Question

The mechanism of creep for a single crystal as depicted in the schematic is 

 

• A. Nabarro-Herring creep
• B. Grain boundary sliding
• C. Dislocation creep
• D. Coble creep

Hint:

To distinguish between diffusional creep mechanisms, remember the path: - {Nabarro-Herring creep} \(⇒\) Diffusion {through the lattice} (bulk). - {Coble creep} \(⇒\) Diffusion {along the grain boundaries}. The schematic shows a single crystal, immediately pointing to a lattice-based mechanism.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Creep is the time-dependent plastic deformation of a material under a constant stress, typically at high temperatures. Different mechanisms can be responsible for creep, including diffusional creep (Nabarro-Herring, Coble) and dislocation creep. The schematic provides visual clues to identify the specific mechanism.
Step 2: Detailed Explanation:
Let's analyze the provided schematic:
System: It is explicitly stated to be a single crystal, which means there are no grain boundaries. Stress: A tensile stress (\(\sigma\)) is applied vertically.
Fluxes: The arrows show two types of flux.
Vacancy Flux (1): Vacancies are shown moving from the horizontal faces (perpendicular to the tensile axis) towards the vertical faces (parallel to the tensile axis).
Mass Flux (2): Atoms (mass) are shown moving in the opposite direction, from the vertical faces under tension to the horizontal faces under compression.
Path: The curved arrows for the fluxes are shown passing through the bulk of the crystal, not along any specific linear feature like a grain boundary.
This process, where stress-induced diffusion of atoms through the crystal lattice leads to elongation in the direction of tensile stress, is the definition of Nabarro-Herring creep.
Now let's evaluate the options:
- (A) Nabarro-Herring creep: Involves vacancy diffusion through the crystal lattice. This perfectly matches the schematic.
- (B) Grain boundary sliding: This mechanism requires a polycrystalline material (multiple grains) and is not depicted. The schematic shows a single crystal.
- (C) Dislocation creep: This mechanism involves the movement of dislocations (climb and glide). The schematic shows atomic diffusion, not dislocation motion.
- (D) Coble creep: This is also a diffusional creep mechanism, but it involves the diffusion of atoms along grain boundaries. Since the schematic is for a single crystal (no grain boundaries), this mechanism is not possible.
Step 3: Why This is Correct:
The schematic illustrates the flow of vacancies and atoms through the bulk of a single crystal under stress, which is the precise definition of Nabarro-Herring creep. The absence of grain boundaries rules out Coble creep and grain boundary sliding.