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).

Macroscopic Defects

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.

Summary

The important information in this chapter is summarized below.

Imperfections in Metals Summary

Microscopic Imperfections

Point imperfections are in the size range of individual atoms.

Line (dislocation) imperfections are generally many atoms in length. Line imperfections can be of the edge type, screw type, or mixed type, depending on lattice distortion. Line imperfections cannot end inside a crystal; they must end at crystal edge or other dislocation, or close back on themselves.

Interfacial imperfections are larger than line imperfections and occur over a two dimensional area. Interfacial imperfections exist at free surfaces, domain boundaries, grain boundaries, or interphase boundaries.

Slip occurs when a crystal is subjected to stress and the dislocations march through the crystal until they reach the edge or are arrested by another dislocation.

Macroscopic Defects

Bulk defects are three dimensional defects.

Foreign particles included in the prime material (inclusions) are most common bulk defect

Gas pockets

Shrinking cavities Welding or joining defects