CALPHAD ( abbr. From the English. CALculation of PHAse Diagrams ) - a method of calculating phase diagrams. Under equilibrium phase diagrams usually understand diagrams of the dependence of the composition of the chemical system on temperature. The phase diagram shows the regions of existence of compounds and solutions (i.e., phases) and the regions of their coexistence. Phase diagrams are a very powerful tool for predicting the state of a system under various conditions. They first appeared as a graphical method for generalizing experimental information on equilibrium [1] . The CALPHAD approach is based on the fact that the phase diagram is a manifestation of the equilibrium thermodynamic properties of the system, which are composed of the properties of the phases that make up the system. Thus, it is possible to calculate the phase diagram by initially assessing the thermodynamic properties of all phases of the system.
Content
Methodology
The CALPHAD method combines all experimental information on phase equilibria in the system and all thermodynamic information obtained during thermochemical and thermophysical studies. Then the set of thermodynamic properties of each phase is described by a mathematical model containing customizable parameters. The parameters are calculated by optimization - fitting the model to all the information, including coexisting phases. After this, the phase diagram and thermodynamic properties of the phases making up the system can be recalculated. The concept of the CALPHAD method consists both in obtaining a consistent description of the phase diagram, and in reliable prediction of the set of stable phases and their thermodynamic properties in those areas of the phase diagram where experimental information is absent, as well as metastable states by modeling phase transformations .
There are two critical factors to using the CALPHAD method successfully.
Thermodynamic Phase Modeling
The first factor is finding a realistic and convenient mathematical model for the Gibbs energy of each phase. Gibbs energy is used because most experimental data were obtained at certain values ββof temperature and pressure. In addition, any other thermodynamic quantity can be derived from the Gibbs energy . Obtaining a complete description of the Gibbs energy of a multicomponent system in an analytical form is impossible. Therefore, it is necessary to determine the main features on which to build a mathematical model. The discrepancy between the model and the real system, ultimately, seems to be a power series expansion of temperature, pressure and phase composition. The adjustable parameters of the model description are refined before reproducing the experimental data. The power of the CALPHAD method is that the description of the subsystems that make up the multicomponent system can be combined to fully describe it.
Calculation of Equilibrium
The second critical factor is the availability of computer programs for calculating the equilibrium and various types of schemes and databases with expert information. Currently, there are many different types of models used for different types of phases, there are several thermodynamic databases (both commercial and freely distributed) of various materials (steels, superalloys, semiconductor materials, aqueous solutions, oxides, etc.). There are also several different types of programs that use different types of equilibrium calculation algorithms. The most developed of them allow using not only temperature, pressure and composition, but also many other types of conditions for the existence of the system when calculating the equilibrium, since often the equilibrium can be determined with a constant volume or with a given chemical potential of an element or composition of a certain phase, etc.
Usage
The CALPHAD approach has been slowly developing since the 1960s until the 1980s when complex systems based on thermodynamic databases appeared. Now there are several commercial ( FactSage , MTDATA , PANDAT , Thermo-Calc , NUCLEA / GEMINI , etc.) software products, as well as free software code - OpenCalphad . They are used both in research and in production. The use of these programs can significantly reduce time and material costs by optimizing experimental work as a result of thermodynamic forecasting of the behavior of multicomponent systems (which would be practically impossible without the CALPHAD approach). There is a magazine called CALPHAD , which highlights scientific advances in the field of thermodynamic modeling of phase diagrams. Scientific papers describing the use of the CALPHAD method are published in many other journals.
See also
- Phase diagram
- Gibbs Energy
Literature
- L Kaufman and H Bernstein, Computer Calculation of Phase Diagrams, Academic Press NY (1970) ISBN 0-12-402050-X
- N Saunders and P Miodownik, Calphad, Pergamon Materials Series, Vol 1 Ed. RW Cahn (1998) ISBN 0-08-042129-6
- HL Lukas, SG Fries and B Sundman, Computational Thermodynamics, the Calphad Method, Cambridge University Press (2007) ISBN 0-521-86811-4