MICRESS Forum

Hi MICRESS Forum,

I have a non thermocalc simulation and was just wondering how the driving force is calculated. From looking at the 2006 paper "Multiphase-field approach for multicomponent alloys with extrapolation scheme for numerical application", the driving force update is dS*m*dC + dS*dT. I can get these values from the micress simulation and calculate dG for an example gridpoint, but the dG calculation comes out much bigger than the dG calculated at each grid point from MICRESS. Is this equation of dS*m*dC + dS*dT how MICRESS calculates driving force or is there another way it is calculated? I see in the inputs for phase interactions there are two reference points, are these used when calculating dG perhaps? Hopefully my query makes sense, any help would be much appreciated.

Many thanks,
Matthew

Hi Matthew,

In principle, your formula for calculation of the driving force is correct. m is either the solidus or liquidus/solvus slope as specified in the linearisation data, and dC is the difference c-c0, i.e. the deviation of the local composition from the reference point. According to the "quasi-equilibrium assumption, it makes no difference whether m*dC is calculated for one or the other side of the interface, i.e. m₁dC₁=m₂dC₂. The driving force is written to the .driv output file, and does not directly contain a curvature contribution (only indirectly via the Gibbs-Thompson equation via a locally changed composition), but has an extra stress contribution in case of stress coupling.

However, there are some subtleties. One is that driving forces are typically averaged across the interface to avoid driving force gradients which may destroy the interface profile. So, to see the unaltered local values of the driving force in the .driv output, one should set the avg. parameter to 0.0 in the dG-options. Another special case is when the "lintq" mode is used which is according to the Thermo-Calc linearisation principles. Then further parameters are added, especially dS₁ and dS₂ instead of a common dS, extra derivatives to temperature dcdt for both phases, and a driving force offset dG₀ which corresponds to the distance of the parallel tangents and which adds up to the total driving force.

What is the reason why you have the impression that your formula for calculation of the driving force is not correct? Can you describe where you see deviations?

Bernd