Re: Analyzing average compositions in dendritic and interdendritic regions
Posted: Tue Feb 08, 2022 7:16 pm
Dear Ahmet,
First of all, I think it is dangerous to base your observations on the peak concentration. Both, in experiments as in simulation, if you have Al-rich precipitates, the peak concentration depends on the discretization size (beam size, grid size) and the relative particle sizes. Instead, you should definitively use some integral measures for your comparison.
I can't follow your argument, that with fast cooling segregation should be less strong because "the diffusion distances is shorter", and "therefore there might be no time to re-arrange this concentration spike/gradient". The reason is that the microstructure length scale (primary/secondary spacing) is also correspondingly smaller, so that from this point of view there should be no difference. You can only have a difference from this source, if the morphology is not scaling with the diffusion length, i.e there is a systematic change or morphology, e.g. from more dendritic to more cellular.
A contribution which does not scale in this way is curvature and the curvature undercooling. There must be a systematic shift of the solidification temperature to lower values with increasing cooling rate. This temperature shift can affect the solubility of Al in the primary phase, leading to more or less strong segregation. If you want to evaluate this effect with Thermo-Calc, you would have to keep the phase fractions constant while lowering the temperature (so that the resulting driving force resembles the effect of curvature on the equilibrium compositions).
Another quantity which also does not scale with the cooling conditions is the physical interface thickness (which we assume to stay the same) and which causes solute trapping. This effect is not automatically included in MICRESS (essentially it is even prevented by using the thin interface correction models). However, I believe that solute trapping is still not relevant under all three condition which you compare.
I agree fully to your statement that the homogenization time strongly decreases for higher cooling rate microstructures, so that even the relatively short reheating or elevated process temperature can be responsible for reduced segregation found in the experiments in case of SLM.
Bernd
First of all, I think it is dangerous to base your observations on the peak concentration. Both, in experiments as in simulation, if you have Al-rich precipitates, the peak concentration depends on the discretization size (beam size, grid size) and the relative particle sizes. Instead, you should definitively use some integral measures for your comparison.
I can't follow your argument, that with fast cooling segregation should be less strong because "the diffusion distances is shorter", and "therefore there might be no time to re-arrange this concentration spike/gradient". The reason is that the microstructure length scale (primary/secondary spacing) is also correspondingly smaller, so that from this point of view there should be no difference. You can only have a difference from this source, if the morphology is not scaling with the diffusion length, i.e there is a systematic change or morphology, e.g. from more dendritic to more cellular.
A contribution which does not scale in this way is curvature and the curvature undercooling. There must be a systematic shift of the solidification temperature to lower values with increasing cooling rate. This temperature shift can affect the solubility of Al in the primary phase, leading to more or less strong segregation. If you want to evaluate this effect with Thermo-Calc, you would have to keep the phase fractions constant while lowering the temperature (so that the resulting driving force resembles the effect of curvature on the equilibrium compositions).
Another quantity which also does not scale with the cooling conditions is the physical interface thickness (which we assume to stay the same) and which causes solute trapping. This effect is not automatically included in MICRESS (essentially it is even prevented by using the thin interface correction models). However, I believe that solute trapping is still not relevant under all three condition which you compare.
I agree fully to your statement that the homogenization time strongly decreases for higher cooling rate microstructures, so that even the relatively short reheating or elevated process temperature can be responsible for reduced segregation found in the experiments in case of SLM.
Bernd