ATOMIC-LEVEL SIMULATION OF NUCLEATION OF MARTENSITE

Martensitic transformations in solids are military (cooperative) transformations that take place in nearly sound velocity by shear of the parent structure. Therefore it is extremely complicated to follow them experimentally and molecular dynamic simulations are a very useful tool to study them. By a study of a hypothetical 2D lattice a few events of nucleation of martensitic transformation were identified by Kastner and Shneck [3] and studied in detail. It was found that the entropy decreases during the nucleation and it was suggested that this forms a free energy barrier to the transformation, namely, the nucleation barrier may be entropic even if there is no potential energy barrier.

To assess this suggestion we explore the potential energy of small clusters of atoms in a static lattice in several computation cells. The clusters were rigidly sheared in different orientations and magnitudes, or left to find the minimum potential energy structure while embedded in a parent phase.

The calculations revealed a subtle balance between the driving force (the potential energy of the nucleus) and the resistance (elastic energy) of the surrounding austenite. After defining trustworthy computational cells, we determined the orientation of the preferred shear and found that there are definite combinations of size, shape and twining of clusters that reduce the potential energy while other combinations increase the energy and will probably not grow. We assume that these first clusters represent critical nuclei. This finding assesses the hypothesis that the free-energy nucleation barrier may be entropic, even if there is no potential energy barrier.