Place: 3rd Sino German Symposium 2011, Xi'an, China
The TCP (Topologically Close Packed) phases appear in many important multi-components systems of industrial relevance, like steels and Ni-based superalloys. Under service conditions, the precipitation of these brittle intermetallic compounds degrades the materials properties, especially mechanical properties and oxidation resistance.
The study of their thermodynamic stability is thus important to evaluate thermodynamic properties and phase diagrams and to implement computer aided design tools in order to avoid their precipitation. To this purpose, a coupled first principles/CALPHAD approach is particularly useful.
In this work, DFT calculations on binary and ternary TCP phases were performed and employed to construct a thermodynamic database. The Compound Energy Formalism (CEF) within the Bragg-Williams approximation was used to obtain thermodynamic properties and phase diagrams from the DFT results. Within the CEF, several approximations involving a different number of sublattices can be adopted and are here discussed. Results for several properties like site occupancies, lattice parameters, enthalpy, entropy, etc. were calculated and compare well with available experimental data.