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

Think of a dislocation segment pinned at both ends, similar to a garden hose pinned at two points.

1. Schottky mechanism: generates vacancies, a point defect, not a dislocation.
2. Burger's vector: only measures an existing dislocation's displacement, it creates nothing.
3. Twist: describes a grain-boundary type, not a way to multiply dislocations.
4. Frank-Read source: under shear stress the pinned segment bows out, loops back on itself, releases a dislocation loop, and resets, repeating the cycle and generating many new dislocations.

So dislocations are generated by the Frank-Read source mechanism.

Answer 2

A useful way to sort these four options is to ask whether each one is a point-defect process, a purely descriptive quantity, a boundary type, or an actual dislocation-multiplication process.

1. Schottky mechanism: belongs to the point-defect category (vacancy formation), a completely different class of defect from a dislocation.
2. Burger's vector: belongs to the descriptive-quantity category. It tells you the size and direction of the slip associated with a dislocation but is not itself a physical process.
3. Twist: belongs to the boundary category, describing misorientation across a grain boundary, unrelated to multiplying dislocations inside a grain.
4. Frank-Read source mechanism: belongs to the multiplication-process category. Under an applied stress \( \tau \), a dislocation segment of length \( l \) bows into a semicircular loop once \( \tau \) exceeds the line-tension resistance, then pinches off a closed loop and regenerates, continuously supplying new dislocations to the crystal.

Only the process in the fourth category actually produces new dislocations, so the source of dislocation generation is the Frank-Read mechanism.

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