how to simulate the evolution of voids in a material

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dongxiao
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how to simulate the evolution of voids in a material

Post by dongxiao » Wed Apr 06, 2016 5:59 pm

Hi, I'm a new user of Micress, and I want use Micress to simulate the evolution of voids in steel. Let's assume that a pure iron is under irradiation. After irradiation, there are many small vacancies in the iron, and I think the vacancy will aggregate to form voids(bigger vacancy). I don't think this progress is a phase transition, the driving force is mainly surface energy. So when I come to the step to be asked to input phase diagram, I don't know haw to handle it. Could you give me some advice?

Bernd
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Joined: Mon Jun 23, 2008 9:29 pm

Re: how to simulate the evolution of voids in a material

Post by Bernd » Wed Apr 06, 2016 8:02 pm

Dear dongxiao,

Welcome to the MICRESS forum.

Generally, MICRESS has strong restrictions for handling voids: All objects or "grains" in MICRESS are always accociated with a phase, and the phase-field equation describres the motion of phase (or grain) interfaces with time, taking into account a chemical driving force (which comes from thermodynamics) and curvature. For moving, the interface always has to transfer matter from one phase (or grain) to the other.
Furthermore, at present, another restriction is that all phases have the same molar volume, i.e. even if we use a gas phase, it always is treated to have the same number of atoms per volume as the solid or liquid phases (there is an input of molar volume for each phase in some circumstances, but this is only for the correct calculation of latent heat or for unit conversion of the chemical driving force). From the application view, this is a strong restriction, but from the implementation view this avoids all problems which are linked with moving away material if e.g. one phase is expanding. Therefore, practically all phase-field models known from literature make this assumption. However, Janin, one of our team, is currently working on overcoming this restriction. So, hopefully, in future we will have the possibility to correctly handle the volume of e.g. gas pores.

Thus, in summary, the strict answer to whether MICRESS can handle pores in a straighforward way is NO. But of course there always exist work-arounds. For example, one can construct a pseudo-phase diagram which would behave in a similar way as we expect it from the pores. The key question is what we expect to be the fundamental processes:

- what makes the irom atoms move so that there is a coarsening of the pores? Can it be described by some sort of diffusion equation (vacancies in iron)?

- is the total pore volume conserved? Then a pseudo-element which can dissolve in iron could be a valid approximation.

- which is the length-scale of the process? Is it atomistic, nanoscopic or microscopic? Does the meanfield approximation of the phase-field model apply?

Some years ago I did a simulation of a cobald oxide ceramic which develops pores or voids when a membrane of this material is exposed to an oxygen partial pressure difference. This process could be astonishingly well simulated with MICRESS using a pseudo-phase diagram approach, although the gas phase had the wrong molar volume. But always in such approaches there an element or species must exist (in that case oxygen) which can change between the two phases.

Without knowing the answers to the above questions, a first trial could be to have a binary "alloy" Fe-VA, where the VA concentration level corresponds to a certain irradiation damage. A stoichiometric "pore" phase consisting of 100% VA could nucleate and ripen according to the surface energy and diffusion kinetics of VA in Fe. Such an approach could be accomplished relatively easy and would have to be calibrated by experimental knowledge.

Best regards

Bernd

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