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Crystal defects and diffusion

Which correctly describes the relation between the Burgers vector and the dislocation line of an edge dislocation?

  1. AThey are perpendicular to each other
  2. BThey are parallel to each other
  3. CThey always make an angle of 45°
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Answer: A. They are perpendicular to each other

An edge dislocation has the form of an extra half-plane of atoms inserted into the crystal, and its Burgers vector b is perpendicular to the dislocation line. An edge dislocation glides in the direction of b, and its slip plane is uniquely defined as the plane containing both b and the line. b is parallel to the line in a screw dislocation; because a screw dislocation has no unique slip plane, it can cross-slip onto another slip plane. Most real dislocations are mixed dislocations in between, with the proportions of edge and screw character changing continuously along the line, so there is no fixed 45°.

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  1. A metal has a vacancy formation energy of 0.90 eV. Using n/N = exp(−Q/kT), which value is closest to the equilibrium vacancy concentration (the fraction of lattice sites that are vacant) at 1000 K? Take Boltzmann's constant as k = 8.617×10⁻⁵ eV/K.
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