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: Notice the gap between theory and reality.
If a crystal were perfect, sliding one entire atomic plane over another would require an enormous theoretical shear stress, yet real metals deform plastically at stresses that are often a thousand times lower.
Step 2: Explain the gap using a simple analogy.
Moving an entire heavy rug across a floor at once is hard, but pushing a small wrinkle across the rug is easy, and the wrinkle achieves the same net displacement bit by bit. A dislocation is exactly this kind of wrinkle in the crystal lattice, a line defect that lets atoms shift one row at a time instead of all at once.
Step 3: Rule out the distractors.
Shear stress is the driving force, not the defect itself, stacking faults and impurities are different kinds of imperfections that actually tend to block or slow down slip rather than enable it. It is specifically the glide, or movement, of dislocations through the lattice that produces the observed plastic slip.
\[ \boxed{\text{Dislocations}} \]