In physical metallurgy and heat treatment, the temperature at which new, strain-free grains nucleate and grow to replace deformed grains during recovery/annealing is termed what?

Introduction / Context:

Cold working introduces a high density of dislocations and stored energy into metals, increasing strength and reducing ductility. Subsequent heat treatment can reverse these effects through recovery and recrystallisation, restoring ductility by forming new, strain-free grains. Recognizing the temperature associated with this transformation is essential for process design.

Given Data / Assumptions:

• Material has been plastically deformed (cold worked).
• Heating occurs below the melting point.
• Observation concerns the onset of new grain formation.

Concept / Approach:

The recrystallisation temperature is the temperature (often expressed as a range) at which new, equiaxed, strain-free grains nucleate and grow to replace the deformed microstructure within a specified time (commonly one hour in definitions). It depends on prior cold work, purity, alloying, and grain size, frequently approximated as 0.3–0.5 T_m (absolute) for many metals.

Step-by-Step Solution:

Verification / Alternative check:

Metallographic studies show nucleation at grain boundaries and high-energy sites; mechanical tests confirm ductility recovery near the recrystallisation range.

Why Other Options Are Wrong:

• Upper/Lower critical temperature: refer to phase boundaries in steels (A₃, A₁), not to cold-work recovery.
• Eutectic temperature: invariant reaction temperature in phase diagrams, unrelated to recrystallisation.
• Solutionizing temperature: heat-treatment step for dissolving precipitates in alloys (e.g., aluminum), different purpose.

Common Pitfalls:

• Assuming a single fixed value; it depends on prior deformation and composition.
• Confusing recrystallisation with grain growth; growth often follows after recrystallisation completes.

Final Answer:

recrystallisation temperature