Free Volumes in Al-based nanocrystalline alloys

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 nearest- neighbourhood 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.

Aluminium alloys are of central importance as light materials with high strength. Correspondingly it is very attractive to further improve the mechanical properties by structural refinement. In this contex procedures based on plastic deformation or rapid cooling are currently developed and investigated, permitting the production of alloys with nanocrystalline or amorphous structure. The complex processes of structural refinement during the production as well as the resulting material properties are massively influenced by atomic sized free volumes, as for instance lattice vacancies and their agglomerates in the crystalline state and structural free volumes in grain boundaries and amorphous phases. In the present project the most sensitive positron annihilation spectroscopy method was used for the first time, to study atomic sized free volumes in selected model systems during the structural refinement process in Al alloys. The nanocrystallization in rapid quenched amorphous Al88Y7 Fe5 (i), the nanocrystallization and amorphization of AlSm during severe deformation by repeated cold rolling (ii), and the structural refinement of age-hardenable Al- Cu-Mg-Mn due to severe torsion deformation under high pressure (iii) are studied in detail. Beside positron lifetime spectroscopy for studying the size of free volumes, the two dimensional Doppler broadening turns out to be a novel powerful tool to study the chemical environment of free volumes on an atomic scale. In all three investigated Al-based systems it was in this way possible to observe characteristic variations of the environment due to structural refinement. In combination with microstructural investigations with X-ray diffraction and transmission electron microscopy detailed understandings on the atomistic procedures of the fragmentation of precipitates during severe deformation (Al-Cu-Mg-Mn) and on the phenomenon of the grain boundaries segregation, which occur in intermediate phases of the deformation induced amorphization (Al-Sm) and the thermally induced nanocrystallization (Al-Y-Fe) could be obtained. Above all the great application potential of the positron annihilation spectroscopy method even in complex material-physical questions was demonstrated.