13 Schematic illustration of the
effects of recovery, recrystallization, and grain
growth on mechanical properties and on the shape
and size of grains. Note the formation of small
new grains during recrystallization. Source
After G. Sachs.
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TABLE 1.2 Homologous Temperature Ranges for
Various Processes
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Note Deforming lead at room temperature is hot
workingsince the recrystallization temperature
of lead is about room temperature

• Cold working- plastic deformation at room
  temperature
• Hot working deformation occurs above the
  recrystallization temperature
• Warm working is carried out at intermediate
  temperatures, thus it is a compromise between
  cold working and hot working