Hi,
I am new in MICRESS, and in the first step i wanted to start my simulation by modeling solidification of IRON. as i looked up in the scripts you give along with the MICRESS software, i found that we should define the interface mobility for different phases. in Literature several methodes for calculation of interface mobility can be found and this is a littel confusing. it would be great if you could help me to find good refernce that matches the simulation in MICRESS as well.
BR
Mehnoush
Mobility coefficient
Re: Mobility coefficient
Dear Mehnoush,
Welcome to the MICRESS forum.
MICRESS is based on the multiphase-field model which is a kinetic model for phase transformations. Therefore, the interface mobility is a central concept.
However, from a physical point of view, the interface mobility is only important in case of kinetically limited phase transformations. In solidification, typically, transport of solute or heat is rate limiting, depending on whether we speak about solidification of alloys or pure elements ("diffusion limited"). Interface kinetics would only be limiting in case of very fast solidification with interface velocities in the order of 1m/s.
Thus, in almost all solidification cases, the interface mobility is not rate limiting and therefore not a physical parameter (at least if we talk about metallic phases). Then in simulation, in principle, the interface mobility just needs to be high enough so as not to limit the solidification reaction.
Practice often is a bit more complicated, because numerical artifacts which are caused by too coarse spatial discretisation can be corrected by the suitable choice of the interface mobility.
Which type of solidification do you want to simulate? Is it pure iron or are we talking about iron alloys (steels)?
Best wishes
Bernd
Welcome to the MICRESS forum.
MICRESS is based on the multiphase-field model which is a kinetic model for phase transformations. Therefore, the interface mobility is a central concept.
However, from a physical point of view, the interface mobility is only important in case of kinetically limited phase transformations. In solidification, typically, transport of solute or heat is rate limiting, depending on whether we speak about solidification of alloys or pure elements ("diffusion limited"). Interface kinetics would only be limiting in case of very fast solidification with interface velocities in the order of 1m/s.
Thus, in almost all solidification cases, the interface mobility is not rate limiting and therefore not a physical parameter (at least if we talk about metallic phases). Then in simulation, in principle, the interface mobility just needs to be high enough so as not to limit the solidification reaction.
Practice often is a bit more complicated, because numerical artifacts which are caused by too coarse spatial discretisation can be corrected by the suitable choice of the interface mobility.
Which type of solidification do you want to simulate? Is it pure iron or are we talking about iron alloys (steels)?
Best wishes
Bernd
Re: Mobility coefficient
Hi Bernd,
Thanks a lot for your massage.
In first step i will try to model solidification of pure iron because i tought it may be easier. but actually the main goal is modelling the microstructure evolution of 42crmo4 during electrical discharge machining (EDM) process. during EDM the material will heated up to the melting or even evaporation temperature and following it will be cooled down by high cooling rates. a ignition time of a spark is in order of µs
.
BR
Mehnoush
Thanks a lot for your massage.
In first step i will try to model solidification of pure iron because i tought it may be easier. but actually the main goal is modelling the microstructure evolution of 42crmo4 during electrical discharge machining (EDM) process. during EDM the material will heated up to the melting or even evaporation temperature and following it will be cooled down by high cooling rates. a ignition time of a spark is in order of µs
. BR
Mehnoush
Re: Mobility coefficient
Dear Mehnoush,
I don't think it makes much sense to start with pure iron because it is probably not easier, and the resulting microstructure will be completely different.
The simulation of solidification of pure iron requires temperature coupling because heat conduction is rate determining for the solid-liquid phase transformation. If you later like to switch to an alloy, you would have to switch to concentration coupling because then solute diffusion is rate determining for solidification. This means that you would have to start completely from scratch...
However, the above said would not be valid if solidification is so fast that solute diffusion is not relevant anymore. Do you have any estimate about the solidification velocity or cooling rate?
Bernd
I don't think it makes much sense to start with pure iron because it is probably not easier, and the resulting microstructure will be completely different.
The simulation of solidification of pure iron requires temperature coupling because heat conduction is rate determining for the solid-liquid phase transformation. If you later like to switch to an alloy, you would have to switch to concentration coupling because then solute diffusion is rate determining for solidification. This means that you would have to start completely from scratch...
However, the above said would not be valid if solidification is so fast that solute diffusion is not relevant anymore. Do you have any estimate about the solidification velocity or cooling rate?
Bernd
Re: Mobility coefficient
Hi Bernd,
Then, is it better that i directly start with the steel alloy?
the cooling rate is in order of 10^6 . In EDM high heat flux applies in a very small surface during a single discharge. the material constantly is in contact with dielectric (oil or water). in edm is some how like arc welding but with higher heating and cooling rated and also more concentrated heat flux on the surface.
Regards,
mehnoush
Then, is it better that i directly start with the steel alloy?
the cooling rate is in order of 10^6 . In EDM high heat flux applies in a very small surface during a single discharge. the material constantly is in contact with dielectric (oil or water). in edm is some how like arc welding but with higher heating and cooling rated and also more concentrated heat flux on the surface.
Regards,
mehnoush
Re: Mobility coefficient
Dear Mehnoush,
to be honest, I have no experience with this process and with such high cooling rates.
But it should be visible in experimental microstructures whether solute diffusion is relevant or not: If there is solute diffusion, there should be microsegregation. If there are no concentration differences, diffusion does not occur, and the alloy is behaving like a pure element, just the melting point is shifted by alloying.
Do you have experimental information about microsegregation?
Bernd
to be honest, I have no experience with this process and with such high cooling rates.
But it should be visible in experimental microstructures whether solute diffusion is relevant or not: If there is solute diffusion, there should be microsegregation. If there are no concentration differences, diffusion does not occur, and the alloy is behaving like a pure element, just the melting point is shifted by alloying.
Do you have experimental information about microsegregation?
Bernd
Re: Mobility coefficient
Hi Bernd,
thanks alot for your reply,
it is different from one material to another and also it is dependent on the process parameters. but in general i can be assumed that no microsegregation can be seen. the alloying particles are also distributed homogeneously in the matrix after process.
best regards
Mehnoush
thanks alot for your reply,
it is different from one material to another and also it is dependent on the process parameters. but in general i can be assumed that no microsegregation can be seen. the alloying particles are also distributed homogeneously in the matrix after process.
best regards
Mehnoush
Re: Mobility coefficient
Hi Mehnoush,
if there is no microsegregation visible, we can assume that solidification is not solute diffusion controlled. Then, it seems most reasonable to treat it as a pure substance (with modified melting temperature as compared to pure iron) and to assume heat diffusion limitation. Then the suitable MICRESS simulation mode would be temperature coupling, and interface mobility would still not be an issue...
Bernd
if there is no microsegregation visible, we can assume that solidification is not solute diffusion controlled. Then, it seems most reasonable to treat it as a pure substance (with modified melting temperature as compared to pure iron) and to assume heat diffusion limitation. Then the suitable MICRESS simulation mode would be temperature coupling, and interface mobility would still not be an issue...
Bernd
Re: Mobility coefficient
Thanks Bernd.