Point Defects

Point Defects

Point defects are defects associated with one or two atomic positions. There are several types of point defects which we will aim to cover. These defects can be manipulated in the formation of an alloy which is a metal in which impurity atoms have been added intentionally to impact specific characteristics of the material. The diagram below shows several different types of point defect and by the end of this section you should be able to identify and explain all of them.

Vacancies (or vacant lattice)

A vacancy defect is a position in a material which is normally occupied but from which an atom is missing. All crystalline solids contain vacancies and it is not possible to create a material that is free from this defect. The presence of vacancies increases the entropy (randomness) of the crystal.

The equilibrium of number of vacancies Nv for a given quantity of materials depends on, and increases with temperature.

Self-Interstitial -

A self-interstitial is an atom from the crystal that is crowded into an interstitial site. An interstitial site is a small void space that under ordinary circumstances is not occupied. In metals this defect introduces relatively large distortions in the surrounding lattice this is because the atom is substantially larger than the interstitial position in which it is situated. Formation of self-interstitials are not highly probable and therefore exist in small concentrations. They are significantly less common than vacancies.

Impurities in solids:

The addition of impurity atoms results in the formation of a solid solution and/or a new second phase.

Two key definitions:

• • Solvent or host atom - is the element or compound that is present in the greatest amount.

  • Solute - is used to denote an element or compound present in a minor concentration.

Impurity point defects are found in solid solutions, of which there are two types: substitutional and interstitial. A solid solution is formed when the crystal structure of the metal is maintained as the host material is added. This means that no new structures are formed. The impurity atoms are randomly distributed around the solid. Meaning a solid solution is compositionally homogeneous.

Substitutional type -

For the substitutional type solute or impurity atoms replace the host atoms.

Interstitial solid solutions

For interstitial solid solutions impurity atoms fill the voids among the host atoms. For both FCC and BCC structure the types of interstitial sites are - tetrahedral and octahedral. Which is determined by the coordination number. The atomic diameter of an interstitial impurity must be substantially smaller than the diameter of the host atoms because metals have relatively high atomic packing factor. Normally the concentration of interstitial impurity atoms is less than 10%. Due to the fact that even very small impurity atoms are usually larger than the interstitial sites some lattice strain is present on the adjacent host atoms.