Question

Which mechanism involves the movement of atoms to form a mirror image of the crystal structure across a plane?

• A. Slip
• B. Climbing
• C. Creep
• D. Twinning

Hint:

- Slip: Atoms slide by discrete steps; crystal orientation remains identical. - Twinning: Atoms shift by fractional steps uniformly; crystal orientation changes to form a perfect mirror image across the twin boundary. [Image comparing slip showing stepped block vs twinning showing a reoriented mirrored region across a twin plane]

Answer 1

The Correct Option is D

Concept: Plastic deformation in crystalline solids takes place through two primary shear mechanisms: Slip and Mechanical Twinning. While slip involves individual dislocations gliding sequentially along specific planes, twinning involves a collective, uniform shifting of atoms within a specific volume.

Step 1: Detailed analysis of the Twinning mechanism.
Mechanical twinning occurs when a homogeneous shear stress acts on a crystal lattice, causing atoms to shift systematically:
• Rather than breaking bonds sequentially to move dislocations over integer atomic distances (as in slip), every atomic plane within the twinned region shifts by a fractional atomic distance relative to its distance from a reference plane.
• This uniform, cooperative movement reorients the crystal lattice within the deformed zone. The reoriented region forms a perfect, symmetrical mirror image of the surrounding unaltered crystal lattice.
• The dividing plane that separates the unaltered crystal matrix from the newly reoriented region is called the twinning plane or composition plane.

Step 2: Comparison with alternative processes.

• Slip: Atoms shift by full atomic distances on a single plane, leaving the orientation of the crystal lattice completely unchanged above and below the slip plane. It does not produce a mirrored structure.
• Climbing: Describes the non-conservative, vertical movement of an edge dislocation out of its slip plane via vacancy diffusion, which does not cause structural mirroring.
• Creep: Refers to slow, time-dependent plastic deformation that occurs under constant stress at high temperatures, driven by a combination of dislocation glide, climb, and grain boundary sliding. Therefore, the mechanism that produces a symmetrical mirror image across a plane is Twinning, matching option (D).