The production volume of titanium and titanium alloys showed rapid growth from 2009 to 2012 due to demand from aerospace-related businesses, automotive parts aimed at preventing global warming (reducing CO2 emissions and improving fuel efficiency), and marine structures such as runways that take advantage of titanium's unique corrosion resistance and design features (Figure 1). However, the production volume in Japan has been rapidly declining, although there is a global downward trend. One of the main reasons for this is that the reduction of titanium dioxide (TiO2) to high-purity titanium consumes a large amount of electricity, which has become cost-prohibitive under the recent electricity situation in Japan, where electricity rates have increased by 25% from 2010 to 2014 after the Great East Japan Earthquake6). 6) Therefore, it has become important to develop technologies to reduce the cost of titanium production. Research has been conducted to establish new reduction processes, to reduce processing costs, and to establish low-cost elemental alloy designs. In particular, attempts have been made to design the microstructure of titanium and titanium alloys using intrusive elements (oxygen O, iron Fe, and nitrogen N), which are considered problematic in the reduction process of titanium. 5) . On the other hand, there are few studies on the effective utilization of α-type titanium alloys (hcp structure).
The reduction process of titanium involves many steps and is a batch process, so it is costly to produce high-purity titanium materials. The high-purity porous titanium (sponge titanium) created through the reduction process is arc-melted in a vacuum and cast as ingots of pure titanium and titanium alloys. In this reduction process, we are trying to make effective use of oxygen by retaining oxygen instead of increasing the purity as much as possible. It is also easy to manage in terms of operation. In addition, by casting a mixture of pure titanium and titanium dioxide (TiO2), the amount of sponge titanium can be reduced, thus reducing the material cost. Thus, if oxygen can be effectively used as an alloying element, the cost can be reduced. Since oxygen has almost no resource limitations, has high solid solution strengthening ability, and has the advantage of not changing the specific gravity of the material,4) it is industrially and academically significant to clarify the role of oxygen in titanium.
Oxygen content and ductility
In view of the above background, various researchers have attempted to effectively utilize oxygen as an alloying element in titanium and titanium alloys.4) However, not many studies have been conducted for oxygen solid solution >0.5 mass%. 4) However, the oxygen solid solution of >0.5 mass% has not been investigated very much, because it is believed to be brittle and does not show ductility when the oxygen solid solution is >0.50%, as reported by Jaffee et al. In this laboratory, we found that high ductility can be obtained by controlling the anisotropy even when the oxygen solid solution is >0.5 mass% (Fig. 1). The mechanical properties and deformation mechanism of Ti-O alloys with controlled anisotropy (directional Ti-O alloys) were clarified, and it was found that Ti-0.71mass%O with a developed aggregate structure showed a total elongation of about 20%. We found that it is possible to expand the range of solid solution of O that can be effectively used as an alloying element by controlling texture.
図１ Elongations of Ti-O alloy with texture and conventinal titanium
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