stored energy in deformed austenite

solid-solid phase transformations, influence of stresses and strains
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nokkikku
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stored energy in deformed austenite

Post by nokkikku » Wed Nov 25, 2009 2:26 pm

Dear Micress experts,

you know that the deformation in austenite accelerates the gamma-alpha transformation very much. On one hand, it increases the number of ferrite nuclei, which we can assign as the maximum number of new nuclei. On the other hand, the higher number of dislocation raises up the delta G terms in gamma-alpha transformation, which can affect directly the delta G term in the phase field equation.

I heard that we can include the extra term of delta G (from higher dislocation) as the 'stored energy in austenite for recrystallization' even if we do not have recrystallization. I want to know if the value for stored energy will go directly to the delta G term in the phase field equation? If so, we should see faster ferrite growth or bigger ferrite grains, right? And if we put too high stored energy, recrystallization in austenite can be triggered, right?

With many thanks in advance
nokkikku

Bernd
Posts: 1504
Joined: Mon Jun 23, 2008 9:29 pm

Re: stored energy in deformed austenite

Post by Bernd » Wed Nov 25, 2009 5:59 pm

Dear nokkikku,

The recrystallisation model included in MICRESS at present is quite simple: You can define a "stored energy" value in the grain input or for nucleation. This value is a constant for the given grain and not changing with time or position.

In a typical recrystallisation application, you would start with an initial microstructure consisting of grain swith different stored energies, e.g. depending on the orientation of the grains with respect to a virtual deformation direction. Through nucleation of new grains with a stored energy of 0 recrystalisation would start. The nuclei would overgrow the old deformed grains with different velocities, depending on the amount of stored energy.
With this scenario in mind, we implemented an additional driving force resulting from the difference in the stored energy between the grains/phases which takes only effect if one of the grains/phases has a stored energy of 0.

You could use this model to your application if you start from a deformed austenite structure (with stored energy >0) and nucleate ferrite without stored energy. This would increase the driving force for the gamma-alpha transformation by the amount of the stored energy in the austenite. For doing so, you have to define recrystallisation for both phases in the phase input section of the MICRESS input file.
By also allowing nucleation of austenite in austenite (with stored energy 0), you can include a competing recrystallisation of austenite which could be taking place simultaneously with the gamma-alpha transformation.


Bernd

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