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June 22, 2015
Seeking Materials Stronger at Higher Temperatures: The Development of Oxide Dispersion-Strengthened Metals

In the fusion reactor, we convert the energy from particles generated by the fusion reaction that occurs in a plasma into heat and use that heat for producing energy. In the reactor, the part called the blanket that envelops the entire plasma fulfills the role of converting the energy from the plasma into heat. Further, that heat is taken out from the reactor by using a coolant, and the heat generates energy by turning a steam turbine. In this process of removing heat and generating energy, in general, the generation efficiency becomes higher by removing the heat at higher temperature. For that reason, we must raise the temperature of the blanket. Thus, development of materials that will constitute a blanket strong at high temperatures over an extended period of time is demanded.

The material to be introduced here is called a “dispersion-strengthened metal.” Typically, the nano-level gaps (of the order of one-millionth of 1mm) that are in a material produce cracks when there is movement. Then, when the temperature increases, those gaps become easier to move. When different varieties of nano-level-size particles scatter uniformly in such a material, because these particles become obstructions and the movement of gaps is suppressed, they become strong materials even at still higher temperatures.

Among the dispersion-strengthened metals, there is “oxide dispersion- strengthened ferrite steel” which scattered oxide materials in ferrite steel, which is one type of stainless that is well known as a structural material. Ferrite steel is a fundamental material in the design of the fusion reactor, and is used at temperatures from 400 degrees to as high as 500 degrees Centigrade. From this ferrite steel we newly developed materials where yttrium oxide is dispersed, and we discovered that this is a strong material even at the higher temperature of 700 degrees Centigrade. Further, we have hypothesized using this material as the structural material for the advanced liquid blanket, and have shown systematically the compatibility with this oxide dispersion-strengthened ferrite steel and liquid coolant. (See the April 6, 2015, Research Update.)

The dispersion strengthening method is attracting attention as an effective strengthening method that is applicable to many heat-resistant materials, and research is being conducted in research institutes around the world. In addition to ferrite steel, this is expected to be applicable to base materials such as tungsten, vanadium, and copper. The National Institute for Fusion Science, too, plans to report in Japan and abroad our research successes in dispersion-strengthened metals.