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Fission Fusion A Materials Challenge Engineering Essay

At present, there are 437 nuclear power plants in operation around the world, producing 16% of the world's electricity 1 - the largest share provided by any non-greenhouse-gas emitting source. But with the worldwide population estimated to expand from 6 to around 10 billion by the year 2050, the problem of meeting the world's increasing energy needs has received international focus 2-4. The major issue of meeting the energy demand, while at the same time addressing climate change has reaffirmed the need to continue and expand the generation of nuclear power.

The viability of planned next generation (Gen IV) fission and future fusion reactors will ultimately rely upon the development of advanced structural materials that can provide extended service under extremely hostile conditions 5-11. Advanced fission structural materials are exposed to service conditions characterised by high temperatures (~300oC), mechanical stresses and intense neutron radiation fields 12. Even tougher is the search for fusion materials which, as Zinkle summates, poses "arguably the greatest structural materials development challenge in history" 8, whereby operating conditions include a combination of high energy neutron fluxes, intense thermo-mechanical stresses and corrosion induced by high temperature coolants.

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Perhaps most significant is the effect of irradiation damage on a given material. Zinkle 12 describes radiation damage as five main threats occurring at varying operating temperatures and damage levels. These phenomena are shown in fig.1. Damage transpires as energetic neutrons produced during fission and fusion reactions have enough kinetic energy to displace a large quantity of atoms in a structural material from their lattice sites. This creates excess concentrations of vacancy and self-interstitial atom (SIA) defects, quantified in terms of displacements per atom (dpa). Fortunately at temperatures associated with nuclear reactors there is sufficient thermal diffusion of radiation induced defects to enable recombination of many of the vacancies and SIAs, such that when designing radiation resistant materials promotion of very efficient recovery of defects is a key design criteria.

Figure 1: Summary of five radiation threats to structural materials performance 12

The objective of this paper is to critically review recent progress in developing dispersion strengthened steels that show great promise in meeting many of these challenges. In doing so we will take an in-depth look at literature surrounding both ferritic and martensitic oxide dispersion strengthened steels (ODS).

Oxide Dispersion Strengthened (ODS) Steels

2.1 A Brief History

Development of ODS steels for nuclear applications began in Belgium in the late 1960's 13, since then an international effort toward further understanding has taken place. Most notably in Japan where pioneering work by Ukai et al. 14, 15 led to the development of two main options for ODS steels; Ferritic ODS steels (typically containing 12-16%Cr) and Martensitic ODS steels (9-11% Cr). The effort in Japan still emphasises research on the aforementioned Ferritic ODS 14, 16-19 and Martensitic ODS 15, 20-22 steels, however the main focus of research has shifted toward the recrystallisation sequences necessary to produce isotropic properties together with assessment of irradiation responses 19, 23-26 and practical application testing 27. European R&D has been aimed primarily at developing martensitic ODS Eurofer97 for use in fusion applications {{69 Schaeublin,R. 2002; 70 Lindau,R. 2005; 71 Yu,G. 2005}}, whereas US based research has placed emphasis on ferritic ODS steels {{58 Alinger,M J. 2002; 62 Miller,M K. 2003; 63 Alinger,M J. 2004; 65 Alinger,M J. 2009; 66 Kishimoto,H. 2004; 64 Miller,M K. 2004; 67 Hoelzer,D T. 2007; 75 Miao,P. 2007; 76 Klueh,R L. 2002}} (especially MA957) and their response to irradiation{{72 Toloczko,M B. 2004; 73 Yamamoto,T. 2007}}.

The ferritic ODS steels mentioned above are referred to in a recent review by Odette 28 as nanostructured ferritic alloys (NFA), Odette suggests that the highly advanced materials have nm-scale oxides that are much finer than conventional ODS alloys and as such warrant classification separate to standard ODS alloys. Similarly martensitic ODS steels are referred to as nanostructured transformable steels (NTS), again due to the nm-scale cluster size as well as the phase transformation undergone during processing (austenite-martensite). However, there is significant ambiguity over the appropriate classification of these advanced ODS alloys.

This review will adopt the nomenclature presented by Odette as a simplified means of differentiation between the advanced ODS Steels primarily discussed forthwith.

2.2 Key Properties

For an alloy to be applicable to nuclear pressure vessels or other structural materials they must cope well under extremely hostile conditions. To resist the threat posed by radiation damage candidate materials must have

A high density of nm-scale precipitates that trap He to avoid swelling and protect grain boundaries

High creep strength enabling them to operate at temperatures above the displacement damage regime{{43 Odette,G. 2008}}

These characteristics can be found in NFA & NTS alloys. When alloyed with Ti both NFA and NTS are known to contain a high density of nanofeatures (NFs) enriched in Y-Ti-O, which results in remarkable high temperature creep strength and radiation damage resistance [124, 135] CITE. MICROSTRUCTURE IMAGES?

2.3 Mechanical, Thermal and Irradiation properties

NFA and NTS alloys have high static and creep strength primarily due to the interaction of NFs with dislocation structures. The Y-Ti-O clusters hinder dislocation motion by Orowan strengthening, the closer the clusters are together the more stress is required for dislocations to bypass the precipitates. As a result, it is fair to conclude that large concentrations of NFs combined with high dislocation densities will contribute significantly to the overall alloy strength. Reference [Odette] predicts that NF strengthening in MA957 is approximately 400 MPa thus supporting the theory.

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Comparison of NFA (MA957) with NTS (Eurofer97) shows that MA957 is generally much stronger [133]. Ductility varies depending on the alloy composition and processing route but is on average reasonable with NFA managing total strains ~2-20% and NTS managing strains ~5-30% [40,52,117]. As assessed further in section 2.4, MA957 has a wide range of σy depending largely on thermo-mechanical tempering (TMT) conditions.

Potential issues arise when looking at toughness and fatigue data. Literature regarding both is limited; however Klimiankou et al. suggest that inclusion-carbide-free NFAs, with fine equiaxed grains, may have a high strength and high toughness [103]. These areas require further R&D.

Thermal properties are generally outstanding due in part to the insolubility of Y in Fe making NFs stable at high temperatures [118,112]. Macroscale porosity formation occurs during high temperature aging, but these effects are minimal up until ~850˚C [Odette]. However, the formation of Cr-rich α' at irradiation temperatures below approximately 550˚C is the primary concern for NTS and NFAs. The formation of this phase causes irradiation hardening and embrittlement, which increases with decreasing irradiation temperatures [55]. This gives us a temperature window between 550˚C and 850˚C where the effect of irradiation damage is minimal. Conveniently this is similar to the high temperatures advanced fission and future fusion reactors will be operating.

The irradiation damage resistance of NFAs

2.4 Microstructure: The effect of composition and processing

Successful production of NFAs was first achieved by Fischer, who patented a mechanical alloying (MA)/hot extrusion powder processing route which led to the marketing of commercial alloys MA956 and MA957 by the International Nickel Company (INCO) {{60 Fischer JJ}}. A good outline of the MA process is provided by Capdevila & Bhadeshia {{77 Capdevila,C. 2001}}.

2.4.1 Composition

Typical NFA compositions and the roles of alloying elements are outlined below [Odette].

In the selection of compositions careful thought must be given to the alloy and process design as well as the effect of precipitate phases on overall alloy performance. A number of phases including oxides, intermetallics and carbides can form unintentionally in NFA and NTS depending on the composition and fabrication method [50,51,81,83,114,124]. Obtaining the optimal balance of Ti and O is also crucial [62, 65]. A high level of both boosts the formation of coarser TiO2 particles over the desired finer Y-Ti-O attributes. These unplanned phases have the potential to degrade properties, particularly via embrittlement [41, 62, 65] and as such drastically reduce alloy performance.

2.4.2 Processing

NFAs are processed by MA Y2O3 powders with Fe, 12-16% Cr, Ti and W powders by ball milling. A conventional ball mill (fig.2a and 3) consists of a rotating drum containing a mixture of balls and the above powders. The change in forces cause the milling balls to raise and fall cyclically striking other balls and powders, in doing so deforming and mixing the content. The resulting milled powders have fine grain sizes and high dislocation densities {{66 Kishimoto,H. 2004}}. There are many variations to this simple design, 3 of which are shown in fig.2. The resulting milled powders contain a supersaturation of dissolved Y and O, which are then canned and vacuum degassed before consolidation by hot isostatic pressing (HIPing) (fig.3). Consolidation causes the Y and O to precipitate along with Ti, producing NFAs of near maximum density (>98%) {{43 Odette,G. 2008}}. As we have established, the density and spread of Y-Ti-O NFs is an important factor in the overall performance of the alloy. Reference [111] concludes that consolidation conditions are the main processing variables controlling the clustering and precipitation of dissolved Y, Ti and O. For that reason the consolidation time and temperature are key parameters in the overall MA process and require careful planning. The consolidation process is usually followed by sequences of recrystallisation or heat treatment (fig.3) for reasons explained below.

A major problem related with MA of NFAs is that of anisotropic and textured grains formed during extrusion [40,86,113,114] . Studies have shown that anisotropy reduces creep strength by 50% in the transverse extrusion direction compared with the axial direction [40,51,52]. Anisotropic microstructures also produce low toughness for cracks propagating in the axial extrusion direction [40,108]. Therefore it is important to relieve the anisotropy of the microstructure, hence the alloy must be modified by post extrusion TMT, which involves cold/warm working and recrystallisation heat treatments [40, 51, 52]. However, high temperature heat treatments can cause any small cavities and gas bubbles in the precipitate matrix or larger pores at particle interfaces to grow into macroscale porosity. This has serious effects on the high temperature creep properties of the alloy [43,63,88].


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