Hi I'm working on the fatigue crack initiation in heat-treated AM Ti64. As you may know, LAGBs are important for crack initiation and normally we categorize them as the boundaries with a misorientation angle between 2-15 degrees. As you see in the below image (which is SEM/EBSD analysis with 2000X magnification) we have a significant change in the detection of LAGBs (yellow color) by a reduction in the minimum LAGB threshold from 2 to 1 degree. What is your understanding of 1-2 degree misorientation areas especially in titanium alloys?
This is a really good question which confused me before. It is an argument point for people to analyse the recrystallisation process of metals (Book： Recrystallisation and Related Annealing Phenomena).
The low bond angle ranges for the low-angle grain boundary are from 0.5˚ to 1˚ or 2˚, sometimes for EBSD analysis software up to 5˚. The high bond is 10 or 15˚. The angle selection is relative to the dislocation accumulation process, boundary energy or boundary mobility. It is hard to tell you which one is perfect for you. But you need to select a range with a good explanation to show your understanding.
Personally i am not sure whether it makes sense to use as low angles as 1° with EBSD. The technique itself has an angular accuracy of 1° or worse(for most materials). I think even if you have a GB with misorientation of 2°, it is important to ask yourself: is it a real GB, or just few points of the scan in the middle of nowhere? According to my experience, you can have huge differences in GB fraction value depending on the definitions used in EBSD software (like what is grain, what is GB, etc.).
Thanks Oleksandr for your answer. I agree with you that we may have 1 degree error.
Our software is Aztec and as you said we have some spots that shouldnt be LAGBs for sure!
And this is question that now what is the real threshhold for LAGBs in EBSD
I don't think that there is a universal "real" threshold. I am not familiar with your alloy, but most probably its not that easy that all LAGB are responsible for crack initiation. Crack initiation is not an ultimate property of some GBs. A mechanism of stress concentration or lower activation energy for plastic deformation must lie behind it. I guess if you learn WHY people say that LAGBs are responsible for crack initiation then you will get a better idea how to characterize them inh EBSD. I hope this half-philosophical advice is of any help :)
It make sence. Well we know that at the interface of primary Alpha with alpha+Beta phases we have nano voids around LAGBs.
And we know that higher GND dislocation density around LAGB is important for nano crack initiation.
OK, sounds pretty complicated. Why don't you try to perform correlative characterization and find out what is going on by yourself? If possible, you can prepare a sample, make a marker on the surface with FIB or indentor, then perform a detailed EBSD of some area, then apply cyclic load in a regime where you would expect crack initiation but no sample fracture, and then look at the same area with EBSD and directly correlate local plasticity to local microstructural features. You can look at this paper and other papers from this group regarding Ni-based alloys:
Stinville, J.C., Lenthe, W.C., Echlin, M.P. et al. Microstructural statistics for fatigue crack initiation in polycrystalline nickel-base superalloys. Int J Fract 208, 221–240 (2017). https://doi.org/10.1007/s10704-017-0241-z
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