Crystal Imperfections

Crystal Imperfections

The most important crystal imperfections are:

Vacancies

Interstitials

Dislocations

Vacancies:

Vacancies are simply empty atom sites as shown in Figure 1.The lattice vacancies are a stable feature of metals at all temperatures above absolute zero. By successive jumps of atoms, just like playing Chinese checkers, it is possible for a vacancy to move in the lattice structure and therefore play an important part in diffusion of atoms through the lattice.

Vacancies are not only present as a result of solidification but can be produced by raising the temperature or by irradiation with fast moving nuclear particles.

Figure 1. Vacancy crystal defect.

Interstitials:

It is possible that some atoms may fall into interstitial positions or in the spaces of the lattice structure which may not be used by the atoms of a specific unit cell as shown in Figure 2. Interstitials tend to push the surrounding atoms farther apart and also produce distortion of the lattice planes.

Interstitial atoms may be produced by the severe local distortion during plastic deformation as well as by irradiation.

Figure 2. Interstitial crystal defect.

Dislocations:

A dislocation may be defined as a disturbed region between two substantially perfect parts of a crystal. A dislocation is a linear defect around which some of the atoms are misaligned.

Two simple types of dislocation are :

Edge dislocation

Figure 3. Edge dislocation

Screw dislocation

Figure 4. Screw dislocation

Dislocations can be observed in crystalline materials using electron-microscopic techniques. Virtually all crystalline materials contain some dislocations that were introduced during solidification, during plastic deformation, and as consequence of thermal stresses that result from rapid cooling.

The importance of dislocations to the metal user is that dislocation interactions within a metal are a primary means by which metals are deformed and strengthened. When metals deform by dislocation motion, the more barriers the dislocations meet, the stronger the metal.

Deformation by dislocation motion is one of the characteristics of metals that make them the most useful engineering materials. The metallic bond is such that strains to the crystal lattice are accommodated by dislocation motion. Many metals can tolerate significant plastic deformation before failing.