Good morning everyone!
I´m trying for a long time to visualize nickel alloy dendrites in the scanning eletronic microscope without success. My samples are previously prepared (Vibrapolishing - 4 hours and etched in a solution of 3 g Mo-(VI)-oxide, 100 mL H2O, 100 mL HNO3, and 100 mL HCl. The micrographies in light microscope are nice, but in SEM not.
In SEM I use EHT 20 and 25 KV, aperture size 60 and 120 micron and working distance approx. 15 mm, with BSD Camera. It seems to get better, but not enough to visualize the contrast between dendritic and interdendritic area, where I should do a EDX - LineScan and analyze the microsegregation profile.
I would appreciate if anyone has a suggestion!
Thanks in advance.
You can try to optimise imaging conditions: vary EHT (15 kV?), check detectors ETD (SE2), InLens (switches off for EHT >= 20 kV).
The BSE contrast is dominated by composition (Z), are the bright spots sufficient for orientation?
You can try to use an OM image for orientation, see Image Registration.
First, what are the compositions of the phases? If you have a strong compositional difference between the phases, then you may expect them to exhibit contrast in a BSE (backscattered electron) image. If the compositional difference is subtle, then you may have a hard time. However, your SEM image definitely shows a denser/brighter phase.
You may need a lot of current to reduce noise in the BSE image. It seems like you have enough.
You may not want so much voltage. More voltage means a larger interaction volume and that may reduce your contrast. I tend to start around 10 kV and increase the voltage to see what provides the best imaging.
Beware of too much polishing or etching for SEM samples. Extended polishing will often accentuate the boundaries due to the different hardnesses of the phases. Etching operates by preferentially reacting and removing one phase faster than another. That may alter the area fraction of the phases at the surface. And what if that phase is the one that gave you atomic number contrast? You may totally remove it. The etching may provide topography which will be useful to optical microscopy, but it is not so helpful for SEM.
If you are interested in the contrast of dendrites, you can try to oversaturate the bright phases. Then, the contrast from dendrites will be more pronounced. If you do it via the auto contrast brightness function in SEM
then magnify the dendrite so that you don't see any bright particles and run auto C/B, then reduce magnification.
So, what I´ve tried until now and worked better: Stefan Baunack I approached the sample close to the radiation source, working distance of 3-5mm; EHT 20 kV, higher EHTs (25 kV - 30 kV) resulted in high quantities of Mo (false results), when line scans were performed. Why? No idea. The use of a BSD filter, I would say, it is mandatory. Aperture 60 microns instead of 120 microns is better for contrast. Warren Straszheim I worked with two alloys: first CM186LC, easy to detect dendritic and interdendritic area, due to the high quantity of Re,W,Co. The other is a variant of LEK-94 with reduced quantites of Re and the dendrites are not so well defined as CM186LC. Grzegorz Cios I do that after I take the picture (attached), is that you meant? Thank you all!
The physics are definitely different between the optical and SEM images. That always makes research fun.
Looking back at your first BSE image, how do the magnifications compare? It seems the SEM is about twice the mag as the optical.
It looks like you have eutectic regions between the grains. But it also looks like some other phase is on the boundaries. It is not so chemically distinct. And if that is the case, it will be hard to delineate. Perhaps an x-ray map would reveal some segregation.
I don't know the geometry of your SEM chamber and detectors. What is your working distance for EDS? Our detector requires the sample to be at 10 mm. I suspect at those high voltages you are scattering a lot and exciting some SEM part or another. Are your apertures made of Mo or Pt?
I would be interested in seen the linescan and/or some selected spectra.
The imaging conditions (EHT, aperture, working distance) are interdependent (see Beam_Parameters_2025-04-05_1.tif).
I've attached some information to select these parameters for a Zeiss NVision SEM/FIB (NVision_Params_8.3.tif), from my SOP. The references are Zeiss manuals.
For your system (ULTRA 40XB-27-40, from the image metadata) they may be slightly different.
The optimum WD for BSE will depend on the detector type.
For EDS the optimum WD should be given somewhere in the documentation (5 mm, if it is designed for FIB/SEM + EDS, larger for standard SEM + EDS).
You can find the optimum WD by plotting EDS intensity of a main peak vs. WD.
André Baldissera doing it after the image is taken is fine as long as it works for you, however if you do it before image collection the image may look better.