How to calculate t-zero temperature using Thermo-Calc- Python (TCPython) for multiple alloys?

Question

T-zero temperature is the temperature at which the Gibbs energies of two phases are equal. Here, I wish to find the temperature at which FCC and BCC have same Gibbs energy. Thermo-Calc console mode has an in-built function to calculate t-zero. However, if I manually do the calculation in TC-Python the results are different.

import numpy as np
from tc_python import *
w_c=0.008
with TCPython() as start:
    # create and configure a single equilibrium calculation
    calc_result = (
        start
            .set_cache_folder(os.path.basename(__file__) + "_cache")
            .select_database_and_elements("TCFE6", ["Fe","C"])
            .get_system()
            .with_single_equilibrium_calculation()
            .disable_global_minimization()
            .set_condition(ThermodynamicQuantity.mass_fraction_of_a_component("C"), w_c)
    )
    step=2000
    for temp in np.linspace(500,1200,step):
        calc = (
            calc_result
            .set_condition(ThermodynamicQuantity.temperature(),temp)
            .calculate()
        )
        
        gf = abs(calc.get_value_of('GM(FCC_A1)'))
        gb = abs(calc.get_value_of('GM(BCC_A2)'))
        if(abs(gf-gb)<=1):
            print(temp,gf,gb)
            break

There was no temperature that satisfied this criteria. However, in console mode (adv-options t-zero bcc_A2 fcc_A1 )I got 784.79 K. If I use run_poly_command also, I get the set-temperature (473 K) and not t-zero temperature.

from tc_python import *
w_c=0.008
with TCPython() as start:
    # create and configure a single equilibrium calculation
    calc_result = (
        start
            .set_cache_folder(os.path.basename(__file__) + "_cache")
            .select_database_and_elements("TCFE6", ["Fe","C"])
            .get_system()
            .with_single_equilibrium_calculation()
            .disable_global_minimization()
            .set_condition(ThermodynamicQuantity.mass_fraction_of_a_component("C"), w_c)
            .set_condition(ThermodynamicQuantity.temperature(),473)
    )
    calc=calc_result.run_poly_command('adv t-zero bcc_a2 fcc_a1').calculate()
    print(calc.get_value_of('T'))

What command should be used to get t-zero temperature in TC-Python?

Answer 1

I have faced a similar issue and it seems that something happens while calling calculate(). I think it may be because when you use the same single equilibrium calculation for determining the Gibbs energy it does without suspending the other phase and this might be creating the problems, but I am not sure. I would suggest trying two different single equilibrium calculations (one for FCC and another for BCC) each with only one phase entered while all others suspended and then calculate the T-zero temperature separately from the resulting Gibbs energy values by finding the temperature at which their difference is zero, like you have done in the first code.

Another simpler way, in case you want to keep the original code with a single equilibrium calculation over a range of compositions, is to use enable_step_separate_phases() with calculate(), this usually works.