Recent Progress in Developing and Qualifying Nanostructured Ferritic Alloys for Advanced Fission and Fusion Applications

JOM, Nov 2014

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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Recent Progress in Developing and Qualifying Nanostructured Ferritic Alloys for Advanced Fission and Fusion Applications

G.R. ODETTE 0 0 1.-Departments of Mechanical Engineering and Materials, University of California Santa Barbara , 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. - 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)


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G. R. Odette. Recent Progress in Developing and Qualifying Nanostructured Ferritic Alloys for Advanced Fission and Fusion Applications, JOM, 2014, pp. 2427-2441, Volume 66, Issue 12, DOI: 10.1007/s11837-014-1207-5