defects occur on a much bigger scale than the rest of the crystal
defects discussed in this section. However, for the sake of completeness
and since they do affect the movement of dislocations, a few of
the more common bulk defects will be mentioned. Voids are regions
where there are a large number of atoms missing from the lattice.
The image to the right is a void in a piece of metal The image
was acquired using a Scanning Electron Microscope (SEM). Voids
can occur for a number of reasons. When voids occur due to air
bubbles becoming trapped when a material solidifies, it is commonly
called porosity. When a void occurs due to the shrinkage of a
material as it solidifies, it is called cavitation.
Another type of bulk defect occurs when impurity atoms cluster
together to form small regions of a different phase. The term
‘phase’ refers to that region of space occupied by
a physically homogeneous material. These regions are often called
precipitates. Phases and precipitates will be discussed in more