Author Topic: Burguers vector of Vacancy Dislocation Loops not identified  (Read 218 times)

Xtof

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Burguers vector of Vacancy Dislocation Loops not identified
« on: April 17, 2018, 03:42:59 PM »
Hi Alex and Ovito users,

Recently I started working with the DXA modifier of Ovito to determine the Burguers vector of Dislocation Loops (DL) in Fe. I succeeded to define interstitial 1/2<111> and <100> DL with LAMMPS. Ovito identified well their Burguers vector in both cases.
Then, I tried to do the same with vacancy DLs. In the case of a vacancy <100> DL (89 sites), I just removed the atoms and equilibrated the system in LAMMPS. In this case, Ovito was not able to identify a dislocation segment.

Any idea why this occurs? Is it due to a limitation of Ovito in this particular case or could it be due to a wrong description of these objects by MD (wrong potential)?

Many thanks in advance and best regards,
Christophe

Alexander Stukowski

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Re: Burguers vector of Vacancy Dislocation Loops not identified
« Reply #1 on: April 18, 2018, 08:59:32 AM »
Christophe,

In this case you should take a step back and first analyse these defects by hand, not using DXA. Use the CNA to identify perfect bcc atoms and check the morphology of the defective atoms.

Keep in mind that the DXA will only determine that it is a dislocation if the vacancy cluster has collapsed into a loop-like shape (i.e. a defect core with torus topology). It will not find a dislocation if the vacancies form a simple cluster (i.e. a defect core with ball topology). There need to be perfect bcc atoms in the centre of the loop, which is probably easer in case of interstitials.

You wrote you removed the 89 atoms to create a vacancy loop. What was the shape of this group of sites you removed? Spherical or more like a platelet?

Xtof

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Re: Burguers vector of Vacancy Dislocation Loops not identified
« Reply #2 on: April 19, 2018, 03:32:53 PM »
Hi Alex,

I determined the position of the atoms as to form a <100> interstitial DL. Then, I simply removed these atoms and minimized the energy. The resulting group of atoms has the shape of a platelet, like the <100> would have. See in attachment the position of the vacancies according to the WS analysis.

The result of the CNA is also attached. I removed from the view the atoms with perfect BCC structure type. It shows the cluster has a platelet-like shape.

Xtof



Alexander Stukowski

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Re: Burguers vector of Vacancy Dislocation Loops not identified
« Reply #3 on: April 20, 2018, 01:21:17 PM »
The CNA pictures suggests that the vacancy platelet hasn't turned into a vacancy dislocation loop yet. Maybe because they are too small for the two surfaces to collapse spontaneously, or because something else is wrong with the construction.

For the bcc simulations published in our Nature paper, we also cut vacancy loops into the crystal. Those loops had a hexagonal shape, with the habit plane parallel to {111}. The hexagonal region within which we deleted atoms from the perfect crystal had a thickness corresponding to exactly one 1/2<111> lattice vector. Then, during a short MD run, the two surfaces collapsed back to a perfect crystal lattice again due to their attraction, leaving behind a hexagonal 1/2<111> dislocation loop.

But our loops where certainly larger than yours, so the spontaneous collapse may not happen in your case and you need to assist it somehow.

Xtof

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Re: Burguers vector of Vacancy Dislocation Loops not identified
« Reply #4 on: April 25, 2018, 10:00:45 AM »
I tried with 1/2<111> DL and it works. Ovito correctly detects a 1/2<111> dislocation. However, after equilibration, the loop adopts a strange shape, like a deformed platelet. I do not know if that is correct. I guess it has to do with the potential (Ackland 2004).

For <100> vacancy loops, Ovito was not able to detect any loop. Again, it might be due to the potential. As you suggest, a colleague of mine, also mentionned a possible problem with the potential and as consequence, that there is no spontaneous collapse.

Xtof