Effect of Minor Alloying Element on Dispersing Nano-Particles in ODS Steel
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0981-JJ07-09
Effect of Minor Alloying Element on Dispersing Nano-Particles in ODS Steel Y. Uchida1, S. Ohnuki1, N. Hashimoto1, T. Suda1, T. Nagai1, T. Shibayama1, K. Hamada1, N. Akasaka2, S. Yamashita2, S. Ohstuka2, and T. Yoshitake2 1 Graduate School of Engineering, Hokkaido University, Sapporo, Japan 2 Oarai-Research Center, JAEA, Oarai, Japan
ABSTRACT A nano-characterization analysis was performed to clarify the mechanism of fine distribution growth and oxide formation in ODS steels, and furthermore, various minor elements were added to find out their capability to make finer and more uniform distribution. Generally ODS fabrication process is required for hot extrusion, following mechanical alloying (MA) process, but in the present study, annealing was performed up to 1150oC to simplify the microstructure. As a result, it is concluded that MA process, giving high mechanical energy, makes nonequilibrium dissolution of oxide particles in matrix and these forcibly solved particles develop complex oxides after heat treatment at 1150oC. Also, Hf and Zr of Va family on the periodic table were turned out to be more effective for making fine-distributed oxide. This fact suggests that some other elements could have similar effects. INTRODUCTION Oxide dispersion strengthened (ODS) ferritic steels are one of the candidate materials for fusion and fission reactors due to their strength at high temperatures and good resistance for irradiations [1, 2]. These steels have been long investigated and developed for years from various stand points [1-3]. In spite of such contributions, the mechanism of precipitation and distribution of oxide in matrix still remains unknown, and also there are some problems to solve such as, instability of oxides [4] and coarsening of their particles [3] and so on. These facts make it difficult to apply ODS steels to practical use. That is the reason why controlling the oxide distribution is one of the most important aspects in further development of ODS steels. According to the work of Okuda [5], adding either 0.8wt% of titanium or 0.7wt% of zirconium into Fe-12Cr-Y2O3 steels leads to finer and more uniform nano-sized dispersion on ferrite matrix compared to ordinary ODS steels that have no such minor elements. These particles would be complex oxides composed of yttrium and titanium, and yttrium and zirconium, respectively. In addition, the titanium-added ODS steel showed better creep rupture strength than that with zirconium [5]. These results indicate that adding minor elements have a possibility to make dispersion particles finer and improve the mechanical properties of ODS steels. The main purpose of this study is to clarify the mechanism of ODS particles formation and the effect of adding minor elements both mechanically and quantitatively, and newly try to find out other minor elements that are capable of making much finer distribution on ferritematrix.
EXPERIMENTAL All specimens prepared were based on mechanically alloyed ODS powders that have a normal composition of Fe-bal, Cr-9 wt%, C-0
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