Defects in a lattice structure of a crystalline material are :
(i) Point Defect. It occurs when an imperfection is restricted to the neighbourhood of a lattice point. Fig. 1.16 illustrates the three different types of point defects. In Fig. 1.16 (a) one lattice atom is missing, creating a vacancy. Since an atom vibrates about its lattice position,
the tendency of the atom to jump out of its regular position creating a vacancy increases rapidly with temperature. It is possible to increase the vacancy density at a given temperature by rapid cooling or extensive plastic deformation. An atom may occupy an abnormal position as in Fig. 1.16 (6) Interstitial impurity atom). An interstitial impurity can be caused when an atom possesses large enough thermal energy or when its energy is increased by nuclear bombardment. It is possible that a regular lattice position may be occupied by an atom of a different material (substitutional impurity).
(ii) Line Defect. It occurs when an imperfection extending along a line has a length much larger than the lattice spacing also called dislocation. Two common, simple types of dislocations are shown in Fig. 1.17. Edge dislocation occurs when an extra half-plane of atoms
is accommodated by distorting the regular lattice arrangement (as done with the AB half-plane in Fig. 1.17(a), screw dislocation occurs due to movement of the lattice atoms from their regular ideal positions, Fig. 1.17 (6). The line separating the deformed and the undeformed regions is normally called the dislocation line (line AB in Figs. 1.17 (a) and (6)). The dislocation density is defined as the total length of all the dislocation lines per unit volume. Plastic deformation takes place mainly through a movement of dislocations.
(iii) Surface Defect. It occurs when an imperfection extends over a surface. Fig. 1.18 shows a common type of surface defect known as twins, produced due to stressing of metal at low temperature.