Thermodynamic Variables

Thermophysical Properties Variables and Elastic Properties Variables

There are different thermodynamic variables in Thermo‑Calc. Some are state variables, for example temperature, pressure and mole fraction. These characterize the equilibrium state of a system. You set them when you define your system in the POLY module before performing a calculation. They are also used in other modules such as DATA, GIBBS and PARROT. State variables that involve components can be used for the defined components, but not for any species. (To define new components in a defined system, use DEFINE_COMPONENTS).

Other useful thermodynamic variables are derive from state variables with a mathematical function. Some derived variables are pre-defined by Thermo‑Calc, such as normalized energetic and compositional extensive state variables for example. You use these by appending various normalizing suffixes to abbreviations that are associated with the state variables.

Thermodynamic variables can also be divided into extensive variables and intensive variables. An extensive variable is a variable whose value depends on the size of the system, whereas an intensive variable is a variable whose value is independent of system size.

More information about thermodynamic variables is available in the online help in Thermo‑Calc Console Mode. Use the command INFORMATION, with a subject keyword such as one of the following as argument: STATE VARIABLES, INTENSIVE VARIABLES, EXTENSIVE VARIABLES, DERIVED VARIABLES, CONDITIONS (for condition settings), AXIS-VARIABLES (for stepping/mapping variable settings).

Common Thermodynamic Variables

The tables below list common thermodynamic variables. Most of these can be used to define equilibrium conditions in the POLY module unless otherwise indicated.

The variables are divided into the following tables:

  • Intensive variables
  • Energy-related extensive variables for whole system or for a phase
  • Compositional extensive variables (overall amount of components in the whole system, or amount of a component in system or in a phase)
  • Constitutional composition-related extensive variables (amount of a constituent/species on a sublattice site in a phase)
  • Special quantities

How to Read the Tables

  • The Abbrev. column shows the abbreviation that you use in the POLY module for referring to the variable. If the variable must be given any arguments, then these are given in parenthesis directly after the abbreviation. For most variables, this is also the abbreviation that you use to refer to the variable in other modules. However, some variables are referred to with a different abbreviation in the POST module. If this is the case, then this is noted at the bottom of the table.
  • Variables that are specific to a species, in which case the species is specified as an argument (sp in the table below). If there are two or more sublattices in a given phase, then this argument is given as constituent#sublattice. This indicates a constituent on a specific sublattice or site in the phase (referred to by the # sign and a digit). For example, y(BCC,C#2) stands for the site fraction of the C species on the second sublattice site of the BCC_A2 solution phase.
  • The Unit column show which unit that you can use to express the variable in the POLY module. This unit is always the SI unit. In other modules, state variables may be expressed in other units.
  • The Suffix column shows the suffixes you can append to the variables. The R suffix can be used with all compositional extensive state variables, but the suffix does not always change the value of the variable. This is indicated by putting the R in parenthesis.

These types of variables are listed in the following topics: