Molecular-dynamics investigation of the fcc → bcc phase transformation in Fe

Using molecular-dynamics simulation, we study the influence of internal and external constraints on the dynamics of an fcc → bcc transformation. We use a many-body model potential of the tight-binding form. Characterizing this potential by its free-energy change along the Bain path, we find it to exhibit a low-temperature metastable fcc phase besides its ground-state bcc structure. Accordingly, the metastable phase is shown to undergo a spontaneous high-temperature phase transition to the ground state. However, this temperature-induced transition only takes place, if the external boundary conditions of the simulation volume allow to accommodate the tetragonal distortion of the simulation volume, and hence to relax all components of the stress tensor. However, the transformation proceeds spontaneously even at 0 K, when a grain boundary is introduced whose large free volume helps to accommodate the structural transformation.