R.M. Wang; C.Z. Li;S. W. Zhang and M.G.Yan(1)Institute of Aeronautical Materials, Beijing 100095, China 2)Laboratory of Atomic Imaging of Solids, Institute of Metal Research, Chinese Academy of Sciences,Shenyang 110015, China 3)Beijing Laboratory of Electron Microscopy, Chinese Academy of Sciences, Beijing 100080,China Manuscript received 26 August 1996)