A perfect crystal, with every atom of the same type in the correct
position, does not exist. All crystals have some defects. Defects
contribute to the mechanical properties of metals. In fact, using
the term “defect” is sort of a misnomer since these
features are commonly intentionally used to manipulate the mechanical
properties of a material. Adding alloying elements to a metal
is one way of introducing a crystal defect. Nevertheless, the
term “defect” will be used, just keep in mind that
crystalline defects are not always bad. There are basic classes
of crystal defects:
- point defects, which are places where an atom is missing
or irregularly placed in the lattice structure. Point defects
include lattice vacancies, self-interstitial atoms, substitution
impurity atoms, and interstitial impurity atoms
- linear defects, which are groups of atoms in irregular positions.
Linear defects are commonly called dislocations.
- planar defects, which are interfaces between homogeneous
regions of the material. Planar defects include grain boundaries,
stacking faults and external surfaces.
It is important to note at this point that plastic deformation
in a material occurs due to the movement of dislocations (linear
defects). Millions of dislocations result for plastic forming
operations such as rolling and extruding. It is also important
to note that any defect in the regular lattice structure disrupts
the motion of dislocation, which makes slip or plastic deformation
more difficult. These defects not only include the point and planer
defects mentioned above, and also other dislocations. Dislocation
movement produces additional dislocations, and when dislocations
run into each other it often impedes movement of the dislocations.
This drives up the force needed to move the dislocation or, in
other words, strengthens the material. Each of the crystal defects
will be discussed in more detail in the following pages.