[1] Tohoku Univ, WPI Adv Inst Mat Res, Aoba Ku, Sendai, Miyagi 9808577, Japan
[2] CSIC, Natl Ctr Met Res, E-28040 Madrid, Spain
[3] Univ Sheffield, Dept Mat Engn, Sheffield S1 3JD, S Yorkshire, England
[4] Ningxia Orient Tantalum Ind Co Ltd, Ningxia 753000, Peoples R China