Question

Dislocation climb occurs due to

• A. Shear stress
• B. Vacancy diffusion
• C. Applied magnetic field
• D. Grain boundary motion

Hint:

- Dislocation Glide = Driven by Shear Stress (Fast, happens at all temperatures). - Dislocation Climb = Driven by Vacancy Diffusion (Slow, requires high temperatures to activate mass transport).

Answer 1

The Correct Option is B

Concept: Dislocation movement drives plastic deformation in materials and occurs via two primary mechanism paths:
• Glide (Conservative motion): The dislocation line glides within its own defined slip plane containing both the dislocation line and its Burgers vector. This process requires only standard mechanical shear stresses and can happen at low temperatures.
• Climb (Non-conservative motion): An edge dislocation moves *perpendicular* to its slip plane. This requires either growing or shrinking the extra half-plane of atoms, which can only happen if atoms are added to or removed from the edge of the half-plane.

Step 1: Mechanism of the climb process.
Let us analyze how an edge dislocation shifts vertically out of its designated slip plane:
• To climb upward by one atomic spacing, the bottom row of atoms on the extra half-plane must be removed.
• This removal happens when nearby lattice vacancies migrate via thermal diffusion and join the bottom edge of the extra half-plane, effectively absorbing the atoms.
• Conversely, to climb downward, atoms must diffuse to the edge of the extra half-plane, which leaves vacancies behind that diffuse out into the bulk crystal. Because this mass transport relies heavily on atomic jumps, dislocation climb is a thermally activated process that occurs at elevated temperatures (typically above \(0.4 T_m\), where \(T_m\) is the absolute melting temperature). It is the primary microstructural mechanism behind high-temperature creep deformation.

Step 2: Conclusion.
Since mass transport via vacancy diffusion is the fundamental driver for this non-conservative motion, Option (B) is the correct choice.