Over the past several years, advances in the design and fabrication of planar solid oxide fuel cells ͑SOFCs͒ have led to a steady reduction in the temperatures necessary for their operation. Consequently, it appears more realistic now to use low cost heatresistant alloys for interconnect components in the SOFC stack. However, no specific criteria or inclusive study are available as a reference to help select and evaluate suitable candidates from the hundreds of available heat-resistant alloy compositions, which overall demonstrate oxidation resistance at high temperatures. In this work, composition criteria have been proposed for the preselection of heat-resistant compositions, such as Ni-, Fe-, and Co-based superalloys, Cr-based alloys, and stainless steels. The proposed criteria have been employed to establish a database of heat-resistant alloys at Pacific Northwest National Laboratory, where a systematic approach has been initiated to evaluate and modify and/or develop alloys for SOFC interconnect applications. The preselected compositions are further evaluated by referring in-house studies and reference to published data. It appears that it would be difficult for traditional alloys to fully satisfy the materials requirements for long-term operation at temperatures higher than 700°C. However, the applicability can be improved via surface/bulk modification and by the implementation of novel stack designs.

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Nuclear power research facilities require alternatives to existing highly enriched uranium alloy fuel. One option for a high density metal fuel is uranium alloyed with 10 wt% molybdenum (U-10Mo). Fuel fabrication process development requires specific mechanical property data that, to date has been unavailable. In this work, as-cast samples were compression tested at three strain rates over a temperature range of 400 to 800°C to provide data for hot rolling and extrusion modeling. The results indicate that with increasing test temperature the U-10Mo flow stress decreases and becomes more sensitive to strain rate. In addition, above the eutectoid transformation temperature, the drop in material flow stress is prominent and shows a strain-softening behavior, especially at lower strain rates. Room temperature x-ray diffraction and scanning electron microscopy combined with energy dispersive spectroscopy analysis of the as-cast and compression tested samples were conducted. The analysis revealed that the as-cast samples and the samples tested below the eutectoid transformation temperature were predominantly phase with varying concentration of molybdenum, whereas the ones tested above the eutectoid transformation temperature underwent significant homogenization. *Manuscript Click here to view linked References

Please cite this article as: ABSTRACTIn the first part of this series, it was determined that the as-cast U-10Mo had a dendritic microstructure with chemical inhomogeneity and underwent eutectoid transformation during hot compression testing. In the present (second) part of the work, the as-cast samples were heat treated at several temperatures and times to homogenize the Mo content. Like the previous ascast material, the "homogenized" materials were then tested under compression between 500 and 800°C. The as-cast samples and those treated at 800°C for 24 hours had grain sizes of 25-30 µm, whereas those treated at 1000°C for 16 hours had grain sizes around 250 µm before testing.Upon compression testing, it was determined that the heat treatment had effects on the mechanical properties and the precipitation of the lamellar phase at sub-eutectoid temperatures.

In Phase 1 of this study, the mechanical properties of as-cast, depleted uranium alloyed with 10 weight percent molybdenum alloy (U-10Mo) samples were evaluated by high temperature compression testing. Compression testing was conducted at three strain rates over a temperature range of 400 to 800°C. The results indicated that with increasing test temperature, the material flow stress decreases and the material becomes more sensitive to strain rate. In addition, above the eutectoid transformation temperature (~ 550°C), the drop in material flow stress is prominent and shows a strain-softening behavior, especially at lower strain rates. In the second part of this research, we studied the effect that homogenization heat treatment had on the high temperature mechanical properties and microstructure of the cast U-10Mo alloy. Various homogenization times and temperatures were studied ranging between 800 and 1000°C for 4 to 48 hours. Based on the microstructural response in this homogenization study, a heat treatment cycle of 800°C for 24 hours and another at 1000°C for 16 hours were selected as the times at temperature to achieve a fully homogenized sample. Samples from these conditions were then compression tested at a variety of temperatures ranging from 500 to 800°C. The microstructure of these samples were compared to the as-cast samples and to a baseline sample homogenized at 1000°C for 16 hours. The results indicate that below the eutectoid temperature (~ 550°C) all three samples showed strain hardening and followed similar trends. Above the eutectoid temperature, the yield strength of the material decreased linearly. For the as-cast sample and the sample homogenized at 800°C for 24 hours, the n-values were negative, whereas for the samples homogenized at 1000°C for 16 hours the material exhibited a perfectly plastic behavior. The as-cast sample, heat treated at 800°C for 24 hours, showed significant lamellar structure transformation that seems to have precipitated along the grain boundaries in the molybdenum-lean regions. In similar samples, homogenized at 800°C for 24 hours and tested at 650°C, the backscattered-electron scanning electron microscopy images revealed a composite structure of lamellar phase and nano-scale molybdenum-rich and-lean phases along the grain boundaries. These phases may have been responsible for the lowering of the flow stress in the material observed in the Phase 1 work. For comparison, the samples homogenized at 1000°C for 16 hours showed no such transformations.

Low-enriched U-22at% Mo (U-10Mo) alloy has been considered as an alternative material to replace the highly enriched fuels in research reactors. For the U-10Mo to work effectively and replace the existing fuel material, a thorough understanding of the microstructure development from as-cast to the final formed structure is required. The as-cast microstructure typically resembles an inhomogeneous microstructure with regions containing molybdenum-rich and-lean regions, whichmay affect the processing and possibly the in-reactor performance. This as-cast structure must be homogenized by thermal treatment to produce a uniform Mo distribution. The development of a modeling capability will improve the understanding of the effect of initial microstructures on the Mo homogenization kinetics. In the current work, we investigated the effect of as-cast microstructure on the homogenization kinetics.

To serve as an interconnect / gas separator in an SOFC stack, an alloy should demonstrate the ability to provide (i) bulk and surface stability against oxidation and corrosion during prolonged exposure to the fuel cell environment, (ii) thermal expansion compatibility with the other stack components, (iii) chemical compatibility with adjacent stack components, (iv) high electrical conductivity of the surface reaction products, (v) mechanical reliability and durability at cell exposure conditions, (vii) good manufacturability, processability and fabricability, and (viii) cost effectiveness. As the first step of this approach, a composition and property database was compiled for high temperature alloys in order to assist in determining which alloys offer the most promise for SOFC interconnect applications in terms of oxidation and corrosion resistance. The high temperature alloys of interest included Ni-, Fe-, Co-base superalloys, Cr-base alloys, and stainless steels. In the US alone, there are hundreds of commercial compositions produced, over 250 of which are listed in Appendix A. Two initial criteria (oxidation resistance and oxide scale electrical conductivity) were used to reduce the list of alloys to manageable proportions. Thermal expansion and fabrication characteristics were then considered to further reduce the list of stainless steels. Due to their outstanding oxidation resistance and their potential to be used in SOFC components that can exclude alumina scales from the stack electrical path, alloys with a sufficient amount of aluminum were classified into a separate alumina-forming alloy category. The down-selected compositions (approx. 130 in number) and their characteristics and/or applications are listed in the Selected Alloy Compositions tables (Appendix B). Following the down-selection of alloy compositions, materials properties of interest corresponding to the their functional requirements in SOFC stacks were compiled in a tabular form (Appendix C). For comparison, the properties of selected noble metals and intermetallics were also collected and compiled and are listed in a separate table in Appendix C. Analysis of the pertinent literature indicated that, for a wide variety of alloys, there remains a lack of information on specific materials properties. Also, we have observed a large scatter in the reported database. For those cases, we employed general alloying principles as a tool of choice to approximate the unavailable data and to evaluate the reliability and consistency of collected data. Though numerous high temperature alloys look promising, it is anticipated that there will be few, if any, "off the shelf" alloy compositions which could completely satisfy the materials requirements as an interconnect, especially for a long term in a specific SOFC design. Therefore, some concepts of alloy design, including composition, constitution, and structure, as well as their effects on properties relevant to SOFC applications, are elaborated in an attempt to provide guidance for modification of curren...

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