Error when linearisation!

[Bernd]

Dear Chamara,

in general, you have still steps 2) and 3) as explained above to cope with the demixing problem. However, please keep in mind that 750K is a very low temperature for a Ni-based alloy, and nothing will happen anyway at any realistic length and time scale. What do you expect what should happen?

Thus, I would stop simulation before reaching such low temperatures, and not worry about the problem.

Bernd

[CharMIC]

Hej Bernd,

In the simulation I am trying to see the microstructure evolution due to multiple thermal cycles in Laser Metal deposition process where multiple tracks have been deposited on up on another. Thermal thermal profiles are from thermocouple measurement. before the next thermal cycle comes, the temperature drops to a value around 400K and when the next layer comes again it reach to a higher temperature.

I am now running the model without modifying the thermal profiles.

since there was not error message showing "Thermo-calc error..." I wanted to be sure about this error is due to demixing

BR
Chamara

[CharMIC]

Hej Bernd,

I am trying with the step (3) that you suggested. Could you please confirm that I have modified the .GES5 file referring the following image

BR
Chamara

[Bernd]

Hej Chamara,

Did you try step 2) before? For the given conditions it seems much more reasonable, because it could perhaps just avoid demixing without introducing extra complications.

Step 3) means including an additional phase in MICRESS. Given the fact that you are already at a very low temperature where diffusion is already practically halted, and that there is no experimental evidence for further phases, I do not see what you could gain. If there is really a demixing of Laves at low temperatures, the length scale will be such small that you can obtain only a diffuse phase mixture at the best.
Technically, how you create LAVES#2 is correct. However, for selection of the major constituents of LAVES#2, you would need to refer to a defined Thermo-Calc equilibrium calculations where a second Laves phase really proved to be stable...

Bernd

[CharMIC]

Hej Bernd,

I was just trying step 3) to see what happens. Just for learning purpose

I tried step 2) with limits for Nb content (min 20 max 100). It works until 650K. So Its reasonable. I am now making modification to my thermal profiles so that I stop at 700K and restart at 700K for the next heating cycle.

The problem I am having is that the Mo content in LAVES is much lower that the reported values. I get like less than 1wt% where in experiments it gives like 10wt%. I don't know who to fix it. ?

Just for your information, you said that its allowed to used component 0 when using the stoichiometric condition. But I was not allowed to defined component 0 in LAVES phase in MICRESS simulation. I dont know if its a bug or not. I wanted to defined stoichio for component 0 (in this case Ni) since I am running a simulation with a modified .GES5 file as follows. In literature it reports that LAVES as (Ni,CR,Fe)₂(Nb,Mo).

Also in such cases should i defined stoichio for all components or only for Ni (if it allowed by MICRESS),Cr,Fe,Nb and Mo?


BR
Bernd

[Bernd]

Hej Chamara,

To be honest, I was not completely sure whether it is allowed by MICRESS to set component 0 stoichiometric, but I am sure that it HAS NO EFFECT! Thus, please set all other components of LAVES to stoichiometric in order to avoid problems.

With the Mo composition, I don't know. In an old simulation which I did with TTNI7, Laves in IN718 contained about 8 wt% Mo with moderate Si contents, but less than 3% when practically no Si was present. But I stopped simulation directly after solidification and did not check what happens at lower temperatures. I did not explicitly check for composition sets in those times...
I would do a scan with Thermo-Calc and global minimization to find out which composition set are stable in the given concentration and temperature range.

Bernd

[CharMIC]

Hej Bernd,

I see low Mo content even at high temperature (around 1273K). So I don't think its because of low temperatures. When I do scheil simulation in Thermo-calc I get the same results, low Mo content. May be due to errors in database?

BR
Chamara

[CharMIC]

Hej Bernd,

For your information..

I contacted Thermo-calc regarding this low Mo and also High Nb (~40 wt%) in Laves phase. They told me that "one reason for this could be that no parameter is assessed for Cr-Nb-Mo system in the TCNI8 database"

BR
Chamara

[Bernd]

Hej Chamara,

This sound reasonable: If some Nb in Laves would be replaced by Mo, both of your problems would be solved...

Bernd

[CharMIC]

Hi Chamara,

When different composition sets of a phase like Laves are coming into play, it is important to first have a look on phase constitution:

Laves_Constitution.GIF

As you can see in the Thermo-Calc System Definer, C14_LAVES has two sublattices which accept all your elements but C.

In order to get a feeling which lattice constitutions could be stable, you then should make a Thermo-Calc equilibrium calculation close to your local conditions in MICRESS (e.g. interdendritic composition and T=800K):

Laves_Composition_Sets.GIF

In case of a single equilibrium calculation, you can evaluate the phase constitution of C14_LAVES in the Table Renderer. Here, it is important to suppress other phases which are metastable (like delta, sigma, etc.). In this (arbitrarily chosen) case you see that Thermo-Calc automatically has chosen two composition sets with arbitrary numbers. The constitution shows different sublattice site fractions for the elements in the different composition sets.

Thermo-Calc automatically creates new composition sets if global minimization is enabled, which is not possible (and also not wanted) in MICRESS. Therefore, it is necessary to create and correctly define those composition sets already when creating the .ges5 file.

Now, if problems with demixing of the phase compositions exist, like in your case, when just the "normal" C14_LAVES has been used, the user should proceed in the following order:

1.) Correctly define the default major components for the phase according to your knowledge or expectation.
When you create the .ges5 file, just before saving it while you are still inside the GES module, use AMEND_PHASE_DESCRIPTION and specify the phase name ("C14_LAVES"). Now, chose to amend MAJOR_CONSTITUENT, which allows you specifying the major constituent(s) for each sublattice. In case the C14_LAVES#3 is the one you prefer, you should specify Fe on the first and Nb and Mo on the second sublattice. This will assure that the start values for calculation of equilibria are set accordingly. You can control this choice through the MICRESS output "Start Composition for iteration of quasi-equilibrium" which is written to the screen output and .log file during initialisation.

2.) If setting the major constituents is not sufficient to prevent demixing, you should use limits or relative criteria in MICRESS for not allowing to enter into another composition set. Especially the limits should be used with care and only for one element which is the best to distinguish the composition sets. Please have in mind that limits are specified in at% in MICRESS.

3.) If you still have problems with demixing, then the other composition set(s) which cannot be prevented from forming seem to be necessary and should be included into the MICRESS simulation. Then you have to go back to Thermo-Calc and add the composition set to the .ges5 file (if not already present). Use AMEND_PHASE_DESCRIPTION with the suboption COMPOSITION_SET to create a new composition set according to your expectations.

Bernd

Hi Bernd,

I want to model gamma' in IN718. In TCNI8 database it has the name FCC_L12#2. But when I list phases, when I create the guess file I get only FCC_L12. So what should I do to get this FCC_L12#2. I also want to change its major constituents.

BR
Chamara