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Observations and results I: Temperature induced phase transition of 85 atoms of two types

Discussion of results

85 atoms of type "red" and "yellow" are placed into a 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 atomic speeds. Screenshot 1 shows the configuration of the body immediately after the simulation started:

In this simulation, we start at high temperature and proceed to cool down the body. No external forces are applied. Thus the block possesses a spin due to the randomly chosen directions of the initial velocities. Screenshot 2 shows the body at a temperature close to the critical temperature, before the transformation starts.

When a certain critical temperature is reached, the square lattice becomes unstable. We observe the phase transition towards the martensitic lattice in the simulation window. The transition happens very quickly. It is characterized be a significant shear of the lattice, and by significant jumps in the kinetic- and potential energy plots. Screenshot 3 shows the body in the martensitic state.

Screenshot 3 : 85 particle body under temperature control. Body after transformation into martensitic phase.

The phase transition is characterized by a shearing of the cubic lattice, and by the following energetic observations:

• The mean kinetic energy jumps to a higher level. This is due to the latent heat of the crystal lattice liberated in the transition.
• Simultaneously the potential energy jumps to a lower level. This is due to the energetically more favourable position of the atoms in the martensitic lattice at low temperatures.

Note, that in the case of 85 atoms, we observe that the whole block transforms at once: If one row starts to transform, all the others do follow immediately. This is not the case, when more atoms are taken into account!

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