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DFT and CALPHAD: predictive phase diagrams
- Date: 29.09.2010
- Time: 1:00 p.m.
- Place: ICAMS Scientific Retreat, Attendorn, Germany
The CALPHAD (CALculation of PHAse Diagrams) method relies on Gibbs energies databases. When only limited experimental data is available for the Gibbs energies parametric modelling, the predictive power of the CALPHAD calculations will also be limited. For example, for the calculation of the phase transformation kinetics within phase-field simulations not only the thermodynamic equilibrium data is required but information on metastable phases is also necessary. Such information is difficult or impossible to obtain directly from experiment. Ab-initio calculations may supplement experimental databases as they comprise metastable phases and arbitrary chemical compositions. In our work we demonstrate how density-functional theory (DFT) calculations of the heat of formation may be used within the CALPHAD methodology for the calculation of prototypical phase diagrams. We present simulations for the binary Fe-B system. For this system we calculate the heat of formation for an extensive set of structures using DFT calculations and employ the total energies in CALPHAD calculations in order to determine the corresponding phase diagram. We account for the configurational entropy within the Bragg-Williams approximation and neglect the phenomenological excess term that is commonly used in CALPHAD as well as the contribution of phonons and electronic excitations to the free energy. The obtained phase diagram will be discussed with regard to the experimentally determined phase diagram.