Shape of grain boundary

dendritic solidification, eutectics, peritectics,....
dingling312
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Shape of grain boundary

Post by dingling312 » Sun Sep 22, 2019 10:38 am

Dear Bernd,
Thank you for your help. Now I am simulating the suspended smelting Ti-6Al-4V alloy. I found that when there are only three elements in the alloying elements, the grains are round, but when some Fe is added to the alloy, the grains gradually change from circular to polygonal, which is very similar to the experimental results. I would like to ask what you think may be the cause of this change or which equation or parameter in the micress affects the shape of the grain boundaries, thank you!


Ling

Bernd
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Re: Shape of grain boundary

Post by Bernd » Mon Sep 23, 2019 10:15 am

Dear Ling,

probably it has to do with segregation of Fe. However, I would need some more information about the process and simulation conditions in order to be sure to understand what you observe.

Bernd

dingling312
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Re: Shape of grain boundary

Post by dingling312 » Fri Sep 27, 2019 4:27 am

Dear Bernd,
Thanks for your reply。These are the results of the gradual decrease in Fe content. I want to know how micress software affects the boundary morphology based on component segregation. If the temperature gradient I set is a fixed value, can segregation produce local supercooling? If so, can this value be extracted?

Ling
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Bernd
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Re: Shape of grain boundary

Post by Bernd » Sun Sep 29, 2019 11:03 pm

Hi dingling312,

of course segregation produces local supercooling. How much depends on the phase diagram information: the liquidus slope and the difference of the equilibrium composition for each element. You can find these data in the initial linearisation which is written to the .log file.
I would guess that Fe shows a big difference of phase compositions (because of low solubility in the solid) and a bigger liquidus slope.

Bernd

dingling312
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Re: Shape of grain boundary

Post by dingling312 » Tue Oct 08, 2019 10:23 am

Dear Bernd,

Thanks for your reply。Now I want to measure the undercooling of the local boundary. I see the mueS file can provide a interface mobility which defines the interface velocity of the boundary,and the unit of the interface mobility is cm*4/Js。How can I convert this unit into a common unit of velocity like mm/s? Or is there any way to calculate the specific values of local undercooling or temperature gradient at the boundary?

Ling

Bernd
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Re: Shape of grain boundary

Post by Bernd » Tue Oct 08, 2019 3:00 pm

Hi Ling,

I think it is important to have clear what "undercooling" means in this case. Local undercooling simply is the difference between the local temperature and a suitable reference temperature (which typically is the liquidus temperature in case of solidification). However, it depends on the local thermodynamic and kinetic properties in which form this (total) undercooling is realized. If the phase interface is not allowed to move (μ=0), then all the undercooling has form of a "kinetic undercooling". Only if we are close to this condition (kinetic control), the undercooling can be obtained from the kinetic equation v=μ*ΔT.
In the other extreme case, interface kinetics are assumed to be "diffusion limited", and the kinetic undercooling ΔTkin=0. This is what we typically have with solidification, and to get it correct in MICRESS, we use "mob_corr" or calibration techniques. Then, the total undercooling consists of a "chemical undercooling" ΔΤchem and/or a curvature contribution ΔTcurv. In case of elastic coupling, an elastic contribution would add up further.
What you can observe (and measure) in MICRESS is the total undercooling (as temperature) and the difference of the total undercooling and the chemical undercooling in form of the driving force ΔG=ΔS*(ΔTtot-ΔTchem). This value which you can find in the .driv output equals the sum of the other contributions: ΔG=ΔS*(ΔTkin + ΔTcurv + ΔTelast).

Thus, as you see, there is no type of undercooling which can be identified locally in the general case, and which reflects the effect of segregation of an additional element. What you can do is to refer to a specific scenario like a stationary planar front, which show a total undercooling which depends on the segregation of the individual elements. Or you could refer to other measures like the Growth Restriction Factor (GRF) which describes how much undercooling is needed to advance a phase front with given kinetics.

Bernd

dingling312
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Re: Shape of grain boundary

Post by dingling312 » Thu Oct 10, 2019 8:53 am

Hi,Bernd
Thank you for your explanation of undercooling. Now I use bulk with seed density to nucleate the grains as shown in the figure, and try to further nucleate at the position marked by the red line. I tried to add seed types (interface or triple), but other types of seeds could not be generated. I want to know if this idea is feasible or if my parameter settings are wrong.

Ling
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Bernd
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Re: Shape of grain boundary

Post by Bernd » Thu Oct 10, 2019 2:01 pm

Hi Ling,

In your current setup, you would get seeds only at interfaces between 2 grains of phase 1, and furthermore there would be no undercooling (same phase nucleation)!
If your aim is to nucleate at the interface between liquid and phase 1, then you should set the reference phase to 0 and the substrate phase to 1. But still, there will be no undercooling apart from the local kinetic undercooling of the already existing 0/1 interface!

Alternatively, you could use nucleation with option "region" for specifically defining a zone ("red rectangle") where you check for nucleation, or use "front" which is available in the latest version 6.4: Here you can define a distance from the bulk dendrites at which nucleation will be checked, i.e. nucleate" in front of" the bulk dendrites.

Bernd

dingling312
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Re: Shape of grain boundary

Post by dingling312 » Thu Oct 10, 2019 2:57 pm

Hi Bernd,

I modified the substrate and reference phases according to your opinion. Now I can nucleate through the interface, but the interface nucleation is formed at the beginning of the calculation and cannot grow up. And I hope that the interface nucleation is generated during the calculation process, is it necessary to adjust the noise?

Ling
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Bernd
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Re: Shape of grain boundary

Post by Bernd » Fri Oct 11, 2019 12:21 am

Hi Ling,
That is what I told you: Nucleation is only possible under conditions of kinetik undercooling (i.e. at the beginning).
Can you tell me what is the aim of your simulation setup?

Bernd

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