Thermodynamic assessment of the Hf-Os system

The Hf-Os system was critically assessed by means of the calculation of phase diagram (CALPHAD) technique. The solution phases (liquid, body-centered cubic and hexagonal close-packed) were modeled with the Redlich-Kister equation. A three-sublattice model (Hf, Os)0.5(Hf, Os)0.5Va3 is applied to describe the intermediate B2 compound HfOs in order to cope with the order-disorder transition in the Hf-Os system. Using this model, the ordered B2 and the disordered Hf-rich bcc are described by a single Gibbs energy function. The intermetallic compound HfOs2, which has a homogeneity range, was treated as the formula (Hf, Os)(Hf, Os)2 by a two-sublattice model with Hf and Os on the first and second sublattices, respectively. The compound Hf54Os17 was treated as stoichiometric compound. A set of self-consistent thermodynamic parameters of the Hf-Os system was obtained.