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Recrystallization

by Markus Kasim (Submitted: 02/05/2007)

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WHAT IS IT?

Recrystallization comes from two words: re and crystallization. It means that there was a preceding crystallization process occurring in a material before the similar process occurs for the second times. Recrystallization happens in all kinds of material: metal, ceramic, polymer, as long as the material has crystal structure.

Recrystallization is a phenomena occurring during annealing. Annealing is one of the heat treatment processes, that a material is heated at the elevated temperature, then held at a period of time, and then slowly cooled. There are three phenomenon occur during annealing process: recovery, recyrstallization, and grain growth. The term recrystallization is defined as the formation of new set strain-free and equiaxed grains.

The main purpose of this two-dimensional model is to comprehend recrystallization phenomenon and identify how the processing parameters affect the microstructure resulted. A related model in NetLogo Models Library, MaterialSim Grain Growth, shows how the configuration and sizes of grains change over time. Some parameters in the former were developed and reused in this model.

HOW IT WORKS

We can divide the simulation into two parts. The first one is to create the initial grains, and the second one is to run the recrystallization.
Here are the steps to create initial grains:
1. Choose a random atom.
2. Check whether its neighbors have an atom on it or not. If its neighbors are still unoccupied, then go to the next step. Otherwise, the next step will not be executed.
3. In all unoccupied sites at its neighbors, create atoms which are identical with the chosen atom. Overall, it will create a small cluster of atoms which have same color and orientation.
4. The step will stop when an atom finds its neighbors have been occupied by other atoms, so there will be no double atoms in a same place. It means that this random function will create various size of grains, as the initial atom could be anywhere in the material.
5. Step 1-4 is executed simultaneously with the other atoms which have different color and orientation. Note that the number of grains is determined by choosing it from the slider. The initial grain diameter will depend on the number of grain we determine.

And the steps for recrystallization are:
1. Check these conditions:
a. The material is plastically deformed
b. The annealing temperature is above the recrystallization temperature
If these two conditions are satisfied, then go to the next step. Otherwise, recrystallization will not occur.
2. Check the nucleation sites: grain-corner, grain-boundary, and deformed matrix. The one chosen “On” will be the place where recrystallized atom nucleate.
3. Randomly choose an atom from the nucleation sites which are set “On”. Then, choose a new shape, orientation, and color for that atom. This is the recrystallized atom.
4. Now we are able to distinguish the type of atoms: initial atom, and recrystallized atom. Then, check whether the atom type on its neighbors is initial atom or not. If it is not an initial atom, then replace the atoms with recrystallized atoms which are identical with the chosen one. Overall, it will create a small cluster of recrystallized atoms which have same new shape, color, and orientation.
5. The step will stop when an atom finds no initial atoms anymore at its neighbors. This is the end of recrystallization.

HOW TO USE IT

Simulation starting point:
1. Set the dimension of material and atom shape
? WIDTH: set x (horizontal) dimension of the material
? HEIGHT: set y (vertical) dimension of the material
? ATOM-SHAPE: choose the shape of atom. This chooser has many different shapes, such as circle, hex, and line.
2. Creating the grain
? NUM-OF-GRAIN: choose the number of initial grain
? SETUP: set the simulation and create initial atoms
? MAKE GRAIN (CONTINUOUSLY): create initial grains continuously
? MAKE GRAIN (PER STEP): create initial grains, one time step at a time.
? Draw the grain:
- DRAW button: activates drawing
- ERASE ALL: erases the screen and sets all atoms to red
- BRUSH-SIZE: set the radius of the brush
- DRAW-COLOR: change the numeric value of the drawing color.
3. Modify processing parameters
? PLASTICALLY-DEFORMED: the material is needed to be plastically deformed if we want the recrystallization to be occurred
? TEMPERATURE: choose the annealing temperature
? GRAIN-CORNERS: set whether the recrystallized atoms are nucleated at grain corner or not. Grain corner is the intersection among three grains.
? GRAIN-BOUNDARIES: set whether the recrystallized atoms are nucleated at grain corner or not. Grain boundary is the intersection between two grains.
? DEFORMED-MATRIX: set whether the recrystallized atoms are nucleated at deformed matrix or not. Deformed matrix located inside a grain.
4. Recrystallization
? GO!!! (CONTINUOUSLY): execute recrystallization continuously
? GO!!! (PER STEP): execute recrystallization, one step at a time
5. Plot and Monitors
? TIME VS %REC: this is a plot of simulation time vs. fraction recrystallized. The plot will follow a sigmoidal shape.
? INIT-GRAIN-SIZE: grain size of the initial atoms
? REC-GRAIN-SIZE: grain size of the recrystallized atoms
? TIME: time steps of the simulation so far

THINGS TO NOTICE

The input variables in this model:
? Dimension of material
? Number of initial grain
? Plastic deformation
? Annealing temperature
? Nucleation sites
And the output variables are:
? Initial grain size
? Recrystallized grain size
From this model we also can see the change of microstructure during recrystallization, and the plot of time vs. fraction recrystallized. This plot will be a sigmoidal curve, the one which is a common shape of a solid state transformation. Recrystallization is a solid state transformation.

THINGS TO TRY

? Dimension of material brings effect to the characteristic of recrystallization plot. Compare between a big and a small size of material by setting from WIDTH and HEIGHT slider
? Initial grain size affects the recrystallized grain size. Try to make different sizes of initial grains. Choose from the NUM-OF-GRAIN slider. Then, figure out what is the effect of the initial grain sizes by looking at the monitor INIT-GRAIN-SIZE and REC-GRAIN-SIZE
? See the effect of plastic deformation in recrystallization by choosing from switch PLASTICALLY-DEFORMED
? The higher the temperature, the faster recrystallization will occur. Try to figure it out by varying the temperature. Note that recrystallization will not occur when the annealing temperature is below recrystallization temperature.
? Try to choose different nucleation sites. You will see that the more possible nucleation sites in the material will produce smaller recrystallized grain size. Grain corner has less nucleation sites than grain boundary and deformed matrix.

EXTENDING THE MODEL

? This model assumes that the composition of material is pure and perfect. Pure means no impurities in the material. Perfect means no vacancy in the material.
? The rate of nucleation and growth during recrystallization is identical at any sites in the material.
? No deformed structure is shown in this model.

NETLOGO FEATURES

? It uses a hexagonal grid (as opposed to a square one)
? It uses different shapes for different visualization purposes.

RELATED MODELS

? MaterialSim Grain Growth
? Crystallization Basic
? Crystallization Directed

CREDITS AND REFERENCES

Some parts in this model are adopted from MaterialSim Grain Growth Model, see http://ccl.northwestern.edu/materialsim/.

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