Defects in solids

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Defects in solids

• Scope know the different types of defect, and
  how these defects can affect on the materials
  properties
• Defect can be either desirable or undesirable.
• In general, a defect simply refers to a
  disruption in the crystalline order of an
  otherwise periodic material

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Types of defects in crystal structure
2.Line defect Dislocation
1.Point defect Vacancy atoms Interstitial
atoms Substitutional atoms

• 3.Area defect
• Grain boundaries

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Point defect

• Vacancy atoms
• Interstitial atoms
• Substitutional atoms

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vacancy defect

• The equilibrium number of vacancies ND depend on
• Temperature T
• Activation energy QD
• Total number of atomic sites

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Examplethe effect of temperature on the vacancy
concentration

• Calculate the concentration of vacancies in
  copper at room temperature (25oC). What
  temperature will be needed to heat treat copper
  such that the concentration of vacancies produced
  will be 1000 times more than the equilibrium
  concentration of vacancies at room temperature?
  Assume that 20,000 cal are required to produce a
  mole of vacancies in copper.

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Impurities

• impurity addition of an atom of a different
  species than the host or matrix
• Example Defect does not necessarily imply a bad
  thing
• addition of C to Fe to make steel
• addition of Cr to Fe for corrosion resistance
• Alloys other types of atoms are deliberately
  added to give the material certain properties
• Example alloy of silver and copper (silver is
  highly corrosion resistant but very soft
• Copper enhance the strength of the silver.

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Solid solution

• Solid solution is a particular type of alloy
• Two types substitutional and interstitial
• solvent the host material, usually the
  element or compound
• present in the greatest amount.
• solute the minor phase, added to the solvent.
  Usually the
• element or compound present in minor
  concentrations.
• phase is a region of uniform composition or
  crystal structure
• What would a solid solution look like?

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Factors affect on solid solution

• 1. Atomic size factor2. Crystal structure3.
  Electro negativity (they will form an
  inter-metallic compound)4. Valences (metal with
  high valence have a stronger tendency to dissolve
  than another metal

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Comparison between a substitutional
solidsolution and an intermetallic compound
Intermetallic compound (1)Specific
stoichiometry, (2) Crystal structure is such
that this stoichiometry is allowed (3)bonding is
partially metallic and partially covalent (or
ionic)
Solid solution (1) random placement of solute
atoms (2) metallic bonding
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LINE Defect- dislocation

• Dislocation is a linear or one-dimensional
  defect.
• Dislocations result from solidification from the
  melt, from mechanical work (e.g., rolling,
  drawing, compressive impact, tensile or shear
  stress), or from thermal stresses
• It is very difficult to prepare a
  dislocation-free crystal!!!
• Type of dislocation
• 1. edge dislocation a missing half plane of
  atoms
• 2. Screw dislocation layers twisted with respect
  to each other
• 3. A combination of the two Mixed

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Types of dislocation
Edge dislocation Misalignment of atomic planes
due to the extra half plane
screw dislocation Crystal is "cut halfway through
and then slide sideways helical path through
structure hence screw.
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Significance of dislocation

• Dislocation is the main reason for plastic
  deformation in materials.
• Plastic deformation is due to the motion of a
  large number of dislocations

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Slipping system

• Slip plane planes where dislocation move
• Slip directions Within the slip planes there are
  preferred crystallographic directions for
  dislocation movement
• Slip system The set of slip planes and
  directions constitute
• The slip planes and directions are those of
  highest packing density
• BCC and FCC crystals have more slip systems as
• compared to HCP,

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Slip in Single Crystals - Resolving the
AppliedStress onto the Slip System

• Resolved shear stress
• Critical resolved shear stress - The shear stress
  required to cause a dislocation to move and cause
  slip

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Critical resolved shear stressSchmids Law
Maximum value of (cosf cos?) corresponds to f ?
45o ? cosf cos? 0.5 ? sy 2tCRSS
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Slip in single crystal
Each step (shear band) result from the generation
of a large number of dislocations and their
propagation in the slip system with maximum
resolved shear stress.
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Polycrystalline Materials

• Most materials are polycrystalline and are made
  of many single crystals
• during solidification the crystal nucleate and
  grow from the liquid in a random orientation
• the grains impinge on each other when the
  solidification
• is complete
• junction of grains are grain
• boundaries

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3.Surface defect grain boundaries

• Grain boundaries
• are boundaries between crystals.
• are produced by the solidification process, for
  example. have a change in crystal orientation
  across them.
• impede dislocation motion.

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Plastic deformation

• Plastic deformation permeate deformation in
  materials
• Plastic deformation is due to the motion of a
  large number of dislocations.
• Plastic Deformation of Polycrystalline Materials

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Plastic Deformation of Polycrystalline Materials

• Slip directions vary from crystal to crystal ?
  Some grains are unfavorably oriented with respect
  to the applied stress (i.e. cosf cos? low)
• Even those grains for which cosf cos? is high
  may be limited in deformation by adjacent grains
  which cannot deform so easily
• Dislocations cannot easily cross grain
  boundaries because of changes in direction of
  slip plane and disorder at grain boundary
• As a result, polycrystalline metals are stronger
  than
• single crystals (the exception is the perfect
  single crystal without any defects, as in
  whiskers)

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SUMMARY
Point, Line, and Area defects arise in solids.
The number and type of defects can be varied
and controlled. Defects affect material
properties (e.g., grain boundaries control
crystal slip) Defects may be desirable or
undesirable (e.g., dislocations may be good
or bad, depending on whether plastic
deformation is desirable or not.

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