Question

The dislocations can be generated by using

• A. Schottky mechanism
• B. Burger’s vector
• C. Twist
• D. Frank-Read source mechanism

Hint:

Frank-Read source is the primary mechanism responsible for dislocation multiplication during plastic deformation.

Answer 1

The Correct Option is D

Step 1: Understanding dislocations.
Dislocations are line defects in crystals that play a crucial role in plastic deformation. Their generation and multiplication directly influence material strength.
Step 2: Frank-Read source mechanism.
The Frank-Read source is a well-known mechanism for the multiplication of dislocations. It involves a pinned dislocation segment that bows out under applied stress and produces new dislocation loops repeatedly.
Step 3: Eliminating other options.
(A) Schottky mechanism: Related to point defects such as vacancies, not dislocations.
(B) Burger’s vector: Describes the magnitude and direction of a dislocation but does not generate it.
(C) Twist: Refers to a type of grain boundary, not a dislocation generation process.
Step 4: Conclusion.
Thus, dislocations are generated by the Frank-Read source mechanism.

Answer 2

The question asks which mechanism actually creates new dislocations during plastic deformation, rather than merely describing or measuring one. Let's examine what each option is actually about.

1. Schottky mechanism: This describes the formation of paired vacancies (a cation and anion vacancy in an ionic crystal, or a single vacancy in a metal) to keep the lattice electrically neutral. It is a point-defect generation process and has nothing to do with line defects.
2. Burger's vector: This is simply the quantity that specifies the magnitude and direction of the lattice distortion caused by an existing dislocation. It is a descriptor, not a source, so it cannot be the mechanism that generates new dislocations.
3. Twist: A twist boundary is a type of grain boundary formed when two crystal regions are rotated relative to each other about an axis perpendicular to the boundary. It describes an interface, not a process that multiplies dislocations within a grain.
4. Frank-Read source mechanism: Here, a dislocation segment is pinned at both ends (for instance by other dislocations or precipitates). Under an applied shear stress, the segment bows outward, wraps around itself to pinch off a full dislocation loop, and then re-forms at the original pinned length, ready to repeat the process. This is a genuine multiplication mechanism that keeps generating new dislocation loops as long as stress is applied.

Only the Frank-Read source actually creates new dislocations, the other three options either describe a different kind of defect or merely characterize an existing one.

Therefore, the correct answer is Frank-Read source mechanism.