FWF - Freies Volumen in Al - Defects and free volumes in nanocrystalline materials

Al-based nanocrystalline alloys have attracted substantial interest due to their outstanding mechanical properties. In order to contribute to an atomistic understanding of the interfacial structure and processes during the various states of nanocrystallization, the present project will deal with studies of interfacial free volumes in Al-Sm and Al-Y-Fe alloys by means of positron lifetime and coincident Doppler broadening spectroscopy. The combination of these two techniques yields unique and novel information on both the size of free volumes and the chemical nearestneighbourhood of the free volumes which cannot be obtained otherwise. Since positrons escaping from the nanocrystallites are trapped predominantly at the interfaces between the nanocrystallites and the intergranular amorphous phase, positron annihilation is particularly well suited for studying of free volumes at the nanocrystal glass interfaces, the chemistry of which is a major factor of the nanocrystallization process of Al-based alloys. The following major issues will be dealt with: Study of the solute pile-up at the crystal amorphous interfaces which are formed between the Al nanocrystallites and the amorphous matrix. Study of the variation of the size and the chemistry of free volumes with processing routes of nanocrystallization (i.e., thermally induced or deformation-mediated) and with state of processing (i.e., variation with annealing and deformation). Variation and optimization of the alloy composition with respect to a high number density of primary crystallites, a high volume fraction of nanocrystallites, and a high thermal stability. The atomic-scale studies on free volumes obtained from positron annihilation will be combined with a structural characterization using x-ray diffraction (XRD) and transmission electron microscopy (TEM). For an unambiguous assessment of the potentials of coincident Doppler broadening as new analytical method for studying of interfaces in multicomponent complex materials, the results will be compared with studies of analytical high-resolution electron microscopy.