CRYSTAL IMPERFECTION

CRYSTAL IMPERFECTION

 Real crystals deviate from the perfect periodicity of atoms which is assumed in an ideal crystal. This deviation is chiefly responsible for the changes in the mechanical and electrical properties of the real crystals. This deviation of atoms from an orderly array of lattice points is termed as defect or imperfection. An understanding of these lattice defects is very important to explain the mechanical behaviour of metals. For example the actual streng­th of polycrystalline material is about 10³ to 10⁵ times lower than the the­oretical strength of an ideal crystal.

Crystal defects or imperfections could be of three types--point defects, line defects and area defects.

 Point Defects: When the deviation from the periodic arrangement of the lattice is locali­zed to the vicinity of only a few atoms, it is called a point defect, or point imperfection. The point defects in crystals could be of three types a vacan­cy, an interstitial and an impurity atom. These defects are . A vacancy or vacant lattice site, exists when an atom is missing from a normal lattice position, (a). In pure metals, a defi­nite number of vacant lattice sites exist at temperatures greater than abso­lute zero. This number increases rapidly with increase in temperature. Rapid quenching from a higher temperature to room temperature increa­ses the number of vacancies than that possible under equilibrium conditions. Number of vacancies can also be increased by extensive plastic deforma­tion (cold working) or as a result of bombardment with high energy nuclear particles. When the density of vacancies becomes relatively large, they may cluster together to form voids.

A point defect is called interstitial when an atom is trapped inside the crystal at a point intermediate between normal lattice positions,  (b).

The presence of a different type of atom either at a lattice position or an interstitial position, the point defect is called an impurity atom.

 Line Defects: If the crystal defect extends through microscopic regions of the crystal, it is called a line defect. The line detects obtain their name because they pro­pagate as lines or as a two-dimensional net in the crystal. Typical examples of line defects are edge and screw dislocations.

The most important line defect is the dislocation. Dislocations exist in all real crystals. Depending upon its nature, the dislocation is termed as either edge or screw type dislocation. An edge dislocation, which is the edge of an incomplete plane of atoms within a crystal, is represented in the cross-section in Figure 2.9(c). In this illustration, the incomplete plane extends part way through the crystal from the top down, and the edge dislocation (which is indicated by the standard symbol T) is its lower edge.

The defect `dislocation' is mainly responsible for the observed low stren­gth in pure metals. A dislocation moves easily on the application of a small amount of force and results in plastic deformation through the phenomenon of slit).

Area Defects: if the crystal defect appears in two dimensions through the microscopic regions of the crystal, it is called an area defect. Typical examples of area defects are stacking faults, twin interfaces and grain boundaries.

Stacking faults are planes where there is an error in the normal sequence of stacking of atom layers. These may be formed during the growth of a crystal or may result from motion of partial dislocations. A partial dis­location produces a movement that is less than a full distance.

Twins are portions of a crystal that have certain specific orientations with respect to each other. The twin relationship may be such that the lattice of one part is the mirror image of that of the other. Twins may occur fre­quently during crystallization from the liquid or the vapour state, by growth during annealing or during phase transformation. -