The process, preceding recrystallization when a cold-worked metal is heated, in which dislocations annihilate and rearrange and internal stresses and electrical resistivity decrease. Hardness changes little.
Japanese name: 回復
Recovery appears in 6 practice questions across 4 topics, including Metals and alloys, Materials processing and joining, Heat treatment and metallic materials.
Field: Metals and alloys
A cold-worked metal stores many dislocations and point defects. On heating, point defects such as vacancies first migrate and disappear, and then dislocations move by climb and cross-slip, so that dislocations of opposite sign annihilate each other or line up into low-energy arrangements. The process in which dislocations align like walls into low-angle boundaries, dividing grains into small subgrains, is called polygonization. The shape and orientation of the grains themselves remain as they were after working, and the fact that no new grains form is what distinguishes recovery from recrystallization.
During recovery, part of the stored energy is released, residual stress falls substantially, and electrical resistivity approaches its value before working. On the other hand, the total number of dislocations does not decrease much, so the drop in hardness and strength is small and much of the strength gained by work hardening remains. Recovery is fastest at the start and slows over time; it does not suddenly take off after an incubation period as recrystallization does. How far recovery goes depends not only on temperature and time but also on the material's stacking fault energy.
In metals with high stacking fault energy, such as aluminum and α-iron, dislocations hardly dissociate and cross-slip and climb are easy, so recovery proceeds readily. Even during hot working, recovery balances work hardening and a stable subgrain structure develops (dynamic recovery). In metals with comparatively low stacking fault energy, such as copper, brass and austenitic stainless steel, dislocations dissociate widely and move with difficulty, so recovery does not progress very far at a given temperature. Much of the stored energy therefore remains in the metal, and recrystallization, for which that energy is the driving force, occurs more readily.
Recovery is put to use when only internal stress needs to be lowered while the shape and strength of a part are kept. Examples are stress-relief annealing after welding or bending, and low-temperature annealing to prevent season cracking of brass. For aluminum alloys, tempers that are work hardened and then partially annealed to restore a little ductility are designated H2x, balancing strength and formability. If the annealing temperature is too high or the time too long, recrystallization begins and the material softens greatly, so the temperature and time of the treatment must be controlled carefully.
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.
Answer: C. A material with a high yield stress and a low Young's modulus
Even after plastic deformation, stresses of about the yield stress remain across the section of a bent sheet, and when the load is removed it springs back by the corresponding elastic strain (roughly σ/E). Springback is therefore larger the higher the yield stress and the lower the Young's modulus. High-strength steel sheet, with the same Young's modulus but higher strength, and aluminum alloy sheet, with a Young's modulus about one-third that of steel, show large springback and are harder to bring to shape. The option of a soft and stiff material is the reverse, and the option saying it is set only by the working speed is also wrong. Countermeasures include overbending by the amount of recovery and pressing the bend firmly in the die so that the bend region deforms plastically.
In recovery only the annihilation and rearrangement of dislocations take place, and the grains remain as they were after working. In recrystallization, new strain-free grains form and replace the deformed structure, so hardness drops greatly.
Point defects such as vacancies and dislocations added by working scatter electrons and raise the resistivity. During recovery these decrease, and point defects in particular disappear readily, so scattering falls and so does resistivity.
It is recovery that takes place during deformation at high temperature, as in hot working. Because the generation and annihilation of dislocations balance, the stress does not rise much as deformation continues and approaches a nearly constant value.
Recovery is the process in which dislocations introduced by working annihilate or rearrange into lower-energy configurations (polygonization, which forms low-angle grain boundaries).
Which correctly describes recovery, which occurs when a col…Countermeasures include overbending by the amount of recovery and pressing the bend firmly in the die so that the bend region deforms plastically.
When a sheet is bent and removed from the die, its bend ang…When a cold-worked metal is heated, recovery occurs first, as dislocations rearrange and internal stresses fall; at higher temperatures, recrystallization follows, with new, nearly strain-free grains nucleating and growing, driven by the stored strain energy.
Which statement about recrystallization when a cold-worked …For lead the guide is 0.4 × (327 + 273) = 240 K, about −33 °C, so rolling at room temperature (25 °C) counts as hot working; recovery and recrystallization proceed during working and almost no work hardening remains.
Taking the recrystallization temperature as roughly 0.4 tim…In hot working, recovery and recrystallization take place during working, so almost no work hardening remains, and the option citing large residual work hardening is wrong.
When carbon steel parts of the same shape are made by casti…Recrystallization Dislocation Work hardening Springback Young's modulus Shrinkage cavity Creep Brass Columnar grains Solid-solution strengthening