Defects in solids

Defects in solids:

In a crystal structure, a lot of defects are found to exist. These defects can be at a point, along a line or over a surface. So crystal defects can be divided into three main categories.

Point defect – A point defect increases the internal energy and decreases its free energy. The main point defects are

i. Impurities – A foreign atom occupies a lattice site of missing atom, is known as substitutional impurity. The space between the large and hot atoms of the crystal is occupied by smaller atoms of slag, is known as interstitial impurity. It affects the electrical and thermal conductivities of metals and alloys.

ii. Vacancies – When an atom is missing from a normally occupied site within a crystal lattice, is known as vacancy. Also di-vacancy and tri-vacancies can be found in crystals. This defect is a result of imperfect packing of atoms during original crystallization.

iii. Schottky defect – An atom is dislocated from its original lattice site to the surface of the crystal.

iv. Frankel defect – An atom leaves its original lattice site and occupies an interstitial lattice site within the crystal. It is more common in open structures.

Line defect – It is due to a linear disturbance of the atomic arrangement in a crystal. This defect is also called dislocation. There are mainly two types of dislocations.

i. Edge dislocation – A line of atoms is displaced by a unit distance. The plane in which edge is dislocated, is known as slip plane. The zone above the slip plane is in compression and those below the slip plane is in tension.

ii. Screw dislocation – The atoms are dislocated in two different planes, mutually perpendicular to each other. It is called screw dislocation because the dislocation follows a helical path.

Plane defects – These are also known as surface defects. The main plane defects are

i. Twin boundaries – It occurs when atoms slip from their original positions in such a way that the adjacent faces of the crystals are mirror images of each other and in between these faces laid the disturbed region (defective region).

ii. Tilt boundaries – It is an array of edge dislocation. It is a low angle boundary defect where the angle of disorientation is represented by θ. It is given by tanθ = b/D, where ‘b’ is the burger vector and ‘D’ is distance of dislocation. ‘θ’ is very small (less than 10°).

iii. Grain boundaries – When two growing surfaces meet and their interface has misalignment of grain boundaries, is known as grain boundary defect. It occurs in polycrystalline materials.

Volume defects – The main volume defects are

i. Stacking defect – It is a kind of packing defect which arises due to improper placing of different layers of polycrystalline material. It is found in HCP and FCC metals. In this defect, the sequence of atomic planes gets disturbed and atomic planes are arranged out of sequence.

ii. Cracks – The deformation may either result in external i.e. on the surface or internal cracks in a crystal.