I am using molecular simulations to perform tensile test on BCC high entropy alloy ( having distorted lattice ). Using the DXA algorithm in Ovito no dislocations are found even in the plastic deformation regime( as determined by the stress-strain curve) . In am having 128000 atoms which I guess is a good enough size to simulate.
Any leads on this is highly appreciated.
Thanks
Keep in mind that the current version of the DXA modifier in OVITO only supports simple crystal structures such as FCC, HCP, BCC, and diamond.
https://www.ovito.org/docs/current/reference/pipelines/modifiers/dislocation_analysis.html
When you choose BCC as input crystal type in the DXA modifier settings, what does the table called “structure analysis results” say?
Thank you for the reply. The structure table says 63.1% BCC and 36.9% Others at a strain of 10% and the algorithm recognizes only 2 dislocation segments (max) with around 50% BCC for higher strains.
Is 63% BCC character recognized by CNA good enough to use for the DXA algorithm?
Thanks
Yes, a recognition rate of 50-60% of crystalline atom structures should be sufficient for DXA to find most of the dislocation defects. But since the dislocation cores can appear wider than usual in such a situation, it can make sense to increase the DXA parameter “trial circuit length” a bit to make it easier for DXA to construct Burgers circuits around potential dislocation defects.
I haven’t seen your MD simulation model, but keep in mind that your simulated crystal volume may be too small or the applied deformation rate too high for dislocations to form. Extreme conditions may promote other mechanisms of plastic deformation such as twinning.
Here are pointers to two more things, which I thought may be of interest to you in this context:
Dear Sir ,
Thank you for your suggestions and guidance.
The no of atoms is 128000 and the initial box dimension is around 120x120x120 A .
I have used Embedded Atom Model with deformation rate ranging from order 10^7 /sec to 10^8 /sec .
I am attaching the snapshot of one of the deformed configuration.
Dear stukowski：
May I ask how can we identify twins in BCC by ovito?
OVITO itself doesn’t provide a function for identifying twin boundaries in bcc crystals. But I think other researchers have developed identification algorithms for these kind of defects. For example, this one has been implemented as extension for OVITO:
https://www.sciencedirect.com/science/article/pii/S2215016116300036
http://jomoeller.github.io/bda/
This method tries to identify the atomic structure at the boundaries of a twin. Of course, you also try to identify deformation twins directly using the Atomic Strain calculation function of OVITO, which can indicate spatial regions that have experienced shear deformation.