Theory of the phase diagram of iron and thallium : the Burgers and Bain deformation mechanisms revised

A theoretical model is proposed for the pressure-temperature phase diagram of Fe and Tl. A unified phenomenological and atomistic description is given for the Burgers and Bain deformation mechanisms. The bcc--hcp and bcc--fcc transformations are associated, respectively, with six- and two-dimensional order parameters, which are expressed as periodic functions of the atomic displacements. The fcc--hcp transformation is interpreted as a displacive transition between two low-symmetry phases. A thermodynamic potential describing the three preceding transformations is constructed, and the corresponding theoretical phase diagram, including the triple point, is worked out. It is compared to the experimental phase diagrams of Re and Tl.