I have a two phase system. The minor phase has a lower average atomic number than the major phase and so I would expect the minor phase to have darker phase contrast. I also see a trace phase (maybe) with brighter contrast, but how can I be sure that it isn't caused by channeling contrast?
Hello, James,
one possible solution is to tilt your specimen by a little angle at different directions, say 2-3 deg, and compare results. If there was a channeling contrast on your specimen, it must disappear since the contrast is dependent on the specimen orientation to the electron beam. The backscattering contrast will likely remain more or less the same.
Hello, James,
one possible solution is to tilt your specimen by a little angle at different directions, say 2-3 deg, and compare results. If there was a channeling contrast on your specimen, it must disappear since the contrast is dependent on the specimen orientation to the electron beam. The backscattering contrast will likely remain more or less the same.
Channeling contrast is weak compared to Z contrast. Can you see grains of main phase? If no, then you probably do not observe channeling contrast. And as it was already advised you can tilt your specimen to check for changes (to minimize channeling contrast you should work with not tilted specimen).
Backscattered contrast is only in first approximation a function of Z. It needs to be corrected by the relative mass of the phases, cf. Reimers book or certainly also that of Goldstein.
e.g. NbC in Ni matrix.
Ni has Z=28, NbC has Z=(41+6)/2=23.5, i.e. NbC should be darker. If you use the corrected equations (after Reuter) you will get
eta(Ni) = 0.295
eta(NbC) = 0.350
i.e. NbC is proposed to have a much higher contrast what you will observe in your images.
For your specific problem it would be good to see the image which you can upload.
Hello James Kelly,
channeling (= orientation) and topographical contrasts are usually small when the sample is placed horizontally in the SEM. A small tilt will change channeling contrast between grains because the Bragg condition is very sensitive to the orientation of the lattice with respect to the primary beam. Channeling contrast is also very much dependent on the perfection of the crystal lattice close to the surface. This means that a deformed or not well prepared surface will not show significant channeling contrast.
Material (= Z) contrast does not depend on the crystallographic perfection of the surface. It is even seen on amorphous surfaces.
Crystalline minerals and ceramics tend to show a strong channeling contrast the steeper the sample is tilted out of the horizontal. For high tilt angles the usual BSE detectors at the ceiling of the specimen chamber are not really effective. If you have an EBSD system installed on your SEM you may use the EBSD detector for BSE imaging without having to index the backsatter Kikuchi patterns. Therefore non-crystalline and bio samples can be imaged as well. A publication on this subject will soon appear in Microscopy Today for free download. (My website www.ebsd.de, however, is in German. You may have a look on my website www.ebsd.info (in English) for more details on EBSD.)
Cheers,
Robert Schwarzer
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