Difference Between Edge and Screw Dislocation

The key difference between edge and screw dislocation is that the edge dislocation is a type of line defect that occurs when an extra half-plane of atoms exists in the middle of the crystal lattice whereas the screw dislocation is also a type of line defect but it occurs when the planes of atoms in the crystal lattice trace a helical path around the dislocation line.

Crystallographic defects are the imperfections of the repeating pattern of a crystal lattice. Moreover, these defects interrupt the regular pattern of the lattice. Line defects are a form of crystallographic defects in which the defects occur in a plane of atoms in the middle of the crystal lattice. There are two types of line defect; they are edge dislocation and screw dislocation, where the atoms of the lattice exist in a misaligned pattern.

1. Overview and Key Difference
2. What is Edge Dislocation
3. What is Screw Dislocation
4. Side by Side Comparison – Edge vs Screw Dislocation in Tabular Form
5. Summary

What is Edge Dislocation?

Edge dislocation is a type of line defect in crystal lattices in which the defect occurs either due to the presence of an extra plane of atoms or due to the loss of a half of a plane of atoms in the middle of the lattice. This defect causes the nearby planes of atoms to bend towards the dislocation. Therefore, the adjacent planes of atoms are not straight. The region in which the defect occurs is the dislocation core or area.

Difference Between Edge and Screw Dislocation

Figure 01: Edge Dislocation

When we apply stress on a crystal having this defect, the dislocation area moves parallel to the direction of stress. However, the movement at the region where a half of a plane of atoms is very small when compared with the region where the other half of that plane of atoms is absent. Edge dislocations can glide in any plane.

What is Screw Dislocation?

Screw dislocation is another type of line defect in which the defect occurs when the planes of atoms in the crystal lattice trace a helical path around the dislocation line. Visualizing this type of defects in crystals is difficult. Unlike in edge dislocation, when we apply stress on a crystal having this defect, the dislocation area moves perpendicular to the direction of stress.

Key Difference Between Edge and Screw Dislocation

Figure 02: Screw Dislocations

However, the stress caused by screw dislocation is comparatively less complex than edge dislocation. Screw dislocations can glide in any plane.

What is the Difference Between Edge and Screw Dislocation?

Edge dislocation is a form of line defect in crystal lattices in which the defect occurs either due to the presence of an extra plane of atoms or due to the loss of a half of a plane of atoms in the middle of the lattice. When we apply stress on a crystal having edge dislocation, the dislocation area moves parallel to the direction of stress. Moreover, Edge dislocations can glide in any plane.

On the other hand, the screw dislocation is a form of a line defect in which the defect occurs when the planes of atoms in the crystal lattice trace a helical path around the dislocation line. When we apply stress on a crystal having this defect, the dislocation area moves perpendicular to the direction of stress. Further, the screw dislocations can glide in any plane. This is the main difference between edge and screw dislocation.

Summary – Edge vs Screw Dislocation

Line defects are forms of crystallographic defects. There are two forms of these defects as edge dislocations and screw dislocations. The difference between edge and screw dislocation is that the edge dislocation occurs when an extra half-plane of atoms exists in the middle of the crystal lattice whereas the screw dislocation occurs when the planes of atoms in the crystal lattice trace a helical path around the dislocation line.

Reference:

1. “Crystal Defects.” Mannich Reaction | Mannich Reaction Mechanism | [email protected] Available here

Image Courtesy:

1.’Dislocation edge d2’By Wikityke (CC BY-SA 2.5) via Commons Wikimedia
2.’Screw dislocation’By Tosaka – Own work, (CC BY-SA 3.0) via Commons Wikimedia