Electrically-Assisted Tubing Processes for Enhancing Manufacturability of Oxide Dispersion Strengthened Structural Materials for Nuclear Reactor Applications

Nanostructured ferritic alloys (NFAs) are a promising class of structural material with extremely radiation damage tolerant for next generation of nuclear reactor applications. They have an iron-based matrix containing a fine, homogenous dispersion of nanosized oxide particles. These particles provide sinks for radiation induced defects and He atoms 3–5, resulting in excellent resistance to radiation damage. In addition, nanosize oxides act as strong, stable barriers to dislocation glide, grain boundary motion, and grain growth providing high temperature strength and creep resistance4. As a result, NFAs can be used at higher temperatures (up to 700°C) than traditional ferritic/martensitic steels (up to 550°C), and also withstand higher irradiation doses, resulting in increased fuel cladding lifetime of more than 20%. For the manufacture of NFA fuel cladding, NFA is formed into a thin-walled tube. However, the low formability of NFA presents challenges to the forming of NFAs. The formability of NFAs is only about 5-7% elongation before necking at 200°C. This makes the tube cracks easily during the process even after annealing. In order to ease the forming of NFAs, the process has to be conducted under high temperature and which greatly increases the manufacturing cost. There is a desire on exploring new technique for the forming of NFAs in order to make this promising material to be more feasible in nuclear component applications. Therefore, the objective of this project is to enhance the formability of NFAs such that high-strength thin-walled claddings can be manufactured successfully and economically to be used in future nuclear reactors. To accomplish the final goal of successfully and economically manufacture of NFA thin-walled claddings, electrically-assisted (EA) forming technique will be applied in the process. This is a technique that uses the generated thermal and athermal effects of an electric current passage to ease material deformation by flow stress reduction. Localized effect can also be achieved to the regions of interest by controlling electric current passage regions. This feature of EA forming makes the process to be more energy efficient and cost effective. Furthermore, electropulsing has also been found its effects on promoting crack healing, improving corrosion resistance, accelerating nucleation, atom diffusion and grain growth as well as enhancing recrystallization and grain refinement. For the successful forming of NFA thin-walled tubes, both understanding of electric current on NFA and designing an effective thin-walled tube forming process are essential. Therefore, in this project, three main tasks are planned: (1) mechanical characterization of NFA coupon specimens in EA forming, (2) demonstration and process mechanics of EA rolling and drawing, and (3) microstructural characterization of EA formed tubes. Task (1) will mainly focus on the effect of electric current on microstructure and mechanical properties, such as flow stress and elongation. Meanwhile, the investigation on the correlation of EA effect and initial NFA microstructure, such as grain size, will be conducted as well. In Task (2), EA technique will be demonstrate on rolling and drawing for NFA part production. Fundamental studies on the capability of EA forming of NFA will be conducted through EA rolling with an existing home-made desktop rolling mill. Then, an EA drawing machine will be designed and built for the manufacture of thin-walled tubes based on the study of EA rolling. The tubes produced in this task will be brought to Task (3) fo