Recent Progress in Developing and Qualifying Nanostructured Ferritic Alloys for Advanced Fission and Fusion Applications
G.R. ODETTE
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1.-Departments of Mechanical Engineering and Materials, University of California Santa Barbara
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Santa Barbara, CA, USA. 2.-
This article summarizes the recent progress on developing a class of potentially transformational structural materials called nanostructured ferritic alloys, which are leading candidates for advanced fission and fusion energy applications. Here, we focus on Fe-Cr-based ferritic stainless steels containing a very high concentration of Y-Ti-O nano-oxide features that enable a host of outstanding high-temperature properties, along with unique irradiation tolerance and thermal stability. Perhaps most notably, these alloys have an unprecedented capability to manage very high helium concentrations, pertinent to fusion service, in a way that transforms this element from a severe liability to a potential asset. In addition to providing some necessary background, we update progress on: (I) the character of the nanofeatures; (II) some unifying insights on key mechanical properties; (III) a quantitative model for nanofeature coarsening; (IV) recent irradiation experiments of the effects of helium on cavity evolution and void swelling; and (V) a powerful new mechanism controlling the transport, fate, and consequences of helium.
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The success of nuclear fission and fusion as
largescale sources of energy for the millennia requires
new structural materials that provide and sustain a
host of high-performance properties. The challenges
presented by irradiation effects are particularly
daunting and, in the case of fusion, are exacerbated
by high quantities of helium.
The objective of this summary review is to update
the status of a transformational class of
iron-chromiumbased ferritic stainless steels.13 We use the
nomenclature nanostructured ferritic alloys (NFAs)
to distinguish NFAs from so-called
oxide-dispersionstrengthened (ODS) steels, such as PM2000, which
contain a variety of coarser-scale oxides, often
associated with Al additions.4 We also distinguish
NFAs, which typically contain 14%Cr (and more
generally 12%Cr) along with small yttrium,
titanium, and oxygen additions, from transformable
ODS steels, which are alloyed with C and 9% Cr.1,3
NFAs, which are often designated by their
percentage of Cr content followed by YWT, as in
14YWT, have many outstanding properties. These
include high tensile, creep, and fatigue strengths
over a wide range of temperatures; truly
remarkable thermal stability up to 1000 C; and unmatched
irradiation tolerance, especially with respect to
managing high levels of helium.1,2,5 There is a large
and growing worldwide interest and literature on
nano-oxide dispersion-strengthened iron-based
alloys that resulted in almost 150 Institute of
Scientific Information Web of Science papers published in
2013 alone, as well as a focus for several special
symposia and journal issues in recent years.
Clearly, this article cannot provide a comprehensive list
of pertinent citations; thus, these are limited to
representative examples.
The outstanding characteristics of NFAs result
from the interrelated presence of an ultrahigh
density of Y-Ti-O rich nano-oxide features (NFs), fine
grain sizes, and high dislocation densities. The NFs
are multifunctional, in that they:13 (I) retard
dislocation climb and glide thus increasing alloy strength;
(II) stabilize grain and dislocation structures; and
(III) act as very deep traps for helium, resulting in the
formation of tiny, high-pressure gas bubbles at their
interface with the matrix.1,2,5 The presence of
nanometer-scale bubbles adds to the irradiation tolerance
of NFAs because they act as stable
sink-recombination centers that self-heal excess vacancy and
selfinterstitial displacement damage defects. Indeed, the
helium bubbles are much more effective in enhancing
recombination than the oxide-matrix interface
itself.1,2,5 The bubbles are also deep traps for
additional helium. Sequestering helium in bubbles
reduces the accumulation of this bond-weakening
element on grain boundaries, which otherwise can
lead to degradation of both creep rupture and fast
fracture toughness properties.5 Helium trapped in a
very high number density of NF-interface small
bubbles also eliminates, or greatly retards, rapid void
swelling.1,2,5 Thus, NFAs may turn high helium
levels from a liability to an asset.
Challenges facing NFAs include: (I)
characterizing NF structures, compositions, oxide-matrix
interfaces, and the various factors that control their
nature; (II) determining the role NFs play in
providing high strength and irradiation tolerance over
a wide range of service conditions; (III) quantifying
the thermal and irradiation stability of NFAs and
NFs; and (IV) alloy designs, thermalmechanical
processing paths, and joining methods that create
sustainable optimized NFA microstructures and
yield outstanding isotropic properties and
defectfree product forms. Other practical NFA challenges
include corrosion and compatibility issues, red (...truncated)