Recrystallization and its impact on tissue performance

　　1. Recrystallization Process

　　When deformed metal is heated at a higher temperature, due to the increased atomic diffusion capacity, the elongated (or flattened) and fractured grains are reborn and grow into new uniform, fine equiaxed crystals through nucleation and growth. This process is called recrystallization. After recrystallization of the deformed metal, the strength and hardness of the metal are significantly reduced, while the plasticity and toughness are greatly improved, and the work hardening phenomenon is eliminated. At this time, all internal stresses disappear, and the physical and chemical properties basically return to the level before deformation. The crystal lattice type of the new grains generated by recrystallization is the same as the crystal lattice type before and after deformation.

 

　　2. Recrystallization Temperature

　　The temperature at which recrystallization occurs in deformed metals is a temperature range, not a constant temperature. The generally mentioned recrystallization temperature refers to the lowest recrystallization temperature (T再), which is usually expressed as the lowest temperature at which a metal with a large amount of cold plastic deformation (70% or more) can be completely recrystallized after heating for one hour. The lowest recrystallization temperature has the following relationship with the melting point of the metal:

 

T再 = (0.35～0.4)T Melting Point

 

　　The temperature unit in the formula is absolute temperature (K). The lowest recrystallization temperature is related to the following factors:

　　(1) Pre-deformation The relative deformation amount of plastic deformation before the recrystallization of the metal is called pre-deformation. The greater the pre-deformation, the more crystal defects in the metal, the more unstable the structure, and the lower the lowest recrystallization temperature. When the pre-deformation reaches a certain size, the lowest recrystallization temperature of the metal tends to a certain stable value.

　　(2) Melting Point of Metal The higher the melting point, the higher the lowest recrystallization temperature.

　　(3) Impurities and Alloying Elements Impurities and alloying elements, especially high-melting-point elements, hinder atomic diffusion and grain boundary migration, and can significantly increase the lowest recrystallization temperature. For example, the lowest recrystallization temperature of high-purity aluminum (99.999%) is 80℃, while the lowest recrystallization temperature of industrial pure aluminum (99.0%) is increased to 290℃.

　　(4) Heating Rate and Holding Time Recrystallization is a diffusion process that requires a certain amount of time to complete. Increasing the heating rate will cause recrystallization to occur at a higher temperature, while a longer holding time will result in a lower recrystallization temperature.

 

　　3. Grain Size After Recrystallization

　　Grain size affects the strength, plasticity, and toughness of metals, so the control of grain size after recrystallization is very important in production, especially for steels and alloys without phase transformation. The main factors affecting the grain size after recrystallization annealing are heating temperature and pre-deformation.

 

 

　　(1) Heating Temperature The higher the heating temperature, the stronger the atomic diffusion capacity, the easier the grain boundary migration, and the faster the grain growth.

　　(2) Pre-deformation The influence of deformation mainly depends on the uniformity of metal deformation. The more uneven the deformation, the larger the grains after recrystallization annealing. When the deformation is very small, it is not enough to cause recrystallization, and the grains remain unchanged. When the deformation reaches 2～10%, a few grains in the metal are deformed, and the deformation distribution is very uneven. Therefore, there are few nuclei generated during recrystallization, and the grain sizes are very different, which is very conducive to the grain engulfment process and rapid growth, resulting in extremely coarse grains. The degree of deformation that causes abnormal grain growth is called the critical deformation. In production, plastic deformation processing should be avoided as much as possible within the critical deformation range. After exceeding the critical deformation, as the deformation increases, the grain deformation becomes more intense and uniform, and more and more recrystallization cores are generated, so the grains after recrystallization become smaller and smaller. However, when the deformation is too large (approximately ≥90%), the grains may become abnormally large again, which is generally considered to be caused by deformation texture.

 

 

　　Since the recrystallization of the heated metal after plastic deformation can eliminate the work hardening phenomenon and restore the plasticity and toughness of the metal, the recrystallization annealing process is often used in production to restore the ability of the metal to undergo plastic deformation, so that it can continue to be deformed. For example, in the production of iron-chromium-aluminum resistance wires, after cold drawing to a certain degree of deformation, hydrogen-protected recrystallization annealing is carried out to continue cold drawing to obtain finer wire materials. In order to shorten the processing time, the recrystallization annealing temperature actually used is 100℃～200℃ higher than the lowest recrystallization temperature of the metal.