1. Department of Applied Mechanics and Aerospace Engineering Waseda University 3‐4‐1 Okubo, Shinjuku‐ku Tokyo 169‐8555 Japan
2. Department of Materials Science Waseda University 3‐4‐1 Okubo, Shinjuku‐ku Tokyo 169‐8555 Japan
3. Kagami Memorial Research Institute for Materials Science and Technology Waseda University 2‐8‐26 Nishiwaseda, Shinjuku‐ku Tokyo 169‐0051 Japan