Question

Which one of the following options is correct?
After cold-working of a metallic specimen, during the recovery stage, its electrical conductivity:

• A. Always increases
• B. Always decreases
• C. Can increase or decrease
• D. Remains unaffected

Hint:

Recovery removes excess point defects and lowers dislocation strain without changing grain structure, so electron scattering drops and conductivity rises.

Answer 1

The Correct Option is A

Step 1: Recall what cold working does to a metal.
Cold working (plastic deformation below the recrystallization temperature) pushes a huge number of new dislocations into the metal and also creates excess point defects such as vacancies and interstitials. All of these lattice defects scatter conduction electrons, so a heavily cold-worked metal has a noticeably lower electrical conductivity (higher resistivity) than the same metal in its annealed state.

Step 2: Recall what happens on annealing after cold work.
Annealing a cold-worked metal proceeds through three stages in order: recovery, recrystallization, and grain growth. Each stage removes some kind of stored lattice defect and imperfection, and each stage therefore also removes some of the electron scattering that was lowering the conductivity.

Step 3: Focus on what happens specifically during recovery.
Recovery is the lowest-temperature, earliest stage. During recovery, the excess point defects (vacancies and interstitials) annihilate, and dislocations rearrange themselves into lower energy configurations such as sub-grain boundaries (polygonization), without any new strain-free grains forming yet and without a major drop in hardness.

Step 4: Connect defect removal to conductivity.
Because recovery removes point defects and lowers the effective density of scattering centres for conduction electrons, resistivity drops and electrical conductivity rises during this stage. This happens for essentially all metals and all degrees of cold work, since removing scattering centres can only help electron transport, never hurt it.

Step 5: Analyze the options.
(A) Always increases: matches the physical mechanism; removing point defects during recovery always improves conductivity. Correct.
(B) Always decreases: the opposite of what happens; defect annihilation cannot make scattering worse. Incorrect.
(C) Can increase or decrease: conductivity change during recovery is not sample-dependent in direction, it is consistently an increase. Incorrect.
(D) Remains unaffected: conductivity is in fact quite sensitive to point defect density, so it does change. Incorrect.

Final Answer:
During the recovery stage of annealing, electrical conductivity always increases as point defects annihilate and internal stresses relax.
\[ \boxed{\text{Always increases}} \]