Diffusionless transformation

dendritic solidification, eutectics, peritectics,....
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Diffusionless transformation

Post by G_azizi » Fri Nov 18, 2022 12:21 am


I am simulating peritectic solidification and in that the solid state transformation from delta-ferrite to austenite takes place. This transformation rate is too high (in order of millimeters/second). But in my simulations it's rate is very low. how should I change the "phase interaction data" parameters or other parameters to get such a transformation rate. Some people believe this transformation is diffusionless (controlled by short range diffusion or massive transformation).
is there any way in MICRESS to simulate such a diffusionless transformation? or in MICRESS all transformations are diffusion controlled.


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Re: Diffusionless transformation

Post by Bernd » Fri Nov 18, 2022 12:49 pm


A diffusionless or "massive" transformation occurs if the driving force for transformation is so high that the pile-up of all elements at the interface can be overrun. Then, the front velocity can be much higher than in the diffusion-limited case. For each element, depending on the physical background, solute redistribution may still take place, or not. The first condition is called "NPLE" or "LENP" (no partitioning - local equilibrium), the second "para-equilibrium", the difference is how much driving force is essentially necessary to get into the condition.

In MICRESS, we handle that by using the "redistribution_control" option, which allows for definition of the corresponding condition for each element:

a) diffusion-controlled:
normal mob_corr
atc mob_corr (including an anti-trapping correction of diffusion)

In this case, diffusion control is assumed even if the driving force is so high that a diffusionless transformation could be possible

b) NPLE/LENP-condition

c) Para-equilibrium
para (alternatively, the Thermo-Calc para-equilibrium model can be used with "paraTQ")

For the γ-α or δ-γ-transformation, typically, diffusion control by the element C is assumed. So, the choice in MICRESS would be to use "normal mob_corr" for C (and other fast diffusing interstitial elements like N, O, H, P), and use "nple" for all other elements. Then, the transformation should occur fast, only controlled by carbon diffusion.

PS: If you would use MICRESS without "redistribution_control" option, the δ-γ-transformation should be fast also, but potentially numerically unstable and with wrong kinetics.


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