1. School of Materials Science and Engineering, Lanzhou University of Technology, No. 287 Langongping Road, Lanzhou 730050 P. R. China
2. State Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals, Lanzhou University of Technology, Lanzhou , P.R. China
3. School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou , P. R. China