Phase stabilities in the Pt-W system: Thermodynamic and electronic-structure calculations

Publisher Website
Google Scholar
Abstract
Ab initio electronic-structure calculations and the Calphad-type analysis of experimental phase diagrams and other thermodynamic information are known to give conflicting results for the enthalpy of several metastable transition metal phases, e.g., fcc W. We have simultaneously used ab initio total-energy calculations and the cluster expansion as well as Calphad methods in a study of the Pt-W phase diagram. The ab initio calculations show that fcc W is dynamically unstable for all long-wavelength shear modes at T=0 K. Even if fcc W becomes dynamically stable at high temperatures, the enthalpy of fcc W derived from Calphad analyses cannot be compared with the ab initio enthalpy of fcc W without a detailed knowledge of the vibrational entropy. Further, we show that the Calphad method can account for the phase diagram data and ab initio results for fcc and bcc Pt-W alloys provided that a metastable fcc W phase is given an unusually large entropy.