Molecular dynamics simulations of the martensitic phase transition process

It is surprising that no full molecular dynamics calculations with correct crystallography exist for the two most commonly studied martensitic phase transitions, iron and NiTi. This is due to the lack of interatomic potentials for these systems, which in turn can be traced to a lack of understanding of the underlying physics. We discuss the discrepancies between theory and experiment regarding crystal structure and elastic moduli in NiTi, in particular that ab initio calculations predict that the stable crystal structure is B33, not the experimentally observed B19′. Despite the lack of NiTi potentials, insight can be gained from studying transitions in related martensitic materials: We present simulations of the body-centered cubic (bcc) to hexagonal close-packed (hcp) transition using a new potential for Zr which, for the first time, properly represents the phase transition. We find that the bcc-hcp phase transition is accompanied by precursor long wavelength instability and rotation of the bcc matrix. We show that strain of the bcc structure is accommodated by martensite formation, especially at temperatures close to the phase transition.