Dislocation dissociation

leave perfect lattice in their wake. If the lattice on one side of a plane is displaced relative to the lattice on the other side, there is some minimum displacement 0 required before the lattices match perfectly again. The magnitude of this repeat distance 0 depends on the lattice and the direction of the displacement and will be the smallest in a close-packed direction of a close-packed plane. In principle, one can discuss dislocations whose b is smaller than o. However, the movement of such a dislocation would shift the lattice on either side of the glide plane by less than o. As a result, the lattices on either side of the glide plane would not match and the movement of the dislocation would create a plane of mismatch in the lattice that would have large energy associated with it. Dislocations with b = 0 are called complete or perfect dislocations. Dislocations with b < 0 are called partial dislocations. The partial dislocations found in nature are limited to those which bound a mismatch plane of relatively low energy. For example, consider the ordered lattice. Note that along the most closely packed (100) direction the repeat distance ao is twice the interatomic spacing d. In the ordered lattice is shown, the energy of the crystal is lowered if the A atoms are surrounded by B atoms and vice versa. In some crystals, like NaCI, the energy difference on going from an ordered to a disordered structure is extremely large, so they always are found in the ordered state. In many intermetallic phases, though, the energy change on ordering is small. Thus it is energetically possible to move a partial dislocation of b = ao/2, leaving a plane of A-A or B-B nearest neighbours behind. In ordered alloys, this planar defect is called an antiphase boundary because the periodicity of the atomic arrangement changes phase here. The antiphase boundary shown is bounded by partial dislocations.