The phenomenon in which new strain-free grains nucleate and grow when a metal storing deformation strain is heated. Strength decreases and ductility is restored.
Japanese name: 再結晶
Recrystallization appears in 6 practice questions across 4 topics, including Heat treatment and metallic materials, Metals and alloys, Magnetic materials.
Field: Metals and alloys
The driving force for recrystallization is the energy of dislocations stored by working. On heating, in regions of high strain, some of the subgrains formed by recovery grow faster than their surroundings and become new grains with high-angle boundaries (recrystallization nuclei). These grains spread by migrating their boundaries into the surrounding dislocation-rich structure, until the whole deformed structure is eventually replaced by grains with little strain. The recrystallized fraction follows an S-shaped curve with time, with a slow start, rapid progress and a slow finish, and is often described by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation X = 1 − exp(−ktⁿ).
The recrystallization temperature is usually defined as the temperature at which recrystallization is complete in 1 hour, and a rough guide is 0.3–0.5 of the melting point in absolute temperature. It is lower for higher purity; for example, high-purity copper recrystallizes at about 120 °C, about 0.3 of its melting point. Small amounts of solute hinder boundary migration and raise the recrystallization temperature substantially. Heavier working increases the stored energy and the number of nuclei, which lowers the recrystallization temperature and makes the recrystallized grains finer. The time to complete recrystallization depends strongly on temperature, and a small increase in temperature shortens it greatly.
If heating continues after recrystallization is complete, large grains consume small ones to reduce grain-boundary energy. Its driving force is grain-boundary energy, making it a separate stage from recrystallization, which is driven by stored energy. The phenomenon in which only some grains become abnormally large is called secondary recrystallization (abnormal grain growth); in grain-oriented electrical steel, normal grain growth is suppressed by precipitates and only Goss-oriented grains are allowed to recrystallize secondarily, aligning the orientation. Freshly recrystallized grains are also often biased toward particular orientations (recrystallization texture), which affects sheet formability.
Hot working and cold working are distinguished by whether working takes place above the recrystallization temperature. In hot rolling and forging, recrystallization (dynamic and static) occurs during or right after deformation, turning the coarse cast structure into fine, equiaxed grains. In cold-working processes, material that has work hardened and lost elongation is recrystallized by intermediate annealing to restore ductility before the next step. Changing the temperature and time of the final anneal adjusts the size of the recrystallized grains, so strength and formability can be tailored through the Hall–Petch relationship.
Answer: A. New, nearly strain-free grains form; strength falls and ductility is restored
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. Because the dislocation density drops greatly, strength and hardness decrease and ductility is restored. The greater the cold work, the larger the stored energy, so the recrystallization temperature is lower. A rough guide for the recrystallization temperature is 0.3–0.5 times the melting point (in absolute temperature), but it varies with the amount of cold work and the purity. If heating continues after recrystallization, grain growth proceeds to reduce the grain boundary energy.
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.
A rough guide is 0.3–0.5 of the melting point in absolute temperature. However, it varies greatly with purity, degree of working and heating time, so the value for each material needs to be confirmed by experiment.
No. Recrystallization replaces highly strained grains with less strained ones within the same phase. It is different from changes that alter the crystal structure, such as the transformation of steel to austenite.
It is the minimum degree of working needed for recrystallization, often a few %. Annealing after working near this level produces few nuclei, so the grains become very coarse, causing surface roughening (orange peel) and a loss of strength.
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 …The formation of new strain-free grains, with a large drop in dislocation density and hardness, is recrystallization, which occurs at higher temperatures.
Which correctly describes recovery, which occurs when a col…Hot and cold working are distinguished not by whether the temperature is high or low but by whether working takes place above the recrystallization temperature.
Taking the recrystallization temperature as roughly 0.4 tim…The amount of cold work also affects the recrystallization temperature and the grain size after recrystallization.
A copper wire 10.0 mm in diameter is drawn down to 8.0 mm. …Grain-oriented electrical steel is given coarse grains in the Goss orientation, with {110} planes parallel to the sheet surface and <001> along the rolling direction, by secondary recrystallization using inhibitors (fine MnS or AlN precipitates).
Grain-oriented electrical steel (Fe with about 3% Si) used …Recovery Dislocation Work hardening Magnetocrystalline anisotropy Eddy-current loss Shrinkage cavity Brass Columnar grains Solid-solution strengthening Stainless steel