Dislocations

1
Dislocations

• Basic concepts
• edge dislocation
• screw dislocation
• Characteristics of Dislocations
• lattice strains
• Slip Systems
• slip in single crystals
• polycrystalline deformation
• Twinning

2
Edge Dislocation

• In edge dislocations, distortion exists along an
  extra half-plane of atoms. These atoms also
  define the dislocation line.
• Motion of many of these dislocations will result
  in plastic deformation
• Edge dislocations move in response to shear
  stress applied perpendicular to the dislocation
  line.

3
Edge Dislocation

• As the dislocation moves, the extra half plane
  will break its existing bonds and form new bonds
  with its neighbor opposite of the dislocation
  motion.
• This step is repeated in many discreet steps
  until the dislocation has moved entirely through
  the lattice.
• After all deformation, the extra half plane forms
  an edge that is one unit step wide
• also called a Burgers Vector

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Edge Dislocation
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Edge Dislocation Examples

• Ni-48Al alloy edge dislocation
• the colored areas show the varying values of the
  strain invariant field around the edge
  dislocation
• Shear was applied so that glide will occur to the
  left.
• Computer simulation

6
Screw Dislocation

• The motion of a screw dislocation is also a
  result of shear stress.
• Motion is perpendicular to direction of stress,
  rather than parallel (edge).
• However, the net plastic deformation of both edge
  and screw dislocations is the same.
• Most dislocations can exhibit both edge and screw
  characteristics. These are called mixed
  dislocations.

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Screw Dislocation
8
Screw Dislocation Examples

• Ni-48Al alloy
• l001, 001(010) screw dislocation showed
  significant movement.
• Although shear was placed so that the dislocation
  would move along the (010) it moved along the
  (011) instead.
• Computer simulation

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Screw Dislocation
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Mixed Dislocations

• Many dislocations have both screw and edge
  components to them
• called mixed dislocations
• makes up most of the dislocations encountered in
  real life
• very difficult to have pure edge or pure screw
  dislocations.

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Mixed Dislocations
12
Mixed Dislocations
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Characteristics of Dislocations

• Lattice strain
• as a dislocation moves through a lattice, it
  creates regions of compressive, tensile and shear
  stresses in the lattice.
• Atoms above an edge dislocation are squeezed
  together and experience compression while atoms
  below the dislocation are spread apart abnormally
  and experience tension. Shear may also occur near
  the dislocation
• Screw dislocations provide pure shear lattice
  strain only.

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Characteristics of Dislocations
15
Characteristics of Dislocations

• During plastic deformation, the number of
  dislocations increase dramatically to densities
  of 1010 mm-2.
• Grain boundaries, internal defects and surface
  irregularities serve as formation sites for
  dislocations during deformation.

16
Slip Systems

• Usually there are preferred slip planes and
  directions in certain crystal systems. The
  combination of both the slip plane and direction
  form the slip system.
• Slip plane is generally taken as the closest
  packed plane in the system
• Slip direction is taken as the direction on the
  slip plane with the highest linear density.

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Slip Systems

• FCC and BCC materials have large numbers of slip
  systems (at least 12) and are considered ductile.
  HCP systems have few slip systems and are quite
  brittle.

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Slip in Single Crystals

• Even if an applied stress is purely tensile,
  there are shear components to it in directions at
  all but the parallel and perpendicular
  directions.
• Classified as resolved shear stresses
• magnitude depends on applied stress, as well as
  its orientation with respect to both the slip
  plane and slip direction

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Slip in Single Crystals
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Polycrystalline Deformation

• Slip in polycrystalline systems is more complex
• direction of slip will vary from one crystal to
  another in the system
• Polycrystalline slip requires higher values of
  applied stresses than single crystal systems.
• Because even favorably oriented grains cannot
  slip until the less favorably oriented grains are
  capable of deformation.

21
Polycrystalline Deformation

• During deformation, coherency is maintained at
  grain boundaries
• grain boundaries do not rip apart, rather they
  remain together during deformation.
• This causes a level of constraint in the grains,
  as each grains shape is formed by the shape of
  its adjacent neighbors.
• Most prevalent is the fact that grains will
  elongate along the direction of deformation

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Polycrystalline Deformation
23
Dislocation Movement across GBs

• As dislocations move through polycrystalline
  materials, they have to move through grains of
  different orientations, which requires higher
  amounts of energy, if the grains are not in the
  preferred orientation.
• As they travel between grains they must be
  emitted across the grain boundary, usually by one
  half of a partial dislocation, and then
  annihilated by the second half at a time slightly
  after the first one.
• LINK TO HELENA2.gif

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Twinning

• A shear force which causes atomic displacements
  such that the atoms on one side of a plane (twin
  boundary) mirror the atoms on the other side.
• Displacement magnitude in the twin region is
  proportional to the atoms distance from the twin
  plane
• takes place along defined planes and directions
  depending upon the system.
• Ex BCC twinning occurs on the (112)111 system

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Twinning
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Twinning

• Properties of Twinning
• occurs in metals with BCC or HCP crystal
  structure
• occurs at low temperatures and high rates of
  shear loading (shock loading)
• conditions in which there are few present slip
  systems (restricting the possibility of slip)
• small amount of deformation when compared with
  slip.

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Twinning