Question

Assertion A: The ductility of metals is affected by dislocations of atomic structure, it increases the ductility of metals.
Reason R: Dislocations mostly occur at the lower stress than the predicted shear strength.
In the light of the above statements, choose the most appropriate answer:

• A. Both A and R are correct and R is the correct explanation of A
• B. Both A and R are correct but R is NOT the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

Think of a dislocation like a wrinkle in a rug. Instead of pulling the whole rug at once (theoretical strength), you can move the rug easily by "pushing the wrinkle" across it!

Answer 1

The Correct Option is A

Concept: Ductility is the ability of a material to undergo significant plastic deformation before rupture. In metals, this is governed by the movement of line defects called dislocations.

Step 1: Evaluate Assertion A.
The motion of dislocations (slip) is the primary mechanism for plastic deformation in crystalline solids. Without dislocations, metals would be extremely brittle. Therefore, the presence and mobility of dislocations directly enable and increase ductility. Thus, A is correct.

Step 2: Evaluate Reason R.
The theoretical shear strength required to move entire planes of atoms simultaneously is much higher than the actual stress required to deform a metal. Dislocations allow deformation to occur one row of atoms at a time, requiring much lower stress. Thus, R is correct.

Step 3: Establish the connection.
Ductility increases because dislocations provide a low-energy pathway for atomic movement (as stated in R). This explains why metals can stretch and deform significantly under relatively low loads. Thus, R explains A.