Understanding the performance of nano-structured ferritic alloys through micro-mechanical testing

Oxide dispersion strengthened (ODS) steels are one of the most promising candidate materials for fuel cladding tubes, in GenIV nuclear reactors, and plasma facing components for tritium breeding blankets in fusion reactors. Although first developed during the 1960\u27s, recent research has demonstrated an improved high temperature strength and irradiation resistance over the more conventional high chromium reduced activation ferritic/martensitic (RAF/M) steels. This improved performance is obtained through microstructures that contain a high density of insoluble nanoscale oxides dispersed in the ferrite matrix (typically 2 -10 nm yttrium- titanium oxides). However, concerns remain over their use in future nuclear application and the following questions are of key issue; Due to the mechanical alloying and powder processing manufacturing routes typically used the microstructures can display significant inhomogeneity in local chemistry, grain size and oxide distribution. How this affects local mechanical properties needs to be measured. The exact mechanisms of strengthening (solid solution, grain refinement or hard particle hardening) are not well understood and individual contributions need to be assessed for better alloy design. How the mechanical properties of these alloys are affected by radiation damage is not well documented and is must be known if they are to be used in a nuclear environment. This work uses state of the art nanoindentation and micro-cantilever bending experiments on a series of systematically varied nanostructured ferritic alloys based on a Fe-14Cr-3W-0.2Ti-0.25Y2O3 in both as processed and irradiated conditions to answer these questions Nanoindentation was used to investigate the hardness and elastic modulus of each specimen, and effect of indent size on hardness. Indentation modulus was found to be similar for each sample but the nanocrystalline samples showed higher levels of hardness, confirmed using microscopic techniques. The variation in hardness was seen to increase in oxide containing samples. This was investigated using EBSD and EDX, and was determined to be caused by a pronounced heterogeneity of the microstructure. While Hall-Petch strengthening and changes in the local chemistry had some effect on the measured hardness, the most likely cause of the large variation in local hardness was heterogeneity in the nanoscale oxide population. By using TEM and atom probe tomography this inhomogeneous dispersion of the oxide particles can be demonstrated. Nanoindentation was also used to measure the strain rate sensitivity and creep rate of each specimen, where it is shown that the oxide containing alloy has superior performance. To simulate neutron damage samples were implanted with 2MeV protons to a peak damage level of 0.2dpa at 20μm depth below the surface. Samples were then rotated 90 degrees and polished to produce a cross section through the damaged layer. The cross-sectional surface of the irradiated layer was then exposed and inclined linear arrays of 250 nm deep indents were placed across the damage profile. 14WT (non-oxide containing) revealed a clear proton damage profile in plots of hardness against irradiation depth, No appreciable hardening was observed in either 14YWT specimens, attributed to fine dispersion of nanoscale oxides providing a high number density of defect sink sites. Micro-cantilevers were fabricated out of the irradiated layer cross-section. Larger micro-cantilevers, with 5 μm beam depth, were place with their beam centre at 0.026 dpa. Smaller micro-cantilevers, with 1.5 μm beam depth, were placed with their beam centre at 0.2 dpa. Both the large and the small micro-cantilevers fabricated in 14WT (non-Oxide containing) displayed significant irradiation hardening (from 0.5GPa to 1.04GPa yield stress) while in the oxide containing variant, irradiation hardening is suppressed. This paper will demonstrate that when the size effects inherent in these tests are accounted for the data produced is comparable to bulk mechanical testing and suggest some key next steps if these alloys are to be pursued as nuclear materials

Room temperature and high temperature micromechanical testing of SiC- SiC fiber composites for nuclear fuel cladding applications

Silicon carbide ceramics are a candidate material for the use in nuclear power generation and are suggested to be used in novel accident tolerant fuel (ATF) cladding designs due to its favorable properties, in particular reduced (compared to Zircaloy) oxidation under accident conditions, good neutronic performance, high temperature strength and stability under irradiation. Due to its inherent brittleness, it is suggested to be used in the form of SiC-fiber reinforced SiC-matrix composite. In order to reliably model behavior of highly non-uniform and anisotropic composite materials the knowledge of the individual properties of fiber and matrix, and, crucially, the fiber-matrix interfaces, is required. In addition, nuclear fuel cladding materials are exposed to elevated temperatures during their operation, and therefore the understanding of the temperature dependences of the relevant properties is essential. Micromechanical testing techniques, such as nanoindentation and microcantilever beam fracture, allow determination of such localized properties, and can be implemented in the wide range of temperatures. Please click Additional Files below to see the full abstract

Effects of Irradiation Temperature and Dose Rate on the Mechanical Properties of Self-Ion Implanted Fe and Fe-Cr Alloys

Pure Fe and model Fe-Cr alloys containing 5, 10 and 14%Cr were irradiated with Fe+ ions at a maximum energy of 2MeV to the same dose of 0.6dpa at temperatures of 300 C, 400 C and 500 C, and at dose rates corresponding to 6 x 10-4 dpa/s and 3 x 10-5 dpa/s. All materials exhibited an increase in hardness after irradiation at 300 C. After irradiation at 400 C, hardening was observed only in Fe-Cr alloys, and not in the pure Fe. After irradiation at 500 C, no hardening was observed in any of the materials tested. For irradiations at both 300 C and 400 C, greater hardening was found in the Fe-Cr alloys irradiated at the lower dose rate. Transmission electron microscopy and atom probe tomography of Fe 5%Cr identified larger dislocation loop densities and sizes in the alloy irradiated with the high dose rate and Cr precipitation in the alloy irradiated with the low dose rate.Comment: 42 pages, 6 figure

Exposure to androstenes influences processing of emotional words

There is evidence that human-produced androstenes affect attitudinal, emotional, and physiological states in a context-dependent manner, suggesting that they could be involved in modulating social interactions. For instance, androstadienone appears to increase attention specifically to emotional information. Most of the previous work focused on one or two androstenes. Here, we tested whether androstenes affect linguistic processing, using three different androstene compounds. Participants (90 women and 77 men) performed a lexical decision task after being exposed to an androstene or to a control treatment (all compounds were applied on the philtrum). We tested effects on three categories of target words, varying in emotional valence: positive, competitive, and neutral words (e.g., hope, war, and century, respectively). Results show that response times were modulated by androstene treatment and by emotional valence of words. Androstenone, but not androstadienone and androstenol, significantly slowed down the reaction time to words with competitive valence. Moreover, men exposed to androstenol showed a significantly reduced error rate, although men tended to make more errors than women in general. This suggests that these androstenes modulate the processing of emotional words, namely some particular lexical emotional content may become more salient under the effect of androstenes

Preparing potential teachers for the transition from employment to teacher training: an evaluative case study of a Maths Enhancement Course

In response to a UK government drive to improve maths teaching in schools, the South West London Maths Enhancement Course (MEC) has been set up though collaboration between three Higher Education institutions (HEIs) to provide an efficient route for non maths graduates in employment to upgrade their subject knowledge and give a smooth transition into teacher training (PGCE). An evaluation of the scheme, measured against Teacher Development Agency (TDA) objectives and success criteria agreed by university staff, involved thematic analysis of focus group discussions and interviews with students and staff during both the MEC and PGCE courses. This has revealed a high level of satisfaction and success related to a number of underlying issues, particularly around student recruitment, curriculum design, peer support and staff collaboration. The model offers an example of practice transferable to a range of programmes aimed at supporting students in the transition between levels and institutions

Development of novel melt spinning based processing route for oxide dispersion strengthened steels

Melt spinning of an Fe-5Y and Fe-1Y-1Ti (wt%) alloys produced a relatively uniform spatial distribution of Y and Ti in solid solution and ribbons with consistent yield (> 60% by weight), fast processing time ( 100 g feedstock material) and repeatability. Heat treatment in the presence of Fe2O3 as an oxygen source (Rhines pack method) at 973 K validated the potential of forming < 20 nm Yrich oxides in the 1 Fe-5Y ribbons. Pulverized Fe-1Y-1Y ribbons were consolidated to bulk using the field assisted sintering technique (FAST) incorporating nano-sized Fe3O4 powder as the oxygen source. After FAST at 1273 K, 50 MPa and 30 min a comparatively high number density of sub-micron Y and/or Ti-rich oxides were developed. Further formation of fine-scale oxides took place during post-FAST annealing, resulting in an approximate 20% increase in hardness at temperatures below 573 K, but with a reduced hardening effect above 673 K due to a small fraction of persistent porosity and mechanically weak prior ribbon boundaries that were decorated with Ti-rich oxide

Effects of single- and simultaneous triple-ion-beam irradiation on an oxide dispersion-strengthened Fe12Cr steel

Oxide dispersion-strengthened (ODS) steels are main candidates for structural applications in future fusion reactors. Understanding their irradiation-induced behaviour is a key in building optimised components with enhanced radiation resistance. In this work, the stability of an ODS Fe12Cr steel was investigated by transmission electron microscopy after single- (Fe4+) and simultaneous triple-ion-beam irradiation (Fe8+, He+ and H+) at room temperature to doses of 4.4 and 10 dpa. The irradiations were accomplished at the JANNUS-Saclay facility. Results after single-ion-beam irradiation were also compared with those from a reference Fe12Cr steel produced following the same route. Analyses focused on determining the irradiation-induced loop size and density in the ODS and reference materials, investigating the grain boundary microchemistry and studying the evolution of the secondary phases present. These experiments show that the Y-rich nanoparticles present in the ODS steel are quite stable under these irradiation conditions although evolution of larger Cr-rich carbides could be taking place. Loop sizes are smaller for the ODS steel than for the reference material and appear to increase with dose. Cr segregates at some of the grain boundaries, though this segregation also occurs in the absence of irradiation.This investigation was supported by the Ministerio de Ciencia e Innovación (Contract ENE2010-17462), the European Commission through the European Fusion Development Agreement (EFDA), the EPSRC Grant No. EP/H018921/1, the FP7-EU Program under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative-I3) and the Royal Society International Exchanges Scheme 2011/R1 (ref. IE110136)

Microstructural and mechanical characterisation of Fe-14Cr-0.22Hf alloy fabricated by spark plasma sintering

Fe-14Cr pre-alloyed powder and pure Hf powder were mechanically alloyed to produce powder with nominal composition Fe-14Cr-0.22Hf (wt. %) that was consolidated by the spark plasma sintering (SPS) technique in order to investigate the ability of Hf to produce a nanometric dispersion of oxide particles in a ferritic matrix. Comprehensive microstructural and mechanical characterisation of the as-milled powder and the consolidated material was performed using electron microscopy, X-ray diffraction, atom probe tomography and indentation techniques. It was shown that Hf additions can effectively produce, by internal oxidation, a fine scale dispersion of Hf-O nanoparticles in the consolidated material. A uniform grain structure was produced in the alloy. Although the nanoparticle dispersion was not homogeneous at the finest scale, the resulting dispersion strengthening contributed significantly to the hardness. According to these results, internal oxidation of reactive elements rather than direct addition of oxides may offer additional opportunities in the design and development of oxide dispersion strengthened steels

Living for the weekend: youth identities in northeast England

Consumption and consumerism are now accepted as key contexts for the construction of youth identities in de-industrialized Britain. This article uses empirical evidence from interviews with young people to suggest that claims of `new community' are overstated, traditional forms of friendship are receding, and increasingly atomized and instrumental youth identities are now being culturally constituted and reproduced by the pressures and anxieties created by enforced adaptation to consumer capitalism. Analysis of the data opens up the possibility of a critical rather than a celebratory exploration of the wider theoretical implications of this process

Social Value of Marine and Coastal Protected Areas in England and Wales.

The U.K. government is committed to establishing a coherent network of marine protected areas by 2012 and the recentMarine and Coastal Access Act, 2009 will designate marine conservation zones and provide wider access rights to the coast. To fulfill these goals, this article argues the need for a clearer, shared understanding of the social value of protected areas in creating new designations and managing existing ones. Although marine and coastal environments attract many people and are vitally important in terms of realized and potential social value, the majority of the public in the United Kingdom lacks understanding and awareness regarding them. Combined with this, the social value of marine and coastal protected areas (MCPAs) have been largely ignored relative to conservation and economics, with the latter invariably taking precedence in environmental policymaking. Social value reflects the complex, individual responses that people experience in a given place. Many reasons determine why one area is valued above another, and this research investigates the social value of MCPAs from a practitioner’s perspective through a series of interviews. Understanding why we “socially” value MCPAs will ultimately equip managers with an informed understanding of these spaces, influence management decisions, and, potentially, policymaking. This article defines social value in the context of MCPAs in England and Wales from a practitioner perspective, explores key concepts, and suggests possible improvements in decision-making