A critical test of ab initio and CALPHAD methods: The structural energy difference between bcc and hcp molybdenum

Ab initio calculations of the enthalpy of formation of bcc, fcc, and hcp Ru-Mo alloys have been performed for random, ordered, and partially ordered structures. The lattice stability of the bcc and hcp forms of Mo is isolated in order to compare the hcp-bcc difference calculated by ab initio and CALPHAD methods with experimental measurements of the enthalpy of formation of Ru-Mo alloys. The significance of this comparison in calculating the Mo-Ru phase diagram is illustrated. The results of these considerations suggest a rational method for coupling ab initio and CALPHAD techniques might be utilization of the ab initio methods for calculation of the isostructural energies of formation for binary bcc, hcp, and fcc solutions while retaining the CALPHAD lattice stabilities in the calculation of phase diagrams.