- Vacancy – An atom missing from the regular lattice position.
Vacancies are present invariably in all materials.
- Interstitialcy – An atom trapped in the interstitial point (a
point intermediate between regular lattice points) is called interstitialcy.
- An impurity atom at the regular or interstitial position in the lattice is another type of point defect.
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| Point Defect |
In ceramic materials point defects occur in pairs to maintain electroneutrality. A cation vacancy - cation-interstitial pair is known as a Frenkel defect. A cation vacancy-anion vacancy pair is known as a Schottky
defect.
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| Frenkel and Schottky Defect |
Line Defects:
- Edge Dislocations
- Screw Dislocations
Dislocations:
Dislocation is the region of localized lattice distortion which
separates the slipped and unslipped portion of the crystal. The upper region of the crystal over the slip plane has
slipped relative to the bottom portion. The line (AD) between
the slipped and unslipped portions is the dislocation. The magnitude and direction of slip produced by dislocation
(pink shaded) is the Burger vector, b, of the dislocation.
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| Dislocations |
Edge Dislocations:
In one type of dislocation, the Burger vector is
perpendicular to the dislocation line and the distortion
produces an extra half-plane above the slip plane.
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| Edge Dislocations |
Screw Dislocations:
The other type of dislocation is screw dislocation where
the Burger vector is parallel to the dislocation line (AD). The trace of the atomic planes around the screw dislocation
makes a spiral or helical path (pink shade) like a screw and
hence, the name. Atomic positions along a screw dislocation are represented in
Fig. (b)
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| Screw Dislocations |
Surface Defects:
Most crystalline solids are an aggregate of several
crystals. Such materials are called polycrystalline. Each crystal is known as a grain. The boundary between
the grains is the grain boundary (the irregular lines in Fig. a). A grain boundary is a region of atomic disorder in the
lattice only a few atomic diameters wide. The orientation of the crystals changes across the grain
boundary as shown schematically in Fig. b.Grain boundaries act as obstacles to dislocation motion.
Hence, the presence of more grain boundaries (finer grain size)
will increase the strength.
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| Surface Defects |
