Question

The mechanism of Nabarro-Herring creep involves

• A. Vacancy diffusion through the lattice
• B. Diffusion along grain boundaries only
• C. Dislocation motion
• D. Cross-slip of dislocations

Hint:

Because both Nabarro-Herring and Coble creep strain rates are inversely proportional to grain size (\( 1/d^2 \) and \( 1/d^3 \)), materials with very large grains or single-crystal structures (like turbine blades) are highly resistant to diffusional creep.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the underlying atomic transport mechanism that drives Nabarro-Herring creep in materials at high temperatures.

Step 2: Key Formula or Approach:
Creep is the slow, progressive, and permanent deformation of a material over time under constant stress at elevated temperatures (\( T \gt 0.4 T_m \)).
The strain rate (\( \dot{\epsilon} \)) for Nabarro-Herring creep is given by:
\[ \dot{\epsilon}_{NH} \propto \frac{\sigma \cdot D_L}{d^2} \]
where:
\( \sigma \) is the applied stress,
\( D_L \) is the lattice (bulk) diffusion coefficient, and
\( d \) is the grain size.

Step 3: Detailed Explanation:

• Nabarro-Herring Creep Mechanism: This mechanism occurs at very high temperatures (\( T \gt 0.6 T_m \)) and relatively low stresses.
Under an applied tensile stress, grain boundaries perpendicular to the tensile axis experience tensile stress, which lowers the vacancy formation energy, while boundaries parallel to the tensile axis experience compressive stress.
This creates a vacancy concentration gradient:
- Vacancies migrate through the bulk crystal lattice from regions of high concentration (tensile boundaries) to regions of low concentration (compressive boundaries).
- Atoms migrate in the opposite direction (from compressive to tensile boundaries), resulting in elongation of the grains along the tensile axis.

• Coble Creep Comparison: Diffusion along grain boundaries only (Option B) is the mechanism of Coble creep, which is dominant at lower temperatures because the activation energy for grain boundary diffusion is lower than that for bulk lattice diffusion.
- The strain rate for Coble creep is proportional to \( 1/d^3 \).

• Dislocation Creep: Dislocation motion (Option C) and cross-slip (Option D) are active at higher stresses, where dislocation climb and glide dominate the deformation process.


Step 4: Final Answer:
Therefore, Nabarro-Herring creep involves vacancy diffusion through the lattice, matching Option (A).