Transcript the structure of metals ppt - Ivy Tech -

Chapter 1 Manufacturing and
Engineering Technology
*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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1.
Ionic bonds-one or more electrons from an outer orbit are
transferred from one material to another (example Na+ and Clform salt)
2.
Covalent bonds- electrons in outer orbits are shared by atoms to
form molecules (H20 water). Typically low conductivity and high
hardness
3.
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.
4.
Van Der Waals forces are weak attractions occurring between
molecules.
*The crystal structure of metals- when metals
solidify from a molten state, the atoms
arrange themselves into various orderly
configurations called CRYSTALS.
1.Body-centered cubic (BCC) least dense
2.Face-centered cubic (FCC) more dense
3.Hexagonal close-packet (HCP) most dense
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minimize
At different temperatures the same
metal may form different structures
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- the appearance of more
than one type of crystal
structure
*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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* 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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*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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Dislocations-defects in the orderly
arrangement of a metal’s 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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*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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*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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• 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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* 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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*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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*Annealing – heating metal to a
specific temperature range for
a given period of time
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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.
o Decrease density of dislocations
o Lowers strength
o Raises ductility
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*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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*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
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