5: Thermal Activation

Thermal Activation in Dislocation Motion

• Thermal activation reduces the required applied stress to overcome energy barriers.

• Thermal energy helps atoms to:

  • Glide through crystal lattices.

  • Recombine partial dislocations.

  • Facilitate screw dislocation cross-slip to another plane.

  • Diffuse atoms/vacancies, enabling edge dislocation climb.

Mechanisms of Dislocation Motion

• Single Atom Impact:

  • Motion of one atom can affect an entire dislocation line under certain mechanisms.

  • Not all atoms on a dislocation line must be thermally activated; some mechanisms leverage the thermal vibrations of a few atoms.

Example: Double Kink Dislocation Motion

• Body-Centered Cubic (BCC) Metals:

  • Screw dislocations are harder to move compared to edge dislocations due to the out-of-plane character of partials.

  • For a right-hand screw dislocation:

    • It advances by overcoming energy barriers that allow segments to shift to new lattice positions through thermal activation.

    • Once a segment jumps the barrier, it's pulled over in the perpendicular direction to the dislocation line.

    • This creates edge dislocation characteristics that can move under lower stress levels.

Creep in Metals

• Definition: Plastic deformation at a constant load below yield stress but accumulates over time.

• Temperature Impact:

  • Creep significance arises around 30% of melting temperature.

  • Homologous Temperature: Ratio of the current temperature to the melting temperature.

• Example Calculations:

  • Steel:

    • Melting Temp: 1500°C (1773 K), thus 30% is 532 K (260°C).

  • Aluminum:

    • Melting Temp: 660°C (933 K), thus 30% is 7°C (below room temperature), indicating that creep can be critical at room temperature for aluminum alloys.

Applications Beyond Metals

• Thermal activation processes occur in ceramics and polymers as well.

• Ceramics Creep Resistance:

  • Creep becomes significant at 40-50% of their melting point, enhancing their utility in high-temperature applications.

• Diffusion Mechanisms:

  • Creep strain due to:

    • Bulk atomic diffusion.

    • Grain boundary diffusion (amorphous regions allow easier diffusion).

Dislocation Glide and Creep

• Thermal activation facilitates dislocation glide and leads to creep strains.

• Creep regime is influenced by:

  • Temperature.

  • Time taken to overcome barriers, particularly for dislocations where climb is necessary to navigate obstacles.

Creep Strain Diagram Overview

• Creep Curve: Strain vs. Time.

  • Stages:

    1. Elastic Strain: Instantaneous response to load.

    2. Primary Creep: Activation of dislocations begins; interactions lead to strain hardening (decreasing strain rate).

    3. Secondary Creep: Balance of recovery (dislocation annealing) and strain hardening occurs.

    4. Tertiary Creep: Localization of strain leads to failure (necking).

Conclusion

• Understanding thermal activation processes contributes significantly to knowledge of material behavior under stress and temperature.