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

Concept: Electrical conductivity is a defect counter.
Free electrons in a metal get scattered by anything that breaks the perfect periodicity of the lattice: phonons (thermal vibration), dislocations, vacancies, interstitials, and grain boundaries. More scattering centres mean a shorter mean free path for electrons, which means lower conductivity. Cold working is basically a defect-injection process, so it always drags conductivity down from its annealed value.

Step 1: List what recovery actually removes.
Recovery happens at a low enough temperature that grains do not yet form new strain-free regions (that is recrystallization's job) and grain size does not yet coarsen (that is grain growth's job). What recovery does instead is let vacancies and interstitials, created in huge excess by the cold work, diffuse and annihilate, and let tangled dislocations rearrange into neater, lower-energy walls.

Step 2: Connect this directly to scattering.
Point defects such as vacancies are strong electron scatterers per defect, even stronger than dislocations in many metals. Removing a large fraction of them during recovery removes a large fraction of the extra scattering that cold work had added.

Step 3: Decide the direction, not just the size, of the change.
Since recovery is purely a defect-removal process (it never adds new defects), the scattering centre count can only go down during this stage, for any metal and any starting amount of cold work. A count that can only go down means resistivity can only go down, and conductivity can only go up. There is no scenario in normal recovery where conductivity falls or where it depends on the particular metal.

Step 4: Compare with the four options.
"Always decreases" and "remains unaffected" both contradict the fact that defect density is dropping. "Can increase or decrease" wrongly suggests the direction is uncertain, when in fact recovery is one-directional in what it removes. Only "always increases" matches a process that strictly removes scattering centres.

Step 5: Conclude.
\[ \boxed{\text{Always increases}} \]