how to turn off the Gibbs-Thomson effect

ripening phenomena, dislocations, grainboundary topology
Ls_Thu
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how to turn off the Gibbs-Thomson effect

Post by Ls_Thu » Mon Oct 28, 2019 2:51 pm

Dear Bernd,

May I ask how to turn off the Gibbs-Thomson effect during the transformation in MICRESS(let the curvature have no effect to the elements distribution )?

Thank you so much!

Sai Li

Bernd
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Re: how to turn off the Gibbs-Thomson effect

Post by Bernd » Mon Oct 28, 2019 9:20 pm

Dear Sai,

I don't know why you want to do that, but it is not possible without changing physics! The most logical procedure would be to set the interface energy to 0. However, this would completely destroy the stability of the interface. The best you can do is to reduce interfacial energy to a certain extent, and try to replace it by interface stabilisation (second optional parameter after interface energy). If you go beyond a factor of 10, you go at your own risk. But it is clear that effects like curvature undercooling or ripening will be also reduced...

Another approach would be to change thermodynamic data: If you increase both, liquidus and solidus/solvus slopes) by the same factor, the partitioning will be the same, but curvature will have less impact on composition. There is however the side effect that altering composition will have an increased influence on the driving force...

Of course, the Gibbs-Thomson effect is only relevant for interactions between different phases. Can you explain a bit more your problem? How is it linked to recrystallisation/grain growth?

Bernd

Ls_Thu
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Re: how to turn off the Gibbs-Thomson effect

Post by Ls_Thu » Tue Oct 29, 2019 9:08 pm

Dear Bernd,

Thank you so much for your kind reply !

To make the question more clear, I will describe the simulation we are doing:

We plan to simulate the process (one of the processes ) of dissolution of spheroid cementite in ferrite and we expect to see the carbon content in ferrite near to the cementite is higher than the site far from the cementite, but the simulation provided an opposite result. When we changed the cementite from circle into plant, we can observe the expected results. Is it the effect of Gibbs-Thomson effect ? And how can we finish such a simulation?


Thank you again for your kind help !

Regards,

Sai Li

Bernd
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Re: how to turn off the Gibbs-Thomson effect

Post by Bernd » Tue Oct 29, 2019 10:13 pm

Dear Sai Li,

How you describe it, it does not make sense! If the Gibbs-Thomson effect would be the (direct) culprit, it would increase the carbon composition rather than decreasing it!
However, it may be an indirect effect: Gibbs-Thomson changes the composition of another element, and this gradient influences the C content by cross diffusion effects: Thus, C just "mirrors" the gradient of other elements. Despite the "wrong" C gradient, C can still diffuse "uphill", so that everything is correct.
Essentially it is easy to distinguish whether you see a direct effect on C or an indirect effect by other elements: Typically, other elements are substitutional and diffuse much slower than carbon. If the carbon depression close to the cementite shows the diffusion length of substitutional elements, it is a cross diffusion effect. Otherwise it would be linked to carbon itself.

Bernd

Ls_Thu
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Re: how to turn off the Gibbs-Thomson effect

Post by Ls_Thu » Sun Nov 03, 2019 3:46 pm

Dear Bernd,

Thank you so much for your help !! We rechechk the results, the carbon content near the cementite indeed higher than the matrix alpha! But we found a new problem...when we simulated the austenization from the cem+bcc microstrucure (austenite nucleate at the cem/fer nterface and the growth direction is vertical to the interface), if we set the geometry of cementite is circle (so is austentite), the carbon contente of alpha/gamma interface in ferrite is very high (even higher than the cem/fer interface on the other side). however, when we set the interface is straight (all phases are plates ), the carbon contente of alpha/gamma interface in ferrite is very low.

May I ask why it is and which condition is reasonable ?

Apology for the unclear description !

Regards,

Sai Li

Bernd
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Re: how to turn off the Gibbs-Thomson effect

Post by Bernd » Sun Nov 03, 2019 7:23 pm

Dear Sai Li,

Your description indeed is unclear - can you show the outputs for the concentration distributions, or some sketch which clarifies your setup and what you observe?

Bernd

Ls_Thu
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Re: how to turn off the Gibbs-Thomson effect

Post by Ls_Thu » Tue Nov 12, 2019 3:34 pm

1.PNG
the sketch for simulation
1.PNG (29.61 KiB) Viewed 1584 times
Dear Bernd,

Apology for the unclear description and late reply !

The attachment is the sketch of the simulation. We did the two kinds of model, the difference of them is only the geometry of the cementite and austenite (I is plate and II is circle).

But in model I, the carbon concentration at point A is very low and it is lower than point B (which is reasonable in our opinion ). However, in model B the carbon concentration at point A increased significantly and was higher than point B.

Thus we are very confused about this phenomenon and want to learn why it is~


Thank you so much for your kind help!

Regards,

Sai Li

Bernd
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Re: how to turn off the Gibbs-Thomson effect

Post by Bernd » Tue Nov 12, 2019 4:57 pm

Dear Sai Li,

I think it is important in such a complex system not only to think of local equilibria at the interfaces, but also of kinetics.
If we assume that the models I and II like you sketched them represent an initial microstructure in MICRESS, and temperature is chosen slightly above the global equilibrium temperature (at the given phase fractions and composition), then in model I we would expect growth of fcc while cementite is shrinking. This means, carbon is consumed by the fcc/bcc interface, and the composition at point A will go down.
Considering system II at the same temperature, curvature of both, the fcc and cementite phase will shift equilibria and change kinetics. Given that curvature at the typical length-scale of cementite may be quite high, we now can observe fcc shrinking and cementite either shrinking or growing (depending on the exact values of the radii and the interface energies). In both cases, the carbon composition at point A will be much higher, because shrinking fcc releases carbon. In point B composition will depend on whether cementite grows or shrinks.
Furthermore, considering the direct effect of curvature on equilibrium concentration , in system II you really should have a Gibbs-Thompson effect which increases carbon composition at point A (relative to model I). However, it also should increase composition in point B.
This is just one scenario, and depending on how close or far the system is from global equilibrium it could behave differently, even more if the initial compositions are not close to equilibrium! I would suggest to consider a system which which reaches a stable end configuration. Then you can study the equilibrium compositions, and thus the pure Gibbs-Thompson effect, without kinetic interferences. However, you also should bear in mind that there may be numerical artifacts which potentially could come from a bad simulation setup...

Bernd

Ls_Thu
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Re: how to turn off the Gibbs-Thomson effect

Post by Ls_Thu » Tue Nov 19, 2019 8:58 pm

Dear Bernd,

Thank you so much for your kind reply !

I understand the numerical artifacts for setup, but is it possible for us to get the background principle and calculation process for MICRESS so that we could find the meaning for each parameters ?

Thank you again for your kind help!

Regards,

Sai Li

Bernd
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Re: how to turn off the Gibbs-Thomson effect

Post by Bernd » Tue Nov 19, 2019 11:24 pm

Dear Sai Li,

Can you please tell us which input parameters of MICRESS you are unsure about? Perhaps it would make things easier if you could append your input file.

Bernd

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