Line Defects
Line defects are linear and one dimensional defect in which one surface of the crystal slips with respect to the other by a distance of about one atomic spacing. The lattice distortion are centered along a line so is called line defect. They are also known as dislocations. Line defects are caused by mishandling, decreasing grain growth and mechanical deformation.
There are three types of line defects:
- Edge Dislocation
- Screw Dislocation
- Mixed Dislocations
1. Edge Dislocation
This defect occurs when an extra half plane is introduced into the crystal structure. This defect centres around the dislocation line that is defined along the end of the extra half plane of atoms. The line perpendicular to the plane of page and below the half plane at B is called the dislocation line. The displacement distance of atoms around the dislocation is called Burger Vector b. This is perpendicular to the dislocation line. The atoms above the dislocation line are squeezed together and below are pulled apart. The magnitude of distortion decreases as the distance from dislocation line increases. (a) Perfect crystal lattice without dislocation Extra half plane Dislocation line is perpendicular to the plane of the paper at B (b) Crystal lattice with an edge dislocation Fig. : Edge Dislocation When slip is such that extra half plane is at top of the crystal, then it is called positive edge dislocations. When the extra half plane is at the bottom of the crystal then it is called negative edge dislocations.
Edge dislocation can have two type of motions:
- Glide motion i.e. movement of edge dislocations parallel to the closed packed plane.
- Climb motion denotes the movement of edge dislocation perpendicular to closed packed planes.
2. Screw Dislocation
When a truely crystalline material is subjected to shear stress, the discrete atomic plane may get converted into the surface of a helix around a dislocation line, this type of crystalline distortion from true crystalline material is known as screw dislocation.
In screw dislocation there is no specific glide plane. Hence glide motion and climb motion are not defined. The mass transportation takes place in screw dislocation is by cross-slip motion.
Atomic bonds near the dislocation line experience a shear deformation, which results in development of shear stress and shear strain field.
3. Mixed Dislocations
Most of the times it is seen that the dislocations in the structure are not entirely edge dislocations or screw dislocations, but components of both the dislocations exists. Such dislocations are called as mixed dislocations.
NOTE: Number of dislocations increases in cold working whereas it decreases in hot working.
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