Thermodynamic modeling of Laves phases in the Ta–V system: Reassessment using first-principles results

The Ta–V system is an interesting system exhibiting the existence of Laves phases among elements of the same group in the periodic system, and with an unusual order of stability of Laves phase structures at 0 K. Whereas the hexagonal C14 Laves phase is stable at 0 K and at high temperatures, the C15 structure is the most stable Laves phase at intermediate temperatures.The ab initio calculations of the total energies of formation for both C14 and C15 Laves phases employed two- and three-sublattice models to revise the thermodynamic description of the system published recently. The remodeled Gibbs energies of Laves phases require less fitting parameters than those obtained in previous treatments and the corresponding phase diagrams provide an excellent agreement with the experimental data on the phase stability and phase boundaries found in the literature. The new fitting procedure proposed involves a temperature dependent heat capacity for the C15 structure and allows us to compare the optimized heat capacity differences between the Laves phase and Standard Element Reference structure with those determined experimentally and theoretically.

Research highlights
► Ab initio calculated total energies of end-members are used in sublattice models.
► Laves phases (LP) are modeled by two- and tree-sublattice models.
► Heat capacity is used for the explanation of temperature dependence of Gibbs energy.
► Stability of C14 LP at low and also at high temperature is explained by such a way.
► New reassessment combining CALPHAD and ab initio techniques is successful.