The following points highlight the three main defects occurs due to the imperfection in solids.
1. Point Defects:
Those associated with one or two atomic positions. They are:
(i) Vacancies.
(ii) Self-interstitials
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(iii) Substitutional impurity
(iv) Interstitial impurity
Equilibrium, no. of vacancies are given as:
Nu = Nexp (-Qv/kτ), Qv = energy required for vacancy formation,
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T = temperature
Thus, as temperature increases vacancies increases exponentially.
Where,
a. Frankel Defect:
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When the atom in the lattice point goes and occupies the interstitial void of other atom then it is called Frenkel defect.
b. Schottky Defect:
In the combination of cation and anion if there is a vacancy defect it is called schottky defect.
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Mechanical & electrical properties of metals such as yield, fracture strength electrical conductivity are adversely affected by these defects, they are known as “structure sensitive properties”.
Where properties such as melting point, specific heat, coefficient of thermal expansion are structure insensitive properties.
2. Linear Defects:
i. Edge Dislocation:
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This defect occurs when an extra half plane of atoms is introduced into the crystal structure. This defect centers on the dislocation line that is defined along the end of the extra half plane of atoms. Edge dislocation is perpendicular to the plane of the page. Around edge dislocation there is some lattices distortion.
Atoms above the dislocation lines are squeezed together and these below are pulled apart. Magnitude of this distortion decreases at distances away from the dislocation line. For extra half plane of atoms included on bottom portion of the crystal, the symbol is (T).
ii. Screw Dislocation:
This Dislocation is formed by shear stress, the upper front region of the crystal is shifted one atomic distances to the right relative to the bottom portion.
The screw dislocation derives its name from the spiral or helical path traced by the atomic planes around the dislocation line.
The magnitude and direction of the distortion associated with a dislocation is expressed in terms of “Burgers Vectors”(b). For an edge dislocation burgers vector are perpendicular to dislocation line and are parallel to dislocation line for screw dislocation. They are neither perpendicular nor parallel to mixed location. For metallic materials, the burgers vector for a dislocation will point in a close packed crystallographic direction, and its magnitude equal to interatomic setting.
3. Interfacial Defects:
These are following types:
i. External Surfaces:
It is the boundary at which the crystal structure terminates. Atoms at the surface are not bonded to maximum. Number of nearest neighbours and therefore are in a higher energy state. To reduce this energy they tend to minimize the surface area e.g., liquid droplet become spherical.
ii. Grain Boundaries:
Grains boundaries are boundaries separating two crystals, have different crystallographic orientation within a polycrystalline material. When the mismatch in orientation between adjacent boundaries is small then they are called small angle grain boundary or tilt boundary.
When the angle of misorientation parallel to the boundary (grain) it is called a ‘Twist Boundary’.
The atoms are bonded less regularly along a grain boundary and thus boundaries are in a high energy state. The magnitude of this energy is a function of degrees of misorientation (being larger for high angle boundaries). Grain boundaries, are chemically more reactive than the grain themselves.
Furthermore, the impurity atoms preferentially segregate along these boundaries because of their high energy state. The total interfacial energy is lower for coarse grain material as compared to fine grained ones because of less total boundary area.
iii. Twin Boundaries:
It is a special type of grain boundary such that atoms on one side of the boundary are mirror images of atoms on the other side of the boundary. The region of material between these boundaries is called ‘twin’.
Twins are produced by atomic displacements by mechanical shear force (mechanical twins) and also during annealing heat treatment following deformation (Annealing twin). Annealing twins are typically found in FCC crystal structure, while mechanical twins are found in BCC and HCP metals.
iv. Stacking Faults:
These are found in FCC metals when there is interruption in the ABC ABC ABC—stacking sequences of closed packed atoms.
v. Phase Boundaries:
Boundaries separating two different phases having different mechanical properties and chemical characteristics.
vi. Ferromagnetic Domain Walls:
Boundary that separates region having different directions of magnetization is termed as domain wall.
Bulk or Volume Defects:
Pores, cracks foreign inclusion, they are normally introduced during processing and fabrication steps.