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LINEAR DEFECTS

   A dislocation is a linear or one-dimensional defect around which some of the atoms are misaligned.

  ?Edge dislocation: A linear crystalline defect associated with the vicinity of the end of an extra halfplane of atoms within a crystal. The Burgers vector is perpendicular to the dislocation line.

 It is a linear defect that centers on the line that is defined along the end of the extra half-plane of atoms. This is sometimes termed the dislocation line, which, for the edge dislocation in Figure 4.3, is perpendicular to the plane of the page. Within the region around the dislocation line there is some localized lattice distortion

   Fig. 4-3 :

  The atoms above the dislocation line in Figure 4.3 are squeezed together, and those below are pulled apart; this is reflected in the

Slight  curvature for the vertical planes of atoms as they bend around this extra halfplane.

   The magnitude of this distortion decreases with distance away from the dislocation line; at positions far removed, the crystal lattice is virtually perfect.

    Sometimes the edge dislocation in Figure 4.3 is represented by the symbol ?, which also indicates the position of the dislocation line. An edge dislocation may also be formed by an extra half-plane of atoms that is included in the bottom portion of the crystal; its designation is a ?.

?Screw dislocation. A linear crystalline defect associated with          the lattice distortion created when normally parallel planes are        joined together to form a helical ramp. The Burgers vector is          parallel to the dislocation line.