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@PHDTHESIS{Chen:1011540,
author = {Chen, Jie},
othercontributors = {Gonzalez-Julian, Jesus and Linsmeier, Christian},
title = {{M}icrostructural design and performance of
self-passivating {WC}r{Y} alloy for the first wall in fusion
reactors},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2025-04556},
pages = {1 Online-Ressource : Illustrationen},
year = {2025},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2025},
abstract = {Magnetic confinement fusion is a promising future energy
source to meet the increasing global energy demand.
Achieving controlled fusion necessitates sophisticated
reactor design and careful material selection to withstand
the harsh fusion environment. First wall of the future
fusion power plant, the inner surface that directly faces
the hot plasma, is likely to be made of tungsten or
tungsten-based material. This thesis focuses on a specific
tungsten alloy as the first wall candidate for DEMOnstration
power plant (DEMO): W-11.4Cr-0.6Y $(wt.\%)$ or
W-31.11Cr-0.96Y $(at.\%),$ developed in the Institute of
Fusion Energy and Nuclear Waste Management (IFN-1, or
formerly IEK-4) in Jülich. This alloy, known as
Self-passivating Metal Alloy with Reduced Thermo-oxidation
(SMART), forms a passivating scale under high-temperature
oxidation, which helps to suppress the oxidation of tungsten
and sublimation of tungsten oxides in the potential reactor
accident. Despite its excellent oxidation resistance, the
standard SMART material (Std-SMART), has less satisfactory
mechanical properties, being hard and brittle, which present
challenges in machining and service. This thesis endeavors
to design new microstructures of SMART materials in hope of
improving their mechanical properties without compromising
oxidation resistance. Various approaches in the
manufacturing process, i.e. milling, sintering or heat
treatment, were employed to achieve these new
microstructures. The new microstructures were evaluated for
their hardness, thermo-mechanical properties (flexural
strength, fracture toughness), ductile - brittle transition
temperature (DBTT), and oxidation behavior at 1000 ℃, and
compared to those of Std-SMART. The key results are as
follows: Four SMART variants featuring different
microstructures were designed in this thesis work. In
Nano-SMART, the grains are refined to the nanoscale on
average, achieved by controlling the grain growth at the
reduced sintering temperature. In ODS-SMART (Oxide
Dispersion Strengthened), yttria powders replace the yttrium
powders during the ball milling and a variety of oxides
(Y2O3, Y-Cr-O and Cr-O) are formed at grain boundaries after
sintering. In PD-SMART (Phase Decomposition), the
homogeneous microstructure of Std-SMART was exposed to heat
treatment at 1000 ℃ and decomposed to Cr-lean and Cr-rich
phases. Similar to PD-SMART, DP-SMART (Dual Phase) adopts a
dual-phase microstructure by sintering mixed Cr-lean and
Cr-rich powders together. For mechanical properties,
Nano-SMART shows higher hardness, attributed to the
increased density of grain boundaries. Conversely, the
presence of Cr-containing oxides in ODS-SMART and chemical
heterogeneity in PD-SMART and DP-SMART contribute to the
softening of SMART materials. Flexural strength of most
SMART variants improves to varying degrees, except for
DP-SMART. The most prominent enhancement is observed in
PD-SMART, where the flexural strength remains around or over
1.1 GPa below 900 ℃, owing to the microstructural
refinement with decomposed Cr-rich phase of small size (~
129 nm). However, fracture toughness does not improve, with
Nano-SMART and Std-SMART remaining comparable (of the order
of ~ 5 MPa∙√m) while the others show deterioration. DBTT
of SMART variants does not decrease; PD-SMART remains
comparable to Std-SMART (at ~ 900 ℃), whereas the others
experience an increase of 50 - 100 ℃. For oxidation
behavior, each SMART variant displays passivation but
differs in mass gain after 20 hours oxidation. Nano-SMART
and DP-SMART exhibit mass gains over and double that of
Std-SMART at 20 h, respectively. ODS-SMART demonstrates a
reduced linear oxidation rate and improved oxide scale
conditions, attributed to the enhanced reactive element
effect resulting from Y2O3 and Y-Cr-O particles. PD-SMART
initially shows a higher oxidation rate within the first 3
hours and then experiences a sluggish mass gain throughout
the remainder of the oxidation period, due to the
development of a thick chromia scale contributed by the
dissolution of Cr-rich phases in the inner oxide layer.
Ultimately, the mass gain is comparable to Std-SMART at 20
h. This thesis provides an empirical understanding of the
microstructural features of SMART materials and correlates
these fractures with their mechanical properties and
oxidation performance, offering valuable insights for future
material design and development.},
cin = {524110 / 520000},
ddc = {620},
cid = {$I:(DE-82)524110_20140620$ / $I:(DE-82)520000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2025-04556},
url = {https://publications.rwth-aachen.de/record/1011540},
}
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