Question

The imperfections responsible for slip are

• A. Shear stress
• B. Stacking faults
• C. Impurities
• D. Dislocations

Hint:

Think of the analogy: moving a heavy carpet. Trying to slide it all at once (perfect crystal) is very hard. Creating a small wrinkle and pushing it across (dislocation motion) is much easier. Dislocations are the "wrinkles" in the crystal lattice.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Slip is the primary mechanism of plastic (permanent) deformation in crystalline materials. It involves the sliding of atomic planes past one another. The question asks for the crystal imperfection that enables this process.

Step 2: Detailed Explanation:

• The theoretical shear stress required to slide an entire perfect plane of atoms over another is very high. However, real crystalline materials deform at much lower stresses.

• This discrepancy is explained by the presence of

(D) Dislocations, which are linear (one-dimensional) defects in the crystal lattice.

• Instead of moving an entire plane at once, plastic deformation occurs by the sequential movement of these dislocations through the crystal. This process requires much less force, analogous to moving a large rug by creating a wrinkle and propagating it across the floor. Therefore, dislocations are the imperfections fundamentally responsible for enabling slip.

• (A) Shear stress: This is the external force that drives the motion of dislocations; it is not the imperfection itself.

• (B) Stacking faults: These are two-dimensional defects (errors in the stacking sequence of atomic planes). They interact with and can impede dislocations, but they are not the primary mechanism of slip.

• (C) Impurities: These are point defects (zero-dimensional) that generally hinder dislocation motion, leading to solid solution strengthening. They make slip more difficult, not easier.

Step 3: Final Answer:
Dislocations are the linear imperfections whose movement through the crystal lattice is responsible for the process of slip.