microsegregation

aspects of evaluating simulation results and their graphic presentation using either DisplayMICRESS or other software tools. Features and possibilities of DisplayMICRESS
RAGHAV
Posts: 45
Joined: Thu Jul 10, 2008 6:21 pm

microsegregation

Postby RAGHAV » Thu Jan 15, 2009 4:07 am

Hello Mr. Bernd,

Could you tell me why can we see a variation of the Al
concentration inside of a grain in solidification of alloy AlCu (Al= 9 % wt. %, Cu = 81 wt. %)?

Could it be due to dendrite formation or at least dendrite like grains?

Bernd
Posts: 760
Joined: Mon Jun 23, 2008 9:29 pm

Re: microsegregation

Postby Bernd » Thu Jan 15, 2009 8:03 pm

Dear RAGHAV,

please excuse me for splitting your question off from "other postprocessing programs"! I think, your actual post is not related to this topic...

For practically all alloys, solidification leads to a microsegregation effect, because the dissolved elements (Al in your case) have a different solubility in the solid and liquid phase. This leads to the fact that, during solidification, the composition of the melt is changing. Because the composition of the solid is always related to the composition of the melt (in first approximation by a constant partition coefficient), also the composition of solid will change during solidification. This "solidification history" is not changing afterwards because, typically, diffusion is very slow in the solid. Only a long homogenisation treatment at elevated temperature could remove such a microsegregation pattern.

In solidification processes, microsegregation occurs practically always, independent whether the solidification is cellular or dendritic. It can be only prevented if one enforces a planar solidification front. This is possible for an extremely slow solidification speed combined with a high temperature gradient, or with extremely high cooling rates, when solute trapping occurs (several 1000 K/sec).

Is this what you describe the result of a simulation or of an experiment?

Bernd

RAGHAV
Posts: 45
Joined: Thu Jul 10, 2008 6:21 pm

Re: microsegregation

Postby RAGHAV » Sat Jan 17, 2009 1:04 am

Hello Mr. Bernd

Actually, this was the result of a simulation.

Thanks for such a explanatary anwer.

MFG
Raghav

Shenyz
Posts: 10
Joined: Mon Jul 25, 2016 4:33 pm
anti_bot: 333

Re: microsegregation

Postby Shenyz » Thu Jun 01, 2017 4:27 am

Dear Bernd:
Hello! :D
Recently I am confused about the solute element distribution during the solidification.As we all know, for practically all alloys solidification leads to a microsegregation effect, because the dissolved elements (Al in your case) have a different solubility in the solid and liquid phase. And there will always be segregation in the dendritic and between dendrites. When I observe the segregation in the dendritic I find that the solute content in the dendritic center is very small which is not inconsistent with the experimental results. And sometimes when I view the solute distribution in the .conc I can see the bright lines in the center of primary dendrites and secondary dendrites. Can you tell me is it right and how to improve this phenomenon?
Besides I find the primary dendrites in my experimental results are more coarse then the simulation results. The primary dendrites in simulation are thin eventhough the subcooling is big. How can I deal with it?
Thank you very much!

Bernd
Posts: 760
Joined: Mon Jun 23, 2008 9:29 pm

Re: microsegregation

Postby Bernd » Thu Jun 01, 2017 4:59 pm

Dear Shenyz,

this is an interesting observation. I believe the bright lines are due to solute trapping which is highest at the center of the dendrite tip because the normal velocity is highest. So far, the effect is real. However, due to the unphysical thickness of the interface in our phase-field approach, the effect is exaggerated. It would be interesting to try out whether it is smaller when you use "atc" correction (in phase diagram input if you have chosen "redistribution_control" in the corresponding phase interaction data). If you give it a try, please tell us about the outcomes. (Edit: please see next post!)

Dendrites in simulation can change morphology for many reasons. One important point is whether they grow at the correct tip undercooling. If the interface mobility is too high (compared to the numerical equivalent for diffusion limited growth), dendrites tend to be too thin (and the other way round). You should try to use "mob_corr" (also in phase diagram input) if possible to get close to diffusion limited growth without the need to calibrate the interface mobility.
Other parameters like the interface energy and anisotropy as well as averaging options for dG also have strong influence on dendrite morphology. Unfortunately, the problem of dendrite growth is too complex to be able to make quantitative morphology predictions for dendrite arms without calibration to experiments...

Bernd

Bernd
Posts: 760
Joined: Mon Jun 23, 2008 9:29 pm

Re: microsegregation

Postby Bernd » Fri Jun 02, 2017 2:34 pm

Dear Sheniz,

my answer was too fast - there is a much simpler explanation for the bright line inside the dendrite: Due to the highest curvature at the tip, equilibrium concentrations are shift :oops:

Example: W-composition in directional dendrites of Ni-superalloy:

A247_DS_concW.png
A247_DS_concW.png (191.34 KiB) Viewed 121 times


If resolution is low, these lines can be just one grid cell in thickness.

Bernd


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