Recrystallization

Course of recrystallization in aluminum . New grains formed during crystallization appear light

In metallurgy and crystallography, recrystallization describes the breakdown of lattice defects in the crystallites through the formation of a new structure due to nucleation and grain growth . The reason for the decrease in strength due to recrystallization is the reduction in dislocations . If the recrystallization takes place during the forming process, this is called dynamic recrystallization, after the forming process is completed static recrystallization takes place. A precursor to recrystallization (especially in body-centered cubic metals) is dynamic or static recovery , which leads to a decrease in strength through the rearrangement of lattice defects.

Recrystallization is technically used in recrystallization annealing , which can be used after cold forming to reduce the solidification . For this purpose, the material is heated above the recrystallization temperature.

Recrystallization Mechanism

During recrystallization, new grains form within a crystal. The nucleation of these new grains starts from the points in the structure where most of the dislocations are located. First, so-called sub-grains are formed, which grow together in the course of recrystallization and expand into the environment. Recrystallization takes place in the structure when a deformation has been applied that is above the critical degree of deformation. The critical degree of deformation or critical degree of stretching is an elongation of around five percent for most metals. Below this there are not enough dislocations from which recrystallization nuclei could arise. Even particularly high temperatures cannot cause recrystallization in this case.

If the degree of deformation is just below the critical degree of deformation, only a few recrystallization nuclei form, which can spread largely unhindered. The grain size after the recrystallization is complete is large. If, on the other hand, there is a high degree of deformation due to strain hardening before the start of recrystallization, there are many dislocations and thus also many recrystallization nuclei that quickly stop each other: The grain size is small. The recrystallization temperature, on the other hand, has a negligible influence on the mean grain size with the exception of special cases with particularly small and particularly large degrees of deformation.

Recrystallization temperature

The recrystallization temperature is the temperature at which a material completely recrystallizes within a period of observation. As a rule of thumb, it is often estimated at 40% or 50% of the absolute melting temperature . In the steel can with thermo-mechanical treatment by the micro-alloying elements titanium and niobium to be increased, which as fine particles during hot forming excrete . E.g .: 0.1% niobium increases the recrystallization temperature by 300 K. If forming takes place above the recrystallization temperature, one speaks of hot forming , below this is cold forming or warm forming if the metal is heated but does not exceed the recrystallization temperature .

Giant grain formation

A special case is high recrystallization temperatures with a high degree of deformation at the same time: This creates a structure of many very small grains and individual, significantly larger, so-called giant grains. The reason: the greater the number of recrystallization nuclei, the higher the probability that some adjacent grains will have a crystal lattice with the same orientation. These grains grow together, have a growth advantage due to their size and as a result consume small, neighboring grains. Giant grains are technically undesirable because they reduce the toughness of a material. When performing recrystallization annealing, the area in which giant grains can form is therefore avoided. For aluminum, the critical area of giant grain formation is at degrees of deformation above about 60 percent and a simultaneous recrystallization temperature of over 500 ° C.

literature

G. Gottstein: Physical basics of materials science. 2nd Edition. Springer, Berlin et al. 2001, ISBN 3-540-41961-6 ( Springer textbook ).
FJ Humphreys, M. Hatherly: Recrystallization and Related Annealing Phenomena. 2nd edition. Elsevier, Amsterdam et al. 2004, ISBN 0-08-044164-5 .
B. Ilschner , RF Singer: Materials science and manufacturing technology. Properties, processes, technologies. 4th revised and expanded edition. Springer, Berlin et al. 2005, ISBN 3-540-21872-6 ( Springer textbook ).

Web links

Physical material properties (detailed lecture notes TU Dresden, of which chapters 14 and 15 are particularly relevant)

Individual evidence

↑ ^a ^b ^c ^d Christoph Broeckmann, Paul Bite: Materials Science I . Institute for Material Applications in Mechanical Engineering at RWTH Aachen University , Aachen 2014, pp. 220–239.
^ Rainer Schmidt: Precipitation phenomena in materials - An introduction to mathematical modeling. 1st edition, Deutscher Verlag für Grundstoffindindustrie, Leipzig 1991, p. 130.

[broeckmann-1] Christoph Broeckmann, Paul Bite: Materials Science I . Institute for Material Applications in Mechanical Engineering at RWTH Aachen University , Aachen 2014, pp. 220–239.

[schmidt-2] Rainer Schmidt: Precipitation phenomena in materials - An introduction to mathematical modeling. 1st edition, Deutscher Verlag für Grundstoffindindustrie, Leipzig 1991, p. 130.