Numéro |
J. Phys. IV France
Volume 124, Mai 2005
|
|
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Page(s) | 17 - 24 | |
DOI | https://doi.org/10.1051/jp4:2005124003 |
J. Phys. IV France 124 (2005) 17-24
DOI: 10.1051/jp4:2005124003
FEG-SEM investigation of
-alumina scales formed on FeCrAlY
alloys oxidised at 1200
C
H. Al-Badairy1, G. Tatlock1, S. Fawcett2, P. Beahan1 and J. Hunt2 1 Department of Engineering, The University of Liverpool, Liverpool L69 3GH, UK
2 UK Centre for Tissue Engineering, Clinical Engineering, The University of Liverpool, Duncan Building, Liverpool L69 3GA, UK
Abstract
This work is part of an ongoing European funded project,
"SMILER", with the aim of improving the performance of alumina forming
Fe-20Cr-5Al-Y alloys for high temperature industrial applications. One
aspect of the project is to investigate the influence of additives on the
oxidation behaviour of these alloys. During this study a LEO 1550 FESEM
(field emission scanning electron microscope), equipped with INCA X-ray
microanalysis facilities was used. Ultra-high-purity model alloys, where the
levels of additives (Hf, Zr, Ti, Si, La and Y) were carefully controlled,
and two commercial Aluchrom YHfAl and Kanthal AMPT alloys were oxidised at
1200C for up to 3100h (100h/cycle).
The YHfAl, (Y+Ti+Zr)- and (Y+Zr+Hf)-containing alloys showed the highest
oxidation rates when oxidised, whereas the La-containing alloy showed the
lowest oxidation rate. However, the La-containing alloy spalled the most,
while the (Y+Zr+Hf)-containing and YHfAl alloys showed little spallation,
and the additives appeared to have a major influence on the spallation of
the -alumina scale formed. On the alloys with La and Si
respectively, the scale spalled at the scale/metal interface (adhesively),
whereas on alloys containing (Y+Hf+Ti) and (Y+Hf+Zr) and the YHfAl alloy,
the scale spalled mainly in a cohesive manner (within the scale). Also, the
added elements affected the scale topography. In general the scale had a
columnar structure at the scale/metal interface, whereas the grains were
equiaxed at the scale/gas interface. However, in the case of YHfAl and
(Y+Zr+Hf)-containing alloys, a sunflower type structure was observed in both
fractured samples, in the regions where the scale spalled cohesively.
Preliminary EDX analyses revealed that, in the case of YHfAl, for example,
the center of the sunflower structure was rich in Mg. This suggests that
there may be inhomogeneities in the metal substrate, prior to oxidation,
where high concentrations of Mg increase the local oxide growth rate.
Key words: alumina forming Fe-20Cr-5Al-Y alloys, oxidation, ultra-high-purity, sunflower type structure.
© EDP Sciences 2005