Lecture 8: Dislocations

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Lecture 8 Dislocations

• PHYS 430/603 material
• Laszlo Takacs
• UMBC Department of Physics

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Edge dislocation

• The crystal is partially cut, a new half-plane of
  atoms inserted coherently with the rest of the
  structure and the bonds between layers
  re-established.
• The Burgers vector is perpendicular to the
  dislocation line.

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Dislocation on the surface of a real crystal
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Sign convention for the Burgers vector Decide
the positive direction for the dislocation
line. Go around the line in a complete loop in
the positive direction. Draw a loop with the
same number of steps in a defect-free
lattice. The vector required to complete the
loop is the Burgers vector.
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Screw dislocation

• The crystal is partially cut, one side shifted by
  a lattice parameter, then the bonds are
  re-established between the two sides.
• The Burgers vector is parallel to the line of
  the dislocation.

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• Dislocations are the primary carriers of plastic
  deformation.

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• Deformation can take place by both edge and screw
  dislocations

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• A mixed dislocation. The dislocation line can be
  bent, the Burgers vector remains the same along
  the dislocation.

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Dislocation in a compound. There are both an
extra Mg and and extra O plane ending at the edge
of the dislocation.
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Dislocation loop

• Dislocations can begin and end at the surface of
  the crystal or at lattice defects. They can also
  form closed loops.
• Notice that the Burgers vector is the same along
  the loop and its nature is edge or screw
  depending on the direction of the dislocation
  line.

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Prismatic loop

• The Burgers vector is perpendicular to the plane
  of the loop, thus it is edge dislocation all
  around. Equivalent to a partial extra lattice
  plane or a finite area of a lattice plane missing.