W.H. Tian; S.L. Hu and M. Nemoto(1) Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, China 2) Department of Materials Forming, University of Science and Technology Beijing, Beijing 100083, China 3) Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812, Japan)Manuscript received in revised form 12 October 1998