Comparative study from drop-tube experiments and ab-initio calculations, of the nucleation of the σ phase in the Re-W and Re-Ta systems during solidification far from equilibrium

Undercooling experiments have been performed on Re-Ta and Re-W alloys using a high drop-tube facility. The A15 structure displays a metastable character at 25at%Re for both systems. For the Re-Ta system, the sigma phase is found to compete with the A15 phase in the Ta-rich domain. The formation energies of the tetrahedrally close packed structures A15, sigma and chi extracted from ab initio calculations are Shown to be strongly more negative in Re-Ta than in Re-W, Moreover, by means of C.V.M. calculations, the metastable character of the sigma phase is related to the site occupation.