How grain boundary diffusion works

technical aspects of .dri file generation (e.g. debug mode ) etc...
Hamid
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Re: How grain boundary diffusion works

Post by Hamid » Tue Oct 05, 2010 4:31 pm

Then, the value for f is 1/8 not 8 as you mentioned above :) Am I right?
Hamid

Bernd
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Re: How grain boundary diffusion works

Post by Bernd » Tue Oct 05, 2010 5:37 pm

No,

\int {\phi_1 \phi_2} = \eta/8
therefore f must be 8 to compensate!

Bernd

Hamid
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Re: How grain boundary diffusion works

Post by Hamid » Tue Oct 05, 2010 6:03 pm

Agree.

Thank you again,
Hamid

CharMIC
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Re: How grain boundary diffusion works

Post by CharMIC » Thu Aug 16, 2018 6:26 pm

Bernd wrote:Dear all,

Grain boundary diffusion is available in MICRESS since more than 2 years, but nearly nobody used it up to now. It also has never been documented in detail. (but in the new manuals we are working on, it will of course be included!)

Grain boundary diffusion in MICRESS is implemented as an increased diffusivity of a phase in the interface region to other phases. This increase is expressed as a reduction of the activation energy, leading to a temperature-dependent factor on the diffusion coefficients. The user has to specify this activation energy correction in J/mol as well as the physical interface thickness which is needed to scale to some real thickness. Thus, grain boundary diffusion is only possible for elements in phases which have also diffusion in the bulk and not e.g. for stoichiometric intermetallics without solubility range!

The input works only in the terse mode, i.e. if each diffusion term is defined with a proceeding element and phase number. First, bulk diffusion has to be defined for this contribution. Afterwards, for the same contribution, grain boundary diffusion can be added by using the keyword "+b", followed by a string with length <number of phases +1> consisting of "n" or "b". This string defines for each phase whether in grain boundaries to the actual phase enhanced diffusion is to be considered. For each "b" character, an actication energy difference and a physical interface thickness has to be entered in extra lines, e.g.(ternary alloy, 3 solid phases):

# How shall diffusion of component 1 in phase 1 be solved?
1 1 multi gg
# How shall diffusion of component 2 in phase 1 be solved?
2 1 multi gg
# How shall diffusion of component 1 in phase 1 be solved?
1 1 +b nnbn
# Grain boundary diffusion settings for component
# 1 at boundary of phases 1 and 2:
Correction for activation energy? [J/mol]
1.8E3
Physical width of the interface? [nm]
2.5
...
end_diffusion_data

Bernd

Hej Bernd,

I did not understand the meaning of syntax "nnbn" could you bit explain what does this order mean.

BR
Chamara

Bernd
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Re: How grain boundary diffusion works

Post by Bernd » Fri Aug 17, 2018 7:18 pm

Dear Chamara,
# How shall diffusion of component 1 in phase 1 be solved?
1 1 +b nnbn
means the following:

For diffusion of component 1 in phase 1, the following boundary diffusion is added ("+b"):
Interfaces with phase 0: no ("n")
Interfaces with phase 1: no ("n")
Interfaces with phase 2: yes ("b")
Interfaces with phase 3: no ("n")

That means, in effect, enhanced diffusion of component 1 in phase 1 inside 1/2-Interfaces is considered. The amount of the enhancement is specified in the following 2 lines (for each "b").
As you see, an interface cannot have an extra diffusivity without reference to a phase (here: phase1)!

Bernd

CharMIC
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Re: How grain boundary diffusion works

Post by CharMIC » Fri Aug 16, 2019 12:41 pm

Hi Bernd

when you have set the parameters correctly, the D_GB physical out put in .diff file should be larger than the bulk diffusion coefficient value ?

Bernd
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Re: How grain boundary diffusion works

Post by Bernd » Mon Aug 19, 2019 10:13 pm

Hi Chamara,

not necessarily, because the output of the grain boundary diffusion coefficient shows the additional diffusivity which comes from the effect of the grain boundary. If you have set the parameters correctly, it should have the same sign as the corresponding bulk diffusion term, but it may be smaller (although in most cases you want it to be much bigger in order to have a pronounced effect).

Bernd

CharMIC
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Re: How grain boundary diffusion works

Post by CharMIC » Tue Aug 20, 2019 11:29 am

Yes. the value that I get is smaller than the bulk value. but has the same sign.

In practice should't it be higher than the bulk value?

Bernd
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Re: How grain boundary diffusion works

Post by Bernd » Tue Aug 20, 2019 2:28 pm

Hi Chamara,

That is true. If the "physical" value is already smaller than the bulk value, the scaled one would typically be even smaller. And even this smaller value would only apply to those cells which are exactly in the center of the numerical interface (Φαβ=0.5) and would be further decreased for all other interface cells! That means you could not expect any notable effect...
For this reason D_GB physical should in almost every practice case be bigger than the corresponding bulk terms.

Bernd

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