Development of a multiphase equation of state for iron and its alloys

We present the construction of a new multiphase equation of state (EOS) for iron (Fe), consisting of the five phases: bcc, hcp, fcc, δ (high-T/low-P bcc), and liquid. The free energy models for the phases are constrained by fitting to experimental data in the lower pressure regime, and to the predictions of ab initio electronic structure calculations of various types at higher pressures and temperatures. We show that our resulting multiphase iron EOS is in excellent agreement with a broad swath of static and dynamic high-pressure experimental data. For Fe properties with conflicting data, such as diamond-anvil-cell melt curves, we have constructed EOS variations to match both lower-temperature and higher-temperature branches. We find that only one of these variations is consistent with the reported shock-melting pressure. This Fe EOS enables us to develop the EOSs of other Fe alloys using a recently implemented mixing algorithm.

This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.