MICRESS Forum

Hi, Bernd,

According to my understanding, the driving force calculation in MICRESS is done from a parallel tangent construction as,

dG = f(belta)-f(alpha)-mu*(c(belta)-c(alpha)), and mu is defined as diffusion potential

When linking to the real molar Gibbs energy from the CALPHAD-type thermodynamic database, I have two questions as follows (take simple binary as the case),

1) How to get a common mu for both alpha and belta phases to construct a parallel tangent?

2) When one of the phase is described by sublattice model, like (A,B)a(A,B)b, in the database, the site fractions, like y(A)', y(B)', y(A)'' and y(B)'', will be introduced. However, only C(A) or C(B) can be used in the concentration evolution. How do you treat this case in your MICRESS software?

Best wishes!

yours,

sunny

Dear sunny,

your understanding of the driving force calculation is perfectly correct! The calculation is done via TQ-subroutines which do the equivalent of performing a "dormant" equilibrium in Thermo-Calc. This is identical to constructing parallel tangents!
It is the same case, if a phase has more internal degrees of freedom and the species can be redistributed between different sublattices. From the MICRESS side, the site fractions are not especially considered because (as well as in Thermo-Calc) internal equilibrium between the sublattices is assumed, so that the diffusion potentials of the species are identical in the different sublattices.

Bernd

Dear Bernd,

Thank you for your information.

I suppose that when the dG or mu is needed in the simulation TQ can calculate and return the values for MICRESS. Thus, there is no need for MICRESS to consider what kind of models used in thermodynamic databases. Then only phase field, concentration and temperature are to be evaluated.

Am I right?

Sincerely,

sunny

In principle, yes!

MICRESS is not directly using the models in the thermodynamic database. But MICRESS is using e.g. information about solubilities of elements in phases, which depends on the sublattices used for description of the phases.

dG cannot be taken directly from TQ because an iteration of the quasi-equilibrium compositions has to be performed using the parallel tangent condition from TQ. This iteration is done inside MICRESS.

The chemical potential mu is not explicitely used inside MICRESS. Diffusion is calculated using the concentration gradient and the diffusion matrix which is directly obtained from TQ.


Bernd