05 Crystal defects

Crystal deffects

Point defects

Line defects

Surface defects

Volume defects

Point deffects

Vacancy

Interstitial defects

Substitutional defects

Schottky defect

a paired set of cation and anion vacancies

Frenkel defect

Line defects

Edge dislocation

Screw dislocation

Edge dislocation

Dislocation line perpendicular to slip direction

Screw dislocation

Dislocation line parallel to slip direction

Burgers vectors

The displacement distance for atoms around dislocations is called Burgers vector. It is measure of magnitude and direction of slip due to dislocation motion

Burgers circuit

The circuit drawn from atom to atom around a region of dislocation is called Burgers circuit. In a perfect crystal the Burgers circuit is closed but in the presence of dislocation, the Burger circuit is open; and the extent to which circuit is open is represented by the Burgers vector

Surface defects

Surface imperfections are two dimensional defects, and are the region of disturbance of few atom diameters. Important surface imperfections are

Stacking fault

Grain boundaries

Twinned boundaries

Stacking fault

Stacking fault results due to breaking the stacking sequence of atomic planes. Thus if fault is created, there is a missing of atomic plane from the normal sequence. For example, the normal sequence of stacking atomic planes is FCC crystal is ABC ABC ABC..... If the plane , A is missing from the above sequence, then the atomic stacking sequence becomes ABC BC ABC .... The stacking in missing is altered. This surface imperfection is called stacking fault

Grain boundaries

Twinned boundary

Volume defects

The volume defects are caused due to conglomerations of vacancies in a small region with crystal or the presence of foreign atoms of large sizes compared on atomic scale. The accumulation of vacancies produce voids, while the foreign atoms produce dissymmetry within crystals. These defects affect properties of metal

Example of volume defect

Frank Read source

Frank Read source

a) dislocation with anchored ends

b) Dislocation bows as stress increases

c) Semi circle, maximum stress

d) Loop grows by sweeping round the ends

e) Loop breaks away

Effect of grain size

Fine grained materials which contain a high degree of grain boundary area is stronger at room temperature than a structure of large grains. However, coarse grained structure is desirable for the components operating at elevated temperature.

The relation between strength and grain size is expressed by

Hall Petch equation

Hall Petch equation

σ = σ₀ + k/√d

σ - yield strength

d - grain size

σ₀ and k are constants for a particular materials