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6.3 CALPHAD Aim: to introduce use of the CALPHAD approach for calculating phase diagrams

Topic description and teaching suggestions: The coupling of thermochemical information and phase diagram information is the basis of the method, now widely used, for optimisation and calculation of phase diagrams in multi-component systems. This topic can be dealt with starting from a description of the thermodynamic models for solution and compound phases (substitutional solutions, sublattice models, quasichemical and association solution models for ionic melts, such as slags and molten salts). The Gibbs energy for each phase in the system is described analytically as a function of composition and temperature by means of models whose parameters are optimized by comparison of experimental and ab initio information. With these functions, it is possible to calculate the equilibrium phase diagram, and extrapolate thermodynamic functions to unknown regions. For that, lattice stabilities are obtained from estimation, extrapolation and from ab initio techniques. One of the most important aspects in recent years has been the merging of solution models with first principles calculations. Examples of the most common softwares, such as ThermoCalc, BINGSS etc. (see below) developed using the CALPHAD approach should be shown. The use of such software for a specific application can be the subject of a class tutorial. Select one or more case studies from those reported in bibliographic references. Simple examples we suggest here are Cu-Ni and Pb-Sn, showing elementary principles of coupling thermochemistry and phase diagrams.

Helpful bibliography:

Books

H.L. Lukas, S.G.Fries, B. Sundman, “Computational thermodynamics- Assessing thermodynamic data and creating multi-component databases using the Calphad method”, Cambridge University Press, 2007 ; see chapter 9 for selected case studies

  • N.

    Saunders and A.P Miodownik,”CALPHAD- Calculation of Phase Diagrams. A comprehensive guide”, Pergamon Materials Series, Pergamon/Elsevier Science, Oxford, 1998

  • U.

    Kattner, “Thermodynamic modeling of multi-component phase equilibria”, JOM 49 (1999) 20- 26

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