Work hardening and Forest Hardening

In summary: Easy to see, hard to fix.In summary, The difference between work hardening and forest hardening is that work hardening occurs due to dislocation interactions while forest hardening explains the actual mechanism of the hardening. Stage I is characterized by easy glide, while Stage II is characterized by forest theory.
  • #1
Chemist20
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0
Anyone know the difference¿¿¿¿¿¿¿¿
 
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  • #2
They are just different models. Depending on the length scale of interest and the application one of them will be more appropriate.
Work Hardening is a simple (i.e., it does not require one to know the material's internal structrue) mathematical model of the hardening phenomena observed in a material's plastic stress-strain behaviour. According to this model, the hardening of the material is described as some function of the plastic work done. Work hardening models do not care if dislocations exist or not.
Forest hardening assumes the existence of dislocations; and the density of these dislocations increases as plastic deformation progresses. Consequently, it becomes harder to move new dislocations across this "forest" of pre-existing dislocations, resulting in hardening of the material. You can now choose to describe hardening as the increase in shear stress required to move a dislocation through the 'forest' as a function of the dislocation density in the forest. Physically, the observation of "work hardening" at the scale of the specimen is the overall effect of "forest hardening" due to dislocations at the micron scale.

Therefore when hardening=func(plastic work done) --> work hardening
and when hardening=func(forest dislocation density) --> forest hardening
 
  • #3
Bavid said:
They are just different models. Depending on the length scale of interest and the application one of them will be more appropriate.
Work Hardening is a simple (i.e., it does not require one to know the material's internal structrue) mathematical model of the hardening phenomena observed in a material's plastic stress-strain behaviour. According to this model, the hardening of the material is described as some function of the plastic work done. Work hardening models do not care if dislocations exist or not.
Forest hardening assumes the existence of dislocations; and the density of these dislocations increases as plastic deformation progresses. Consequently, it becomes harder to move new dislocations across this "forest" of pre-existing dislocations, resulting in hardening of the material. You can now choose to describe hardening as the increase in shear stress required to move a dislocation through the 'forest' as a function of the dislocation density in the forest. Physically, the observation of "work hardening" at the scale of the specimen is the overall effect of "forest hardening" due to dislocations at the micron scale.

Therefore when hardening=func(plastic work done) --> work hardening
and when hardening=func(forest dislocation density) --> forest hardening


But everywhere it says that: "Work hardening, also known as strain hardening or cold working, is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements within the crystal structure of the material". So Work hardening does depend on dislocations, its not like it just ignores them right?
 
  • #4
I think Bavid's explanation is good. The mechanism of work hardening depends on dislocations, and there are always dislocations in materials, but dislocation density will vary depending on cold work and annealing.

There are also stages in work (strain) hardening. I wonder if some folks use 'forest hardening' to refer to Stage II and beyond. Stage I is characterized by easy glide.

http://www.osti.gov/bridge/servlets/purl/10170133-adbMk3/10170133.pdf
STAGE II: FOREST THEORY
The consequence of this secondary slip for the flow stress is that the dislocations produced are mostly "forest" dislocations with respect to the primary slip system. The term "forest" refers to the concept that the flow stress on a given slip plane is determined by the short range interaction of mobile.
. . .
See also - http://aero.caltech.edu/~ortiz/talks/tms-04.pdf
 
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  • #5
@Chemist20: We came to know relatively recently (1950s) that dislocation interactions are responsible for hardening. The idea of 'forest hardening' came after this time, when people actually started to try and model dislocations interacting with a forest of other dislocations.

Work Hardening ignores dislocations in the sense that it does not REQUIRE that you know anything about dislocations and how they interact. Work hardening is therefore empirical, while forest hardening explains the actual mechanism of the hardening. Again, as I said before, it is the same observed hardening phenomenon, only interpreted differently. Work hardening occurs DUE TO [forest dislocation interactions] in the crystal structure.
 
  • #6
Ok, let me see if i got this right:

Forest hardening arises due to dislocation interactions, i.e. The higher density the easier it is for them to interact. Consequently, work hardening takes places, meaning its more difficult to plastic deform the material. Is this right??

Then... Work hardening is like the consequence of forest hardening.


Thanks so much for all your help!
 
  • #7
Yeah, you got that all right.
 
  • #8
Bavid said:
Yeah, you got that all right.

perfect! thanks so much!
 
  • #9
Dislocations are 'units' of plastic shear, as it were. Metals deform plastically by shearing; the movement of dislocations enables shear. Dislocations are line defects that spread over surfaces; like wrinkles in a carpet that is being pulled across a floor. Moving the wrinkle-line causes a bit of shear if it spreads across the entire carpet. Clever nature has found a bit-by-bit way of enabling shear, rather than an all-at-once mechanism which is much, much more difficult.

If other dislocation lines intersect the surface, like a forest of trees, they impede the movement of dislocations that are causing shear on that surface. Dislocations have to cut through the trees. This is the cause of work-hardening. Hence the name 'forest hardening'. You've got it exactly right now, Chemist 20.
 
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1. What is work hardening?

Work hardening, also known as strain hardening, is a process in which a metal or alloy is subjected to plastic deformation, resulting in an increase in its strength and hardness. This occurs due to the dislocations created in the crystal structure of the material.

2. How does work hardening affect the properties of a metal?

Work hardening increases the strength and hardness of a metal, making it less ductile and more resistant to deformation. It also results in an increase in the yield strength of the material, meaning it can withstand higher levels of stress before it permanently deforms.

3. What is the difference between work hardening and forest hardening?

Work hardening and forest hardening are both processes that increase the strength and hardness of a metal. However, work hardening occurs due to plastic deformation, while forest hardening occurs due to the formation of fine precipitate particles within the metal's microstructure.

4. What are the applications of work hardening and forest hardening?

Work hardening is commonly used in metalworking processes such as rolling, forging, and drawing to increase the strength and durability of the final product. Forest hardening is often used in heat treatments to improve the strength and wear resistance of materials, particularly in the aerospace and automotive industries.

5. Can work hardening and forest hardening be reversed?

Work hardening can be partially reversed through a process called annealing, which involves heating the metal to a high temperature followed by slow cooling. This allows the dislocations to rearrange and decreases the material's strength and hardness. Forest hardening, on the other hand, is a permanent change and cannot be reversed without melting and reforming the material.

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