A copper wire becomes progressively harder to bend each time it is flexed back and forth. What is the microstructural cause?
AThe crystal structure transforms from FCC to a denser phase under repeated stress, increasing resistance to deformation
BGrain boundaries multiply as the wire is bent, creating more obstacles to dislocation motion
CDislocation density increases with each bend, and the dislocations tangle and impede further dislocation glide
DMicrocracks form at the bend and act as pinning sites that simulate increased stiffness
Work hardening is caused by an increase in dislocation density. Every plastic deformation event moves dislocations, and moving dislocations can interact, tangle, and lock. At dislocation densities of 10¹⁰–10¹² cm/cm² (versus ~10⁶ in an annealed metal), dislocations encounter each other constantly, forming tangles, forest dislocations, and Lomer-Cottrell locks that resist further glide. The increased stress required to move dislocations past all these obstacles is what we observe as hardening. The crystal structure (FCC for copper) does not change.
Question 2 Multiple Choice
A manufacturer cold-works a copper alloy to increase yield strength, but the parts have unacceptable residual stresses from the deformation process. She heats them to a moderate recovery temperature. What should she expect?
AResidual stresses are substantially reduced and electrical conductivity is largely restored, while yield strength remains mostly preserved
BResidual stresses are eliminated and yield strength fully returns to the pre-cold-work (annealed) level
CNew strain-free grains nucleate throughout the material, eliminating both work hardening and residual stress
DYield strength increases further because recovery rearranges dislocations into higher-energy configurations
Recovery involves dislocation rearrangement within existing grains — dislocations of opposite sign annihilate, and remaining dislocations organize into low-energy subgrain boundaries (polygonization). This substantially reduces residual stresses (the main goal here) and restores electrical conductivity that was degraded by dislocation scattering. However, because the overall dislocation density drops only modestly and grain structure is unchanged, yield strength decreases only slightly. Option C describes recrystallization, which is a different process that nucleates new grains and does reverse most of the work hardening.
Question 3 True / False
Recovery reduces residual stresses and partially restores electrical conductivity in a cold-worked metal without significantly changing grain size or reversing most of the work-hardened strength.
TTrue
FFalse
Answer: True
This is the defining characteristic of recovery that distinguishes it from recrystallization. Recovery occurs at moderate temperatures (roughly 30–50% of melting point in Kelvin) and involves rearranging dislocations within existing grains — not nucleating new ones. The result is a meaningful reduction in residual stress and recovery of electrical conductivity (both were degraded by high dislocation density scattering), while the grain structure and most of the elevated dislocation density responsible for hardening remain. This makes recovery the right thermal treatment when you need stress relief without sacrificing the cold-worked strength.
Question 4 True / False
Recovery and recrystallization are two names for the same process of restoring a cold-worked metal's properties through heating.
TTrue
FFalse
Answer: False
They are distinct sequential stages. Recovery occurs first, at lower temperatures: dislocations rearrange and partially annihilate within existing grains, relieving residual stresses and restoring conductivity, but grain structure and most of the hardening are preserved. Recrystallization occurs next, at higher temperatures: entirely new strain-free grains nucleate and grow, consuming the deformed microstructure. Recrystallization nearly eliminates the work-hardened strength and dramatically restores ductility. Confusing the two leads to errors in annealing process design — for example, heating to a recovery temperature when recrystallization was intended, or vice versa.
Question 5 Short Answer
Why does cold working increase a metal's strength, and why doesn't recovery undo that strength increase the way recrystallization does?
Think about your answer, then reveal below.
Model answer: Cold working increases strength by dramatically increasing dislocation density. The dislocations tangle, forming obstacles that impede further dislocation glide — more stress is required to move them, so yield strength rises. Recovery only partially reduces this effect: it allows dislocations of opposite sign to annihilate and rearranges the remainder into organized subgrain boundaries, but the grain structure and total dislocation density remain largely intact. Recrystallization goes further: it nucleates entirely new strain-free grains that grow by consuming the deformed material, resetting the dislocation density to near-annealed levels and eliminating most of the hardening.
The key distinction is whether new grains form. Recovery is a within-grain rearrangement process; it cleans up the most energetically costly dislocation configurations (tangles and opposite-sign pairs) without changing the grain boundaries. Recrystallization is a grain-scale phase transformation: new low-dislocation-density grains grow at the expense of old high-dislocation-density grains, fundamentally resetting the microstructure. This is why recovery is used industrially when stress relief is needed but the cold-worked strength must be preserved — for example, in spring-temper copper alloys or in cold-drawn wire that will be used as-drawn.