Structure of Metals
IMPERFECTIONS IN METALS
Interfacial imperfections exist at an angle between any two faces of a crystal or crystal
form. These imperfections are found at free surfaces, domain boundaries, grain
boundaries, or interphase boundaries. Free surfaces are interfaces between gases and
solids. Domain boundaries refer to interfaces where electronic structures are different
on either side causing each side to act differently although the same atomic arrangement
exists on both sides. Grain boundaries exist between crystals of similar lattice structure
that possess different spacial orientations. Polycrystalline materials are made up of many
grains which are separated by distances typically of several atomic diameters. Finally,
interphase boundaries exist between the regions where materials exist in different phases
(i.e., BCC next to FCC structures).
Three-dimensional macroscopic defects are called bulk defects. They generally occur on a much
larger scale than the microscopic defects. These macroscopic defects generally are introduced
into a material during refinement from its raw state or during fabrication processes.
The most common bulk defect arises from foreign particles being included in the prime material.
These second-phase particles, called inclusions, are seldom wanted because they significantly
alter the structural properties. An example of an inclusion may be oxide particles in a pure
metal or a bit of clay in a glass structure.
Other bulk defects include gas pockets or shrinking cavities found generally in castings. These
spaces weaken the material and are therefore guarded against during fabrication. The working
and forging of metals can cause cracks that act as stress concentrators and weaken the material.
Any welding or joining defects may also be classified as bulk defects.