Question

With reference to edge and screw dislocations, which of the following statements is/are CORRECT?

• A. Both edge and screw dislocations can leave the slip plane by climb.
• B. Burgers vector of a screw dislocation is parallel to its line vector.
• C. Both edge and screw dislocations can leave the slip plane by cross-slip.
• D. Strain energy per unit length of an edge dislocation is higher than that of a screw dislocation.

Hint:

Remember that edge dislocations have a higher strain energy than screw dislocations due to the lattice distortions they create. Also, the Burgers vector of a screw dislocation is always parallel to its line vector.

Answer 1

The Correct Option is B, D

To understand the correct options, let's examine the nature of edge and screw dislocations:
Step 1: Analysis of the options - Option (A): "Both edge and screw dislocations can leave the slip plane by climb." - Incorrect: Climb is a mechanism that is typically associated with edge dislocations, not screw dislocations. While edge dislocations can leave the slip plane by climb (movement of atoms in the direction perpendicular to the dislocation line), screw dislocations cannot leave the slip plane by climb. They can move in the plane itself.
- Option (B): "Burgers vector of a screw dislocation is parallel to its line vector." - Correct: The Burgers vector of a screw dislocation is indeed parallel to its line vector. For screw dislocations, the dislocation line and Burgers vector are aligned along the same direction, which is characteristic of screw dislocations.
- Option (C): "Both edge and screw dislocations can leave the slip plane by cross-slip." - Incorrect: Cross-slip is a mechanism where dislocations move from one slip plane to another, but it is easier for screw dislocations to cross-slip than edge dislocations. Edge dislocations face more resistance to cross-slip due to the direction of the Burgers vector, which is not parallel to the dislocation line. Therefore, this statement is not entirely correct for both types of dislocations.
- Option (D): "Strain energy per unit length of an edge dislocation is higher than that of a screw dislocation." - Correct: The strain energy associated with edge dislocations is higher than that of screw dislocations. This is because edge dislocations produce a greater elastic distortion in the surrounding crystal lattice, leading to a higher energy per unit length. For screw dislocations, the distortion is less severe, leading to lower strain energy.
Step 2: Conclusion The correct statements are Option B and Option D.