1. School of Advanced Materials Engineering, Kookmin University, Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Korea, Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, Materials Physics & Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and School of Materials Science and Engineering, Seoul National University, Shillim-dong, Seoul, 151-744, Korea