The glass forming ability of the Ti-Be alloys is a matter of great interest, and the detailed calculation for the critical cooling rates was performed using the Davis-Uhlmann kinetic formulations.¹⁾ Such a property was also examined combining the thermodynamic and kinetic approaches,²⁾ where the thermodynamic quantities based on the phase diagram calculation³⁾ were applied for expressing the free energy barrier for the homogeneous nucleation. Their results showed that the glass forming tendency is controlled by the formation of the metastable TiBe phase with a CsCl-type B2 structure. In the latter calculation, the phase stability of the TiBe phase was assessed to avoid the appearance as an equilibrium phase, assuming that the TiBe phase has a similar entropy of fusion value to that of other B2 compounds. Therefore, more precise information on the thermochemical properties of this TiBe phase is quite necessary for the quantitative evaluation of the glass forming ability in this binary system.

Thus, in the present study, a thermodynamic analysis has been carried out using the experimental phase diagram data and the formation energies of the stoichiometric compounds by ab initio calculations. Furthermore, the Gibbs energy of formation for the bcc phase was evaluated in the whole composition range, applying the cluster expansion method (CEM) to the total energy of some bcc-based ordered structures by ab initio calculations. For the bcc phase, the two sublattice formalism, (Ti, Be)_0.5(Ti, Be)_0.5, was adopted to describe the A2/B2 transformation. A good agreement between the calculated and the experimental phase equilibria was obtained. Evaluation of glass forming ability was also attempted utilizing the thermodynamic quantities from the phase diagram assessment.

(1) L.E. Tanner and R. Ray: Acta Metall. 27 (1979) 1727-1747.

(2) N. Saunders and A.P. Miodownik: Mater. Sci. Technol. 4 (1988) 768-777.

(3) N. Saunders: Calphad 9 (1985) 297-309.