When a sufficient load is applied to a metal or other structural
material, it will cause the material to change shape. This change
in shape is called deformation. A temporary shape change that
is self-reversing after the force is removed, so that the object
returns to its original shape, is called elastic deformation.
In other words, elastic deformation is a change in shape of a
material at low stress that is recoverable after the stress is
removed. This type of deformation involves stretching of the bonds,
but the atoms do not slip past each other.
the stress is sufficient to permanently deform the metal, it is
called plastic deformation. As discussed in the section on crystal
defects, plastic deformation involves the breaking of a limited
number of atomic bonds by the movement of dislocations. Recall
that the force needed to break the bonds of all the atoms in a
crystal plane all at once is very great. However, the movement
of dislocations allows atoms in crystal planes to slip past one
another at a much lower stress levels. Since the energy required
to move is lowest along the densest planes of atoms, dislocations
have a preferred direction of travel within a grain of the material.
This results in slip that occurs along parallel planes within
the grain. These parallel slip planes group together to form slip
bands, which can be seen with an optical microscope. A slip band
appears as a single line under the microscope, but it is in fact
made up of closely spaced parallel slip planes as shown in the