Understanding CALPHAD

9.6.2014 Fabian Rendon

CALPHAD is the acronym for CALculation of PHAse Diagrams. By definition, an equilibrium diagram is a diagram that has axes for composition and temperature of a chemical system. Now the diagram shows areas that a particular solution or substance is stable and also the areas where they can two or more can coexist.

Phase diagrams are very important in coming up with the probable state of a system when subjected to different conditions; they can be termed as a graphical representation to define information on states of equilibrium. Therefore the concept of CALPHAD is principled on the fact that a phase diagram is a manifestation of the equilibrium thermodynamic characteristics of the system, which is the total sum of the properties in the individual phases.

Therefore there is a possibility to calculate a phase diagram by examining the thermodynamic properties of all the phases in a system. Through CALPHAD one gathers all the experimental information on the phase equilibrium in a particular system including all thermodynamic information obtained from thermo chemical and thermo physical studies. Then there is a mathematical preposition that has adjustable parameters and is used to define each phase’s thermodynamic properties.

The process involves coexisting phases after optimization of the parameters so as to fit of the model to all the information. There is also the possibility of recalculating the phase diagram together with the thermodynamics properties within. One of the guiding principles of CALPHAD method is to derive a sustainable description of the phase diagram and the thermodynamic characteristics to be able to confidently come up with the right set of stable phases and their thermodynamic characteristics in areas with no experimental information and metastable states during simulations of phase transformations.

So as the method to be successful, the solution must find convenient and reliable mathematical models for the Gibbs energy for each phase. The Gibbs method seems to be reliable enough because of the various experimental information that is determinable at certain temperatures and pressure and the other thermo dynamics quantities can be derived from this.

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