PHASE-STABILITY STUDY OF THE AL-NB SYSTEM

The total energies of formation of AI-Nb intermetallic phases Nb3Al (A 15), Nb2Al (sigma), and NbAl3 (DO22) have been calculated using the li near-muffin-tin-orbital (LMTO) method in the full potential (FP) appro ach. Relaxation and distortion effects have been included and have bee n shown to play an important role in the stabilization of the DO22 int ermetallic phase. In order to get the interaction parameters to descri be the energetics of the bcc and fee solid solutions, FP-LMTO calculat ions have been performed for various superstructures based on the bcc and fee lattices. To discuss thermodynamic properties and the phase di agram of the AI-Nb system, the cluster variation method configurationa l entropy has been introduced using the tetrahedron-octahedron approxi mation in the fee solid solution and the tetrahedron approximation in the bcc solid solution. The thermodynamic data obtained in this work a gree fairly well with the available experimental data. Connections wit h Calphad approaches are also discussed. An attempt to calculate compo sition-temperature phase diagram of the Al-Nh system is presented, usi ng a semiempirical treatment of the liquid phase.