Basics of recrystallization
Plastic deformation of a material at room temperature results in an increase in energy content. The energy growth becomes apparent in such a cold deformation through an increase in temperature as well as an increase in the lattice imperfection density. The further the degree of deformation rises, the more lattice defects, so-called dislocations, are created. These move on so-called slip planes during plastic deformation, but can be prevented from further movement by grain boundaries or immobile dislocations. This causes the sliding resistance to grow, the material becomes progressively firmer and work hardening occurs, which increases with the degree of deformation. From a certain degree onward, damage to the material can occur, which becomes noticeable through the formation of cracks. Through recrystallization, these dislocations can be recovered and thus the properties of the original workpiece can be restored. This makes it possible to further deform the workpiece.
During recrystallization, the material regains its original mechanical properties by breaking down the dislocations. For this purpose, the energy content is reduced. To achieve this, the formation of new crystallites with few dislocations is stimulated.
In order for the crystallite acting as a nucleus to grow into the dislocation-rich structure, it must be of sufficient size. The energy required for the grain boundary enlargement comes on the one hand from inside through dislocation degradation, and on the other hand through heat energy supplied from outside. This is done by heat treatment, so-called recrystallization annealing.
Due to polygonization, subgrains are already formed during crystal recovery. These are needed as nuclei for recrystallization. Thermally activated climbing allows the associated subgrain boundaries to grow together. The orientation difference of the enlarging nuclei subsequently widens. If this procedure is repeated, the size of the nucleus increases as well as the orientation difference. If these become large enough compared to the adjacent structure, the nucleus becomes capable of growth.
Factors influencing the resulting microstructure
Which microstructure is created during recrystallization depends on several factors. The most important ones are the following:
- As the degree of deformation increases, the number of subgrains formed during polygonisation grows. The size, however, continues increasing. This leads to the microstructure becoming increasingly fine-grained.
- The annealing temperature and time have a direct influence on the growth of the nuclei. If the temperature or time increases, the size of the grains also increases.
- The melting temperature of the workpiece has a direct effect on the degradation of the dislocations. The higher the melting temperature and thus the stronger the atomic bonds of the material, the slower the dislocation processes take place. A recrystallization temperature of 40 % to 50 % of the absolute melting temperature can be derived from this.
At high degrees of deformation and recrystallization temperatures, a special case of giant grain formation can occur. In this case, several adjacent grains of the same orientation grow together. This creates a structure of so-called giant grains. This has the effect of reducing the toughness of the material and is therefore undesirable. For this reason, temperatures and degrees of deformation in this range are avoided during recrystallization annealing.
Applications of recrystallization
The changes in mechanical properties after forming can be reversed with the help of recrystallization. Recrystallization also plays a role in so-called hot forming, i.e. forming processes above the recrystallization temperature. The material is recrystallized during such processes, but solidification does not occur at all. Forming processes below the recrystallization temperature are accordingly called cold forming.