Precipitation phase dissolve back into matrix during simulation

solid-solid phase transformations, influence of stresses and strains
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Precipitation phase dissolve back into matrix during simulation

Post by Junzhen » Wed Jul 10, 2019 2:15 pm

Hi, I have a Problem when I am trying to simulate the precipitation during cooling process.
I indeed define the nucleation of precipitation Phase, however, the precipitates dissolve back into the matrix during the Simulation process.
Part of input is shown below. Did I set any wrong parameter?

# Input for seed type 4:
# ----------------------
# Type of 'position' of the seeds?
# Options: bulk region interface triple quadruple front [restrictive]
# Phase of new grains (integer) [unresolved|add_to_grain]?
# Reference phase (integer) [min. and max. fraction (real)]?
# Substrate phase [2nd phase in interface]?
# maximum number of new nuclei 4?
# (set negative for unlimited number)
# Grain radius [micrometers]?
# Choice of growth mode:
# Options: stabilisation analytical_curvature
# min. undercooling [K] (>0)?
# Shield effect:
# Shield time [s] [shield phase or group number] ?
# Shield distance [micrometers] [ nucleation distance [micrometers] ]?
# Nucleation range
# min. nucleation temperature for seed type 4 [K]
# max. nucleation temperature for seed type 4 [K]
# Time between checks for nucleation? [s]
# Options: constant from_file
# Time interval [s]
# Shall random noise be applied?
# Options: nucleation_noise no_nucleation_noise
# Seed for random-number generator initialisation
# -----------------------------------------------
# Max. number of simultaneous nucleations?
# ----------------------------------------
# (set to 0 for automatic)
# Shall metastable small seeds be killed?
# ---------------------------------------
# Options: kill_metastable no_kill_metastable

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Joined: Mon Jun 23, 2008 9:29 pm

Re: Precipitation phase dissolve back into matrix during simulation

Post by Bernd » Wed Jul 10, 2019 7:09 pm

Dear Junzhen,

You specify a critical undercooling of 0 K, but this is not a sufficient condition that the nucleus which you set can really grow. There are other conditions:

- The undercooling must be high enough to overcome curvature. How big the curvature is depends on the length scale (radius r), the interface energy as well as the transformation entropy ΔS.

- If grid resolution Δx is very fine, small grain need to overcome a certain 'numerical curvature' which is written to the screen and .log output when nucleation is checked for the first time (in case of TQ-coupling). The solution in case you would need unrealistically high undercooling values is to use "analytical_curvature" with a correspondingly larger critical radius.

- Furthermore, numerical problems (e.g. too high interface mobility etc.) or thermodynamic problems (ill-defined phase diagram, convergence problems in case of TQ-coupling) can make the precipitate vanish.

Apart from the information that the critical undercooling may be just too low, there is no hint about where the problem comes from in the nucleation data which you have showed. Obviously, nucleation works fine, but the nucleus dissolves afterwards. Thus, the reason of the problem should lie in other parts of your input file...


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