First-Principles Calculations of Phase Stability for High Temperature Intermetallics

Total energy electronic structure calculations are performed for the elements and selected binary ordered compounds of the ternary system Nb-Ru-Zr. These calculations provide binding energies, atomic volumes, bulk moduli, Debye temperatures, and Gruneiesen constants for the selected structures and compounds. Volume dependent pair and many-body chemical interactions are also obtained from the total energy results which, in turn, are used to study partially ordered alloys at finite temperatures. The stability of all the binary intermetallic compounds experimentally observed at low temperatures is correctly predicted by the first-principles calculations. The solid state portion of the Nb-Ru binary phase diagram is calculated using the chemical interactions obtained from the total energy calculations, a Debye-Gruneisen model for the vibrational free energy and the cluster variation method (CVM) for the configurational entropy with a local volume relaxation scheme. The calculations reproduce the experimentally observed ordering temperature of the NbRu 3 intermetallic to within 2%.