1. Materials Center Leoben, Forschungs GmbH, Leoben, Austria
2. Erich Schmid Institute of Material Science, Austrian Academy of Sciences, Leoben, Austria
3. now at: National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, U.S.A.
4. Department of Materials Physics, Montanuniversität Leoben, Leoben, Austria