Dynamic Recrystallization
Dynamic recrystallization is a process that occurs in crystals, most frequently in metals, when they are plastically deformed. It has a profound effect on the structure and properties of the material and these effects can often be seen in metal objects that have been worked in some way prior to use.
Dynamic recrystallization is the process by which recrystallized and rapidly growing grains, produced during a deformation process, replace the original grains that are strained or distorted. This process is thought to result from the formation of new crystallites, which upon the passage of time, due to their higher mobility, induce a rapid reordering of the original grain boundary network. During the process of dynamic recrystallization, grains become equiaxed, nearly isotropic and uniform in size, as the grain boundary during the course of recrystallization, reorient so that grains develop favored orientations over time. The grain growth process can exhibit a range of morphologies, such as cellular, cellular-dendritic, dendritic and net-like growth patterns.
During dynamic recrystallization, grain refinement occurs both due to a decrease in the average grain size (or grain size distribution) and to a decrease in interlinking between the grains, as evidenced by a reduction in grain boundary area. Reducing the grain boundary area serve to reduce the energy associated with the amount of surface area in a material, i.e. reducing the sum of grain boundary areas which leads to a decrease in surface energy, thus increasing the materials strength and other physical properties. Grain refinement due to dynamic recrystallization can also improve the materials hardness, yield strength, fatigue characteristics and thermal stability.
The dynamic recrystallization process occurs when grains have been subjected to mechanical or thermal stresses, or by increasing the temperature of the material to a temperature where a phase change or plastic deformation may occur. Dynamic recrystallization is applicable in a variety of different materials such as metals, alloys, semiconductors, polymers and ceramics. It is a common practice in the metals industry, used to alter the grain size and shape of materials, such as those used in the automobile, aerospace and shipbuilding sectors. The most commonly used method to achieve dynamic recrystallization is to impart a certain energy to the material to generate new grains, by either hot deformation or cold working techniques. In hot deformation, materials such as steel and aluminum are heated to temperatures that exceed their recrystallization temperatures (the temperatures at which dynamic recrystallization occurs). The material is then plastically deformed under pressure. Hot deformation is the preferred method for larger shapes and for larger grain sizes.
In cold working, the material is deformed at room temperature or lower temperatures. This type of deformation can result in a more localized grain refinement, thus allowing certain properties such as strength and ductility to be improved. Cold working can also be used to create new grain boundaries, as well as to increase existing ones.
There are several factors that affect dynamic recrystallization. These include the rate of strain, temperature, the rate of cooling, strain hardening and hardening conditions, the source of stored energy, strain rate sensitivity and the types of deformation processes used. These factors can all contribute to the dynamic recrystallization process, either facilitating it or impeding it, depending on the particular circumstances.
In conclusion, dynamic recrystallization is an important physical process that can have significant effects on the structure and properties of materials, especially metals. The process enables the production of high-quality materials with improved strength and uniform grain sizes. It is important to understand the factors that can affect dynamic recrystallization, in order to optimize the production of desired material properties.
