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Metals and alloys

Which correctly describes recovery, which occurs when a cold-worked metal is heated to a temperature too low for recrystallization?

  1. ADislocations rearrange and internal stresses fall, but hardness and strength drop only slightly
  2. BNew strain-free grains form and replace the worked ones, the dislocation density falls, and hardness drops greatly
  3. CGrain boundaries migrate and the grains coarsen to several times their size, so hardness and strength drop greatly
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Answer: A. Dislocations rearrange and internal stresses fall, but hardness and strength drop only slightly

Recovery is the process in which dislocations introduced by working annihilate or rearrange into lower-energy configurations (polygonization, which forms low-angle grain boundaries). Since the total number of dislocations does not drop much, hardness and strength fall only slightly, but internal (residual) stresses are reduced, and electrical resistivity recovers well as point defects are eliminated. Stress-relief annealing, used to remove only the stresses while keeping the shape of a worked part, exploits recovery. The formation of new strain-free grains, with a large drop in dislocation density and hardness, is recrystallization, which occurs at higher temperatures. Grain enlargement is grain growth, which follows recrystallization; both are stages distinct from recovery.

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