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Observations and results II: Temperature induced phase transition of 230 atoms of two types

Discussion of results

230 atoms of type "red" and "yellow" are placed into a rectangular square lattice. The Lennard Jones parameters are chosen in an appropriate manner: At low temperatures, the square lattice is not a stable configuration. However, if a higher initial temperature is applied to the body, this square lattice becomes stable!

The initial temperature is chosen by fixing the initial velocities. Calculation under adiabatic conditions for an extended lapse of time proves that the austinitic lattice is stable at this temperature level, although significant thermal fluctuations can be observed. The bottom row of atoms (marked blue) is fixed over time so as to avoid the movement of the block as a whole. Screenshot 1 shows the 230 atom body under adibatic conditions in the austenitic phase:

Screenshot 1 : 230 particle body under adiabatic condition. Austenitic Phase.

After some time, the cooling process starts. We observe the decay of the mean kinetic energy  over the time in the small upper window on the right-hand side of screenshot 2:

Screenshot 2 : 230 particle body under temperature control. Cooling the body.

After some time the critical transformation temperature is reached. From this time on we observe a martensitic transformation in progress: The whole block does not transform at once. Rather we observe that some rows transform as one layer, followed by some further rows, which act as a second layer and so forth. Screenshots 3 and 4 illustrate this process.

Screenshot 3 : Transformation in progress: First layer starts to transform.

Screenshot 4 : Transformation in progress: Transforamtion of first and second layer is finished.

We observe how the transformation wave rushes over the body. At the end the whole body has been transformed from the austenitic lattice into the martensitic lattice.

We observe that not all layers possess the same shear-direction: Some are tilted to the right, others to the left. This phenomenon is well known in material science and it is called "twinning". Screenshot 5 shows the body, when the transformation has ended.

Screenshot 5 : 230 particle body, transformed into the martensitic phase.

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