Two-dimensional crystal imperfections are known as surface defects or plane defects. In surface defects, the imperfections should lie about a surface having few atomic dimensions thick. Surface defects are of two types (a) external surface defects and (b) internal surface defects.
(a) External surface imperfections: Every atom present inside the crystal has a large number of surrounding atoms, whereas the atoms present on the surface of the crystal has no neighbouring atoms on one side of the surface. Hence atomic bonds do not extend beyond the surface of the crystal. Because of this, the surface atoms possess larger energy than the interior atoms. This larger energy at the surface causes imperfection at the external surface itself.
(b) Internal surface imperfections: The change in stacking of atomic planes across a boundary in the crystal is known as internal surface imperfection. Some of the internal surface imperfections are explained below:
(i) Grain boundaries: The non-periodicity of atoms between the crystallets (grains) of a polycrystalline material causes grain boundary surface defect. During recrystallization or during solidification of a polycrystalline material, the atoms from adjacent regions of two crystallets eventually impinge on each other, while the atoms between the crystallets are pulled to take up a compromised position between the two crystallets.
The thickness of this non-periodic region is of the order of 2 to 10 atomic distances or more. This boundary region is called a crystal boundary or a grain boundary and is shown in Fig. B.1. The orientation of the crystallets changes sharply at the grain boundary. If the misorientation angle between the crystallets is greater than 10 to 15°, then it is called a high-angle grain boundary. On the other hand, if the misorientation angle between the adjacent crystals is of the order of a few degrees or less than 10°, then it is called a low-angle grain boundary.
(ii) Tilt and Twist boundaries: Tilt boundary has an array of edge dislocations as indicated by (┴) in Fig. B.2. In the figure, ‘h’ is the vertical spacing between two consecutive edge dislocations and ‘b’ is the length of Burger's vector. Here tan θ ≈ θ = , is the angle of tilt or misorientation. This is a low-angle boundary. Twist results from a set of screw dislocations, it is also a low-angle boundary.
Figure B.2 Tilt boundary
(iii) Twin boundaries This is a surface imperfection that separates two mirror orientations of a crystal.
As shown in Fig. B.3, the atomic arrangement on one side of a twin boundary is a mirror refl ection of the atomic arrangement on the other side of the twin boundary. The region between the two boundaries is called the twinned region. The twin boundaries can be seen under an optical microscope.
Figure B.3 Twin boundaries
(iv) Stacking fault: Stacking fault is a surface imperfection in which there is a discrepancy in the stacking sequence of atomic planes. As shown in Fig. B.4, the stacking sequence in close packed FCC structure is ABCABCABC…
Figure B.4 Stacking fault
Suppose that in a small region in atomic layer ‘C’, the atoms are not positioned properly. Then at this region, the stacking sequence is different and here the stacking sequence becomes…ABAB….This is the stacking sequence of HCP structure. Thus the missing atoms in a small area of atomic layer ‘C’ gives rise to a stacking fault in close packed FCC crystal. The crystal will be sound on both sides of the fault.
(v) Ferro-magnetic domain walls: A ferro-magnetic material contains a large number of ferro-magnetic domains. Each domain is magnetised to saturation in a particular direction inside the material. The intensity of magnetic field and hence the magnetic field energy is almost uniform inside the domains. However, the intensity of magnetic field and the magnetic field energy is more at the surface of the domains. This large magnetic field energy on the surface of the domains gives rise to a surface imperfection known as magnetic domain-wall imperfection.
(2) volume defects: The cracks that are formed due to small electrostatic dissimilarity between the stacking layers, or due to sudden thermal waves or by using the material for some application cause volume defects.
The presence of large vacancies or voids due to missing clusters of atoms, non-crystallined regions, and inclusion of foreign particles with a dimension of at least 10 to 30Å are considered as volume detects.