What is Dislocation?

We are referring to dislocations number of times while describing the plastic deformation of the crystals. Let us try to define what is dislocation.

In order to initiate plastic deformation, if every bond between layers of atoms had to be broken at once, then the stress required would be very high. However, defects called “dislocations” exist in the structure of metal crystals which are localized imperfections in the alignment of the layers of atoms in the lattice.

These imperfections can consist of

a) layers being twisted with respect to each other (the result looks like a screw thread and so these are called “screw” dislocations),
b) an extra half plane of atoms (this is an “edge” dislocation) or
c) a combination of these two.

Various forms of dislocations are shown in the figures below.