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@PHDTHESIS{Galiullin:825550,
author = {Galiullin, Timur},
othercontributors = {Spatschek, Robert and Krupp, Ulrich},
title = {{O}xidation behavior of wrought, cast and additive
manufactured {C}o-base alloys for high temperature
applications},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2021-08341},
pages = {1 Online-Ressource : Illustrationen, Diagramme},
year = {2020},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2021; Dissertation, Rheinisch-Westfälische
Technische Hochschule Aachen, 2020},
abstract = {Various high temperature industrial processes require
construction materials that possess an optimal balance of
mechanical strength, corrosion resistance and fabricability.
Among the main candidate materials considered for the
long-term service at temperatures exceeding 600°C are
metallic Fe-, Co- and Ni-base alloys. The present study
followed the oxidation mechanisms of wrought, cast and
additive manufactured (AM) Co-base alloys during air
exposures at 950-1050°C to address the effects of alloy
microstructure, minor alloying elements and specimen
thickness on oxidation kinetics and subscale depletion
processes. Substantial differences in oxidation behavior
between cast and AM (produced by Selective Laser Melting)
versions of alloy MAR-M-509 were correlated with the alloy
grain morphology and size as well as distribution of the
strengthening precipitates. Faster oxidation of the cast
alloy was shown to be mainly induced by: (1) oxidation of
coarse primary Ta-rich carbides, that oxidized rapidly
forming thick oxide nodules and promoting development of
cracks/pores in the adjacent Cr2O3. Cracking of the scale
and its spallation provided direct access of nitrogen to the
Cr-depleted base metal causing internal nitridation. (2)
Incorporation of other alloy constituents (Co, Ni, W) into
the scale was observed from Cr-depleted alloy thus
decreasing the scale grain size and increasing its growth
kinetics. In contrast, the microstructure of the AM
MAR-M-509 featured fine carbide precipitates that were found
to dissolve in the subscale region forming internal oxides
and resulting in more homogeneous oxide scale compared to
the cast alloy. EDX analyses in the subscale zone of AM
specimens further revealed virtually flat Cr concentration
profiles, contrary to the cast alloy which exhibited strong
Cr depletion. Such behavior was proposed to be the result of
enhanced Cr diffusion due to contribution of grain-boundary
transport in the fine-grained AM alloy. High temperature
exposures of the wrought Co-Cr alloys Haynes 25 and Haynes
188 showed significantly faster oxidation of the former
material. Relatively high Mn content in Haynes 25
facilitated formation of Cr/Mn-spinel and increased the
scaling rate of Cr2O3, which was found to exhibit a
significant part of inward growth. TEM and APT studies of
the chromia scale revealed a presence of intragranular
Cr/Mn-particles possibly nucleating simultaneously with
Cr2O3. Substantially better performance of Haynes 188 was
interpreted in terms of differences in composition of Cr,
Ni, Mn and especially La. The chromia scale formed on the
alloy surface featured a distinct duplex morphology with
coarse columnar oxide grains in the outer part and fine
equiaxed grains in the inner layer. Observed sub-parabolic
oxidation of Haynes 188 was attributed to the slow inward
growth of the inner chromia layer as a result of reactive
element incorporation into the scale, which could suppress
outward cation transport. Dependence of oxidation kinetics
on specimen thickness was analysed by oxidizing 0.15-1mm
sections of wrought alloy Haynes 188. Exposures at 950 and
1000°C showed higher oxidation kinetics of thinner
specimens with more pronounced Cr depletion in agreement
with the common observations reported for Fe-Cr and Ni-Cr
alloys. Higher oxidation rate of thin Haynes 188 specimens
could be clearly observed already after initial hours of
isothermal exposures in a thermobalance. The possible
explanation of this effect was referred to low intrinsic
creep strength of thin sections resulting in more pronounced
response to oxide growth stresses. In contrast, during the
testing at 1050°C the increase of oxidation-induced Cr loss
with decreasing section thickness was not monotonous and
showed a maximum in the 0.5 mm thick specimen. This result
was correlated with rapid exhaustion of Mn reservoir in
thinner specimens that appeared to reduce Cr depletion
kinetics due to less pronounced Mn incorporation into the
scale.},
cin = {525820 / 520000},
ddc = {620},
cid = {$I:(DE-82)525820_20160614$ / $I:(DE-82)520000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2021-08341},
url = {https://publications.rwth-aachen.de/record/825550},
}
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