Group 3: estimation of enthalpies for stable and metastable states

The structural energies associated with promoting the elemental metals from their ground states to non-equilibrium states of different structures are an important element to the CALPHAD description of substitutional alloys. In this report, we are concerned with the issue that a priori theory and the CALPHAD community differ strongly as to the behavior of these excitations. It is observed that in all cases so far studied, a priori theory indicates that the excited non-equilibrium elemental phases are unstable to local distortions at zero temperature. In some instances, phases that are stable at high temperatures display such zero temperature instability as well. In the course of observing the locally unstable phases, a theorem based on symmetry is noted concerning the derivative of the energy as a function of lattice distortion, say the tetragonal Bain distortion. It is observed that the derivative of the energy must be zero valued when the distortion passes through structures having different, i.e., higher, crystal symmetry.[?] A first pass is made at resolving the consequence(s) of these observations for phase diagram constructs using the example of Cr in the fcc regime of the Ni-Cr phase diagram.