Crystal recovery
Under crystal recovery is defined as the elimination of the effects of plastic deformation (z. B. cold-forming ) without formation of the microstructure ( recrystallization ). Crystal recovery leads to the release of tension. The grain shape and size of the deformed structure are retained. By increasing the temperature, crystal recovery is promoted due to greater atomic mobility. In the case of aluminum, a noticeable crystal recovery occurs after cold forming at room temperature, in the case of steel only at temperatures above approx. 300 ° C. If the recovery takes place during the forming process, one speaks of dynamic recovery - in all other cases of static recovery.
Mechanisms
The crystal recovery is primarily due to two parallel effects, the healing and the rearrangement of zero and one-dimensional lattice defects.
The healing of zero-dimensional defects occurs through diffusion of interstitial atoms in lattice vacancies. One-dimensional errors, i.e. step and screw dislocations , heal through mutual annihilation of dislocations with alien signs. During the rearrangement of zero-dimensional errors, vacancies or interstitial atoms are embedded in the lattice half-planes of the dislocations, whereby the step dislocations can change their position - they climb . By climbing the steps and crosswise sliding of screw dislocations, these can move into an energetically more favorable position, arrange in regular rows and form so-called small-angle grain boundaries. This polygonization process creates sub-grains within a crystallite with a very low density of dislocations in their interior and network-like structures rich in dislocations at their borders.
Influence on the material properties
After crystal recovery, materials show higher ductility . As the temperature rises, recovery processes in the structure are increasingly favored. This is reflected in the mechanical properties such. B. by a decrease in hardness and tensile strength . If the temperature and / or the degree of deformation are increased further, recrystallization processes , combined with a complete new structure, set in.
The stacking fault energy of a material has a great influence on the extent of crystal recovery. In the case of materials with a low stacking fault energy, recrystallization occurs very quickly. In the case of copper, for example, the reduction in hardness due to recovery without recrystallization is practically immeasurable. Aluminum, on the other hand, has a high stacking fault energy. With pure aluminum, the hardness caused by strain hardening can only be reduced by 40 percent through recovery.
Individual evidence
- ↑ Christoph Broeckmann, Paul Bite: Materials Science I . Institute for Material Applications in Mechanical Engineering at RWTH Aachen University , Aachen 2014, pp. 220–239.