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 struc tures are an important element to the CALPHAD description of substitut ional alloys. In this report, we are concerned with the issue that a p riori theory and the CALPHAD community differ strongly as to the behav ior of these excitations. It is observed that in all cases so far stud ied, re priori theory indicates that the excited non-equilibrium eleme ntal phases are unstable to local distortions at zero temperature. In some instances, phases that are stable at high temperatures display su ch zero temperature instability as well. In the course of observing th e locally unstable phases, a theorem based on symmetry is noted concer ning the derivative of the energy as a function of lattice distortion, say the tetragonal Bain distortion. It is observed that the derivativ e of the energy must be zero valued when the distortion passes through structures having different, i.e. higher, crystal symmetry.[?] A firs t pass is made at resolving the consequence(s) of these observations f or phase diagram constructs using the example of Cr in the fee regime of the Ni-Cr phase diagram.