Modeling of Gamma-Alpha transformation

[Bernd]

Dear Deepu,

1. You do not see it in the .driv output. I think, the only place where you can see it directly is the .cpha output where you observe a different concentration value for the substitutional elements at the interface compared to the bulk of gamma. This shows that diffusion of these elements is not yet at equilibrium.
2. No, reality should be like that...
3. I don't understand under which circumstances you see this behavior. The nucleation conditions as well as the interface mobility should not change the final fraction of ferrite. Also, the diffusion data should not be essential as long as carbon is fast and the other elements are slow. If you see differences between experiment and simulation, it could be due to incorrect thermodynamic data or to wrong model assumptions (you could try para-equilibrium instead of nple). Numerical issues also never can be excluded...

Bernd

[deepumaj1]

Dear Bernd,
Is it possible to increase frequency of output in the TabK file? In the manual, it is told 'This file is updated by default each time a grain is set or disappears'. But can we change this? I want to see the average grain size of ferrite at more intervals. I'm pasting the current output of TabK below. Thanks.

Deepu

# Simulation Avg. radius Gr. density Grain Liquid Phase 1 Phase 2
# time [s] [micrometer] [micrometer**(-2)] Number present? n. avg. r. n. avg. r.
# NB: the radii are calculated from the average fractions
7.54445E-02 7.8428 5.17500E-03 207 No-Liquid 207 7.84278 0 0.00000
920.09 6.9056 6.67500E-03 267 No-Liquid 207 7.84278 60 0.00160
920.64 7.0524 6.40000E-03 256 No-Liquid 207 7.84278 49 0.00156
921.12 7.0801 6.35000E-03 254 No-Liquid 207 7.84278 47 0.00157
921.37 7.0941 6.32500E-03 253 No-Liquid 207 7.84278 46 0.00157
921.85 7.1081 6.30000E-03 252 No-Liquid 207 7.84278 45 0.00158
935.26 7.1223 6.27500E-03 251 No-Liquid 207 7.84278 44 0.00141
1515.4 7.1365 6.25000E-03 250 No-Liquid 206 6.93296 44 8.02101
1578.1 7.1508 6.22500E-03 249 No-Liquid 205 6.89497 44 8.23871
1749.8 7.1652 6.20000E-03 248 No-Liquid 204 6.82006 44 8.58605
1770.0 7.1797 6.17500E-03 247 No-Liquid 203 6.83063 44 8.60882
2437.9 7.1943 6.15000E-03 246 No-Liquid 202 6.74048 44 8.98828
2598.9 7.2090 6.12500E-03 245 No-Liquid 201 6.74143 44 9.04229
2606.0 7.2237 6.10000E-03 244 No-Liquid 200 6.75752 44 9.04480
2869.6 7.2386 6.07500E-03 243 No-Liquid 199 6.74853 44 9.13214

[Bernd]

Dear deepumaj1,

when invoking the .TabK output with "tab_grains" you can use the following optional keywords to change the output behavior:

standard: standard output intervals like e.g. for .TabF, i.e. for each regular output time step
extra: like standard, but including extra outputs when the number of grains changes

Bernd

[CharMIC]

Hi Deepu,

thank you for your questions.

1. The output 'Minimum undercooling for stable growth' is written when nucleation for a seed type is checked for the first time. The value is calculated from the interface energy between the new phase and the matrix phase, the corresponding DeltaS value and a critical radius which corresponds to one grid cell. The meaning of this value is the numerical undercooling which is needed to overcome the stage of a small grain. As you may know, there is a "small grain" model in MICRESS which helps new nuclei to overcome the single-grid stage and to get the size where the normal phase-field equation can take over. If the "stabilisation" model is used, the grain is stabilized until it reaches a fraction of 1 in the central cell. At this stage, stabilisation ends, and the grain is subject to a curvature undercooling which has a value close to the 'Minimum undercooling for stable growth' value.
For practice, the value means that the '# min. undercooling [K] (>0)' value should be higher than 'Minimum undercooling for stable growth' value to assure that growth is not hindered by the numerical grid. This is a quite good criterion for nucleation in the bulk, for nucleation at interfaces or triple junctions growth may be possible also with somewhat smaller undercoolings. One possibility to work around this numerical restriction is to use the "analytical curvature" model.

2,3. This means that for initialisation of the Thermo-Calc workspace, automatic start values are used for the phase compositions. MICRESS uses an extra workspace for each phase interaction. As you may know, Thermo-Calc always uses start compositions from the previous calculation. So, for the first one, an initialisation procedure is necessary. This has nothing to do with the accuracy of the calculations.

Bernd

Hej Bernd,

How can we get this DeltaS. Is it written in the .log file?

BR
Chamara

[Bernd]

Hi Chamara,

Yes, this value is given in the initial linearisation which is written to the .log file. It is the dSf+ value in case that the second phase is the seed phase.

Bernd