Mesomechanics of the α–ε transition in iron

The high pressure α–ε transition in iron is investigated by use of a two dimensional discrete element method. Special attention is focused on mesomechanics at the grain level. Anisotropic structure is simulated by a random distribution of packing orientation. Thermo-dynamic variables such as temperature and Gibbs free energy are treated as internal degrees of freedom of individual elements. The dynamics of the transition is described by the first and second order kinetics having a constant activation energy. The study shows that global metastability arises as a result of nonequilibrium fields caused by the crystal anisotropy and grain boundary effects. Calculated hysteresis under quasi-static conditions are in good agreement with diamond anvil tests. Calculations of the shock-induced transition confirms an earlier suggestion, based on continuum modeling, that the transition relaxes toward the static metastable state. Results also show that the kinetics at the grain level is different from that speculated through use of macroscopic measurements.