Strengthening mechanisms and annealing Flashcards
What are the different processes to strengthening a material?
- Alloying
- Work hardening
- Grain boundary strengthening
- Precipitation hardening
- Grain structure control
How does the work hardening process improve material strength?
How does it work?
* There are local stress fields associated with each dislocation and solute atoms.
* As the yeild point of the material is reach Slip motion will occur at the yeild point.
* Instead of all Slip motion occuring at the same rate and same time, resulting in no more stress needed for motion. Stress is needed to be increased up until UTS. This is because local stress fields of dislocations react with each other.
* Like stress fields repel, while opposite stress fields annihalet each other, found in perfect crystals.
* Therefore, work hardening is the ‘tangling’ of dislocations resulting in more stress needed to get them to move once more.
What is the drawback of work hardening?
- As cold work increases yeild strength, the ability for the material to be elongate will decrease (the material becomes more brittle and less ductile).
- Cold work to work harden also affects grain structure. It elongates and deforms the grains with a high dislocation density.
How does Grain boundary strengthening process improve material strength?
How it works?
* Dislocations will move along a crystal until it hits the grain boundary.
* The dislocation can’t move across the grain boundary easily if they aren’t similarly aligned.
* Dislocations will pile up at the dislocation boundaries. If stress concentration builds up too high, then they may be able to cross.
* This relationship is described by the** Hall-Petch equation. Stating that size of the grain, material quality and types of processes on it, such as work hardening, precipitation hardening, etc, all affect the yeild strength of a material**. Shown below.
* Smaller grain structures will have less dislocations in them and so require more stress build up to allow the dislocations to go across grain boundaries. That is one reason why larger grain boundaries have lower yeilds strength.
What is a drawback of Grain boundary strengthining?
- If the** grain size is too small (like nano) then the dislocations will be too small for the grain and then yeild strength will be go down again and worsen**.
- If the grain sizes are too small then creep can occur which is a negative side effect.
How does the precipitation hardening process work to improve a materials strength?
How it works?
* Precipitates are small impurities in cyrstals when the material can no longer dissolve the impurity.
* Precipitates are regions with different structural arrangements
* Thermal treatment can controll the growth of precipitates.
* Thermal treatment controlled growth: As the precipitate grows it is likely to become coherent, then incoherent (lattice structure dissimilar).
* Effectively the precipitates will start off to grow out to then make a structural arrangement in the middle of the crystal that starts off coherent (which is similar to the original crystal strucutre, but might have different atomic spacing, etc) then with enough age or precipitate hardening it will become incoherent (with a strucutre dissimilar to the crystals strucutre).
* Both coherent and incoherent structures are better at raising yeild strength, but incoherent is slightly better.
How does coherent and incoherent precipitates differ in affecting yeild strength?
Coherent:
* For dislocations to move past them, they have to shear through the precipitates.
Incoherent:
* The dislocations can’t pass through the precipitates as they are so out of phase and align. The precipitates act almost like an internal grain boundary.
* Dislocations are pinned but can bow round the precipitates. thus precipitation spacing affects dislocations ability to move.
What is annealing?
Annealing is a process that can recover and reset or even change the crystals structures again after cold work has be done on a material.
What are the three stages of annealing?
- Recovery: dislocations re-arrange but no change in the grain strucuture
- Recrystalisation: Nucleation and Growth of new grains
- Further grain growth
What happens in the recovery stage of annealing?
The dislocations rearrange to minimise the local stress fields to form a low angle grain boundary. They align as shown below.
What happens in the recrystallisation stage of annealing?
New grains grow out of the free regions in the de-stressed regions. Atoms will diffuse in the material and form new grains, and come out, dislocations don’t affect it because of the dislocation process and the grain structure will be newly organised.
What happens in the further grain growth stage of annealing?
- Depending on the amount of cold work and the annealing temperature, grains might grow to be larger or smaller depending on those factors and desired new grain sizes.
- Larger grains form as each grains boundary will be high energy and stress compared to the centre. So by combining grains the energy and stess is reduced on the material.
- Remember: that diffusion is the process in this stage so we can understand how grain growth will based on the equation shown before in earlier lectures.
