Question

In which crystal structure is dislocation motion most difficult due to limited slip systems?

• A. FCC
• B. BCC
• C. HCP
• D. SC

Hint:

Remember: HCP structures are the least ductile among common crystal systems due to limited active slip systems — this makes them harder but more brittle.

Answer 1

The Correct Option is C

Dislocation motion — which is the primary mechanism for plastic deformation in crystalline materials — depends heavily on the availability of slip systems, which are combinations of slip planes and slip directions.
- FCC (Face-Centered Cubic) structures have 12 highly symmetric slip systems, making dislocation motion very easy and these metals highly ductile (e.g., aluminum, copper).
- BCC (Body-Centered Cubic) structures have more slip systems than FCC but require higher stresses for slip because the atoms are not closely packed.
- HCP (Hexagonal Close-Packed) structures have very few active slip systems (typically 3 to 6), especially at room temperature. This makes dislocation motion difficult, leading to poor ductility and higher brittleness.
- SC (Simple Cubic) is rare in metallic crystals and is mainly theoretical.
Therefore, HCP structures offer the most resistance to dislocation motion due to their limited slip systems.