About the T0 Temperature Property Model

T-Zero Temperature is a Property Model available when using the Property Model Calculator in Thermo‑Calc and it calculates the so-called T-zero line.

The T0 temperature is defined as the temperature where two phases of identical chemical composition have the same molar Gibbs free energy. This temperature is an important quantity in the field of diffusionless phase transformations, e.g. martensitic transformation, since it is the upper limit where diffusionless phase transformations can occur.

Vary Temperature or Conditions

With this Model up to two solutions can be found. When searching for T-zero you can vary the condition of temperature or element composition in units of mass percent (e.g. mass percent Fe). This is managed using the settings Condition to vary and in some cases, Manual search limits.

When Temperature is the varied condition, T-zero is obtained as a solution. When a specific composition is selected as the condition to vary, then the temperature(s) is calculated when the T-zero condition is fulfilled. So the resulting temperature is equal to T-zero .

In most cases the curvature of the Gibbs energies of the two phases is different and results in multiple solutions for T-zero . If you keep the default settings (Condition to vary → Temperature and with no manual search limits) the Property Model Calculator can store up to two solutions. If the solution of interest is not calculated, including Manual search limits and/or changing the condition to vary to one of the listed compositions, allows you to find the solution of interest.

Gibbs Energy Addition

There is also an option to include a Gibbs energy addition to the second phase, which allows for inclusion of an energy barrier for martensitic phase transformations. This is done using the Energy addition second phase [J/mol] setting.

The energy addition field in Graphical Mode corresponds to the Console Mode command ADVANCED_OPTIONS PHASE_ADDITION (see EQUILIBRIUM_CALCUL for information). However, the energy addition can also be composition- and/or temperature-dependent, which is not possible in Console Mode. In addition, Property Models are not available in Console Mode. However, You can access the functionality using methods available with an applicable API such as TC-Python or TC-Toolbox for MATLAB®

The settings are entered on the Configuration window for the Property Model Calculator and described in T0 Temperature Property Model Settings.

For examples, see PM_G_09: T-Zero Temperature and PM_G_14: Ti-Fe T-Zero Martensite