1. Institute of Materials Science and Technology , Technical University of Vienna, Karlsplatz 13/308, A-1040 Vienna , Austria; e-mail:
2. German Aerospace Centre , Linder Höhe, 51147 Cologne, Germany , e-mail:
3. Metallic Structures and Materials Systems for Aerospace Engineering , RWTH Aachen University, Aachen 52062 , Germany; e-mail: