Stability of the body‐centered cubic phase of iron: A thermodynamic analysis

The boundary separating the body‐centered cubic (bcc) and face‐centered cubic (fee) phases in the phase diagram of iron has an exceptionally large curvature, especially if the two bcc‐fcc boundaries are connected through negative pressure. This curvature can be shown to result from the Curie transition from the ferromagnetic (spin ordered) to paramagnetic (spin disordered) state in the bcc phase. An analysis of the thermodynamics of the magnetic properties indicates that they have little effect on the curvature of the metastable bcc‐fcc boundary at pressures and temperatures above the δ‐γ‐melt triple point at 52 kbar and 1990 K. Thus the metastable bcc‐fcc boundary is expected to continue as a straight line with a slope of approximately 6.25°/kbar in the stability field of the melt. This boundary will not intersect the melt curve at 1.5 Mbar and 4000 K to produce a stability field of bcc as proposed by Ross et al. (this issue) unless the lattice instability mechanism proposed by them affects the thermodynamic properties of the phases to cause a substantial increase in entropy change (Δ S ) or decrease in volume change (Δ V ) of the bcc‐close packed boundary with increasing temperature and pressure.