Lecture 9/2: Dislocations

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Lecture 9/2 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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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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• 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.

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Twinning The crystal continues as its own mirror
image beginning with a lattice plane as the
mirror.After a few layers of atoms the
crystal is may return to the original orientation
via another twin boundary, but shifted. Twinning
can result in plastic deformation, just like
dislocation movement does.
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Plastic deformation produced by twinning and by
slip
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Twinning in an fcc latticeThe rectangles
represent 1 1 0 planes.
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The (-1 1 0) plane in the cubic fcc unit cell and
a view of this plane showing the edge of the
close-packed planes.
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Twinning in brass (optical micrograph)
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Twinning in La aluminate(oblique illumination
optical image)
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Stacking faults and twinning in Si (HRTEM)