4.21.3.3.1 Scoping by bulk immersion tests

Exploration of new candidates having superior per­formance compared to CaO and AlN was carried out using static immersion tests. Figure 4 shows the mass loss of various insulator ceramics due to exposure to static Li at high temperatures. As predicted by ther­modynamics, Er2O3 and Y2O3 showed stability supe­rior to that of CaO.2,15 For these materials, formation of LiXO2 (X = Er or Y) during exposure to Li was reported,17,18 although the impact of these changes on the coating properties remains to be assessed. In particular, the effects of Li flow on the stability of the corrosion products on the surfaces will be the key issue.

4.21.3.3.2 In situ coating with Er2O3

Based on the experience with CaO, in situ coating with Er2O3 and Y2O3 was attempted on V-4Cr-4Ti by doping Er or Y in Li and O into V-4Cr-4Ti. Coating with Y2O3 was shown to be difficult, proba­bly because there was almost no solubility of Y in Li.

On the other hand, formation of Er2O3 was con- firmed.19 Because the solubility of Er in Li is much lower than that of CaO, the stability of the coating, once formed, is much higher compared with the CaO in situ coating. The cross-section of the coating with compositional profile and coating thickness with time and temperature are shown in Figures 5 and 6, respectively.

In the effort to optimize the precharging condition of oxygen, the microstructural process for restoring oxygen in vanadium alloy substrate was clarified.2 Figure 7 shows the depth profile of hardness before and after oxygen charging, after heat treatment and

(b)

(c)

„ > [200] 1000 [020]

V — 4Cr -4Ti (NIFS-HEAT-2)

T-o "D і. ra

■W w

МіАіІаігіІіЦіїІцКИНіЦ

(a)

0 50 100 150 200 250 Depth from surface (pm)

300

Figure 7 Depth distribution of hardness and transmission electron microscope microstructure near the surfaces in the V-4Cr-4Ti substrate. (a) Before oxidation, (b) after oxidation, (c) after annealing, and (d) after in situ coating (coating was removed). Reproduced from Yao, Z.; Suzuki, A.; Muroga, T.; Yeliseyeva, O. I.; Nagasaka, T. Fusion Eng. Des. 2006, 81, 951-956, with permission from Elsevier.

after in situ coating, together with the transmission electron microscope (TEM) microstructures near the surfaces. The hardness is known to follow the approx­imate level of O in V-4Cr-4Ti.21 After oxidation for 6 h at 700 °C, the surface was covered with a complex oxide layer. After the subsequent heat treatment for 16 h at 700 °C, the matrix was composed of a high density of needle-shaped Ti—O (mostly TiO2) precipitates oriented in the (100) directions (net structure). This structure was most prominently observed after annealing at 700 °C. This is consistent with the results of the precipitation study, which showed that, although Ti interacts with impurity O already at ^200 °C, Ti-O precipitates start to form at ^700 °C in V-4Cr-4Ti alloys.22 Figure 7 also shows that the net structure near the surface disappeared after exposure to Li for 100 h at 700 °C because of the loss of oxygen. A recent study showed in situ healing capabilities with Er2O3, but further optimization of the process is required to obtain a reliable healing function.23

The mechanism of the net structure formation and supply of oxygen for the coating was elucidated using a kinetic model. The model successfully explained the experimental trends.24