[Ifeffit] Fe K-Edge in Zirconium alloys and flattening effect

[adrien couet]

Dear all,

I am currently a PhD student working on the corrosion of zirconium
alloys used in nuclear reactors.

A very commonly used alloy in the industry is called Zircaloy-4 and has the
following chemical formula in wt%: Zr-1.2Sn-0.2Fe-0.1Cr. The iron and
chromium solubility limit in zirconium is very low. A large portion of the
iron precipitates out as Zr(Cr,Fe) precipitates with approximately 40wt% of
Fe, approximately 0.02wt% of Fe is left in solid solution in the zirconium
matrix. Sn is completely soluble in zirconium.

I am working on XANES of the Fe K-Edge absorption line in this material. To
do so, I use the microbeam (0.2x0.2 microns) at the 2IDD beamline at
the APS in fluorescence mode.
To quantify my results, I needed proper standards of metallic Fe. I quickly
realized that metallic Fe powder mixed with Zr powder did not give me the
same XANES signal as the Zircaloy-4 metal (in which metallic Fe is
embedded). Indeed, if you compare the "Fe powder in Zr" group to
both Zircaloy-4 groups after normalization and flattening they look
different. My thinking is that the Fe in precipitates and the Fe in solid
solution, even if they are both metallic, could have different XANES
patterns.

So I decided to create a thin sample where I could probe either a
precipitate or the matrix to get standards of metallic Fe in my Zircaloy-4
metal.

Both signals (from Fe in a precipitate and Fe in solid solution) look
similar once they are normalized and flattened (see in the attached project
file). However, the data before flattening look significantly different!
The post-edge of Fe in precipitates abruptly decreases whereas the
post-edge of Fe in solid solution increases. I also acquired spectra in
between these two extreme cases. When I scan in between the matrix and the
precipitate, I see an evolution of the post edge, which looks more and more
similar to the precipitate post-edge as I get closer to it. But after
flattening they all look identical and the XANES spectra does not look too
bad.

I have some questions regarding this:
- Is the flattening process changing anything such as the edge energy in a
way that I should not use it when the post-edge slopes are significant as
is the case here (even though my XANES spectra in solid solution and in the
precipitate match after flattening)?
- Have you ever seen this kind of post edge behavior before flattening? Do
you think it could be due to a concentration effect since I am going from
0.02wt% of Fe in solid solution to 40wt% of Fe in the precipitate?

I should mention that I have checked that no other elements present in the
metal have edges that could potentially disturb the Fe post-edge.

Thanks a lot for your help!!!

-- 
Adrien Couet
Graduate Student
Dept of Nuclear Engineering, Penn State University
+1 814 865 9709
axc1019 at psu.edu
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