CALPHAD
Encyclopedia
CALPHAD stands for CALculation of PHAse Diagrams. An equilibrium phase diagram
is usually a diagram with axes for temperature and composition of a chemical system. It shows the regions where substances or solutions (i.e. phases) are stable and regions where two or more of them coexist. Phase diagrams are a very powerful tool for predicting the state of a system under different conditions and were initially a graphical method to rationalize experimental information on states of equilibrium. The CALPHAD approach is based on the fact that a phase diagram
is a manifestation of the equilibrium thermodynamic properties of the system, which are the sum of the properties of the individual phases. It is thus possible to calculate a phase diagram
by first assessing the thermodynamic properties of all the phases in a system.
materials, aqueous solution
s, slag
s, etc. There are also several different kinds of software available using different kinds of algorithms for computing the equilibrium. It is an advantage if the software allows the equilibrium to be calculated using many different types of conditions for the system, not only the temperature, pressure and overall composition because in many cases the equilibrium may be determined at constant volume or at a given chemical potential
of an element or a given composition of a particular phase.
Phase diagram
A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium...
is usually a diagram with axes for temperature and composition of a chemical system. It shows the regions where substances or solutions (i.e. phases) are stable and regions where two or more of them coexist. Phase diagrams are a very powerful tool for predicting the state of a system under different conditions and were initially a graphical method to rationalize experimental information on states of equilibrium. The CALPHAD approach is based on the fact that a phase diagram
Phase diagram
A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium...
is a manifestation of the equilibrium thermodynamic properties of the system, which are the sum of the properties of the individual phases. It is thus possible to calculate a phase diagram
Phase diagram
A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium...
by first assessing the thermodynamic properties of all the phases in a system.
Methodology
With the CALPHAD method one collects all experimental information on phase equilibria in a system and all thermodynamic information obtained from thermochemical and thermophysical studies. The thermodynamic properties of each phase are then described with a mathematical model containing adjustable parameters. The parameters are evaluated by optimizing the fit of the model to all the information, also involving coexisting phases. It is then possible to recalculate the phase diagram as well as the thermodynamic properties of all the phases. The philosophy of the CALPHAD method is to obtain a consistent description of the phase diagram and the thermodynamic properties so to reliably predict the set of stable phases and their thermodynamic properties in regions without experimental information and for metastable states during simulations of phase transformations.Thermodynamic modeling of a phase
There are two crucial factors for the success of the CALPHAD method. The first factor is to find realistic as well as convenient mathematical models for the Gibbs energy for each phase. The Gibbs energy is used because most experimental data have been determined at known temperature and pressure and any other thermodynamic quantities can be calculated from it. It is not possible to obtain an exact description of the behavior of the Gibbs energy of a multi-component system with analytical expressions. It is thus necessary to identify the main features and base the mathematical models on them. The discrepancy between model and reality is finally represented by a power series expansion in temperature, pressure and constitution of the phase. The adjustable parameters of these model descriptions are refined to reproduce the experimental data. The strength of the CALPHAD method is that the descriptions of the constituent sub-systems can be combined to describe a multi-component system.Equilibrium calculations
The second crucial factor is the availability of computer software for calculating equilibria and various kinds of diagrams and databases with the stored assessed information. As there are at present many different kinds of models used for different kinds of phases there are several thermodynamic databases available, either free or commercially, for different materials like steels, super-alloys, semiconductorSemiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
materials, aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
s, slag
Slag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...
s, etc. There are also several different kinds of software available using different kinds of algorithms for computing the equilibrium. It is an advantage if the software allows the equilibrium to be calculated using many different types of conditions for the system, not only the temperature, pressure and overall composition because in many cases the equilibrium may be determined at constant volume or at a given chemical potential
Chemical potential
Chemical potential, symbolized by μ, is a measure first described by the American engineer, chemist and mathematical physicist Josiah Willard Gibbs. It is the potential that a substance has to produce in order to alter a system...
of an element or a given composition of a particular phase.