Draft:Dynamic recovery and recrystallization

In metallurgy, cold working increases strength and hardness and reduces flexibility or ductility. This phenomenon increases as a result of continuing to do cold work, and as a result, the internal energy increases. This makes the substance thermodynamically unstable.

Heating such a material can eliminate crystal defects and restore the microstructure and initial physical and mechanical properties. This phenomenon is known by "recovery" and "recrystallization".

cold working

Hot working and cold working

Recovery

In metallurgy, recovery is a process by which a metal or alloy's deformed grains can reduce their stored energy by the removal or rearrangement of defects in their crystal structure. These defects, primarily dislocations, are introduced by plastic deformation of the material and act to increase the yield strength of a material. Since recovery reduces the dislocation density, the process is normally accompanied by a reduction in a material's strength and a simultaneous increase in the ductility. As a result, recovery may be considered beneficial or detrimental depending on the circumstances.

the removal or rearrangement of defects in their crystal structure

Dynamic and static recrystallization

1. Occurrence during Deformation:

  - Dynamic Recrystallization (DRX): In DRX, the nucleation and growth of new grains occur during deformation. This process allows for new grain sizes and orientations, which can prevent crack propagation.

  - Static Recrystallization: Static recrystallization, on the other hand, occurs after deformation, typically as part of a separate heat treatment.

2. Effect on Material Properties:

  - DRX: After dynamic recrystallization, the ductility of the material increases¹. The final grain size in DRX increases with increased stress.

  - Static Recrystallization: Static recrystallization can lead to very large changes in texture.

3. Dependence on Deformation:

  - DRX: DRX is dependent on the rate of dislocation creation and movement. It also depends on the recovery rate (the rate at which dislocations annihilate). The interplay between work hardening and dynamic recovery determines grain structure.

  - Static Recrystallization: Static recrystallization occurs upon subsequent annealing of a deformed sample.

Dynamic recrystallization (DRX)

Dynamic recrystallization (DRX) is a process that occurs during the deformation of materials, such as metals and minerals, at high temperatures. Unlike static recrystallization, which happens after deformation, DRX involves the nucleation and growth of new grains while the material is being deformed. This process can lead to a reduction in grain size, which increases the risk of grain boundary sliding and decreases dislocation mobility within the material. As a result, the material becomes less hard and more ductile.

Graph of tensile strength and ductility - annealing temperature

DRX can occur in various forms, including CDRX, GDRX, and DDRX, each with distinct mechanisms and characteristics.

DRV and DRX usages

Continuous Dynamic Recrystallization (CDRX):

CDRX is characterized by the gradual transformation of low-angle grain boundaries into high-angle grain boundaries without a significant change in flow stress. This process is driven by the accumulation of dislocations and the subsequent recovery and recrystallization of grains. CDRX typically occurs at higher temperatures and lower strain rates, where the material's ability to recover is enhanced.

CDRX Grain Growth

Geometric Dynamic Recrystallization (GDRX):

GDRX involves the formation of new grains through the elongation and thinning of existing grains until they reach a critical size. This process is often observed in materials with local serrations on their grain boundaries. Upon deformation, these serrations become more pronounced, leading to the formation of new equiaxed grains.

GDRX Grain Growth

Discontinuous Dynamic Recrystallization (DDRX):

DDRX is characterized by distinct nucleation and growth events that result in a "necklace" microstructure. It typically occurs in materials with low to medium stacking fault energy, where dislocation slip and twinning are both active deformation mechanisms. DDRX is influenced by factors such as temperature, strain rate, and initial grain size.

DDRX grain growth

Difference between DDRX & CDRX

summary

In summary, DRX plays a vital role in controlling the microstructure and mechanical properties of materials during hot working processes. The choice between CDRX, GDRX, and DDRX depends on the specific conditions of deformation, such as temperature, strain rate, and material properties. Understanding these mechanisms allows for the optimization of processing conditions to achieve desired material characteristics.