Geometric Dynamic Recrystallization
Encyclopedia
Geometric Dynamic Recrystallization (GDR) is a recrystallization
Recrystallization (metallurgy)
Recrystallization is a process by which deformed grains are replaced by a new set of undeformed grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous...

 mechanism that has been proposed to occur in several alloys, particularly aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

, at high temperatures and low strain rates. It is a variant of dynamic recrystallization
Dynamic recrystallization
Dynamic recrystallization is a type of recrystallization process, found within the field of metallurgy. In dynamic recrystallization, as opposed to static recrystallization, the nucleation and growth of new grains occurs during deformation rather than afterwards as part of a separate heat...

.

The basic mechanism is that during deformation the grains
Crystallite
Crystallites are small, often microscopic crystals that, held together through highly defective boundaries, constitute a polycrystalline solid. Metallurgists often refer to crystallites as grains.- Details :...

will be increasingly flattened until the boundaries on each side are separated by only a small distance. The deformation is accompanied by the serration of the grain boundaries due to surface tension effects where they are in contact with low-angle grain boundaries belonging to sub-grains.

Eventually the points of the serrations will come into contact. Since the contacting boundaries are defects of opposite 'sign' they are able to annihilate and so reduce the total energy in the system. In effect the grain will pinch in two new grains.

The grain size is known to decrease as the applied stress is increased. However, high stresses require a high strain rate and at some point statically recrystallized grains will begin to nucleate and consume the GDRX microstructure.

There are features that are unique to GDRX:
  • The recrystallisation spreads throughout the specimen over a strain range (0.5-1 in Al-Mg-Mn alloys) without any change in flow stress. This is in contrast to discontinuous mechanisms where the flow stress normally decreases by ~25% as the recrystallized grains form.
  • GDRX results in grains that are around 3 times the sub-grain size. Statically recrystallized grains are normally 20-30 times the sub-grain size.
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