Free volumes in nanocrystalline bulk metals

Nanocrystalline metals exhibit novel and enhanced properties compared to their conventional coarse grained counterparts. For prospective application as structural materials their synthesis in bulk form, i.e., as rods or sheets, where at least two dimensions are of cm-size is essential. In this respect techniques based on severe plastic deformation (SPD) such as high-pressure torsion (HPT) or equal channel angular pressing (ECAP) have gained substantial interest. Furthermore, these synthesis routes produce nanocrystalline bulk metals with unique enhanced mechanical properties not found for other nanocrystalline metals. Despite indications for high concentrations of lattice vacancies and/or excess volumes in non-equilibrated grain boundaries in these materials, studies of them by direct and specific methods are rare. Therefore, the present project aims at a detailed defect-specific study and comprehensive understanding of free volume-type defects in nanocrystalline metals which are induced during the extreme conditions of severe plastic deformation. For this purpose, a multi-method approach of the specific and sensitive techniques of positron annihilation spectroscopy (positron lifetime spectroscopy, coincident Doppler broadening technique) and time-dependent dilatometry in combination with structural and thermal characterization (electrical resistivity, differential scanning calorimetry, X- ray diffraction, electron microscopy) will be used. These studies shall yield information on the type and concentration of the predominant defects (lattice vacancies and their agglomerates, dislocations, interfacial free volumes), on their variation with processing parameters, and on their thermal behaviour. The kinetics of vacancy- type defects is investigated by in-situ time-dependent dilatometry and by fast positron annihilation measurements using a high-intense positron beam. In addition, coincident Doppler broadening technique will be applied in order to gain information on the local atomic environments of free volumes in SPD-processed nanocrystalline alloys. From these in-depth studies of free volumes, important contributions for the understanding of the SPD-induced processes of structural refining and alloy formation as well as of the enhanced mechanical properties and the fast atomic diffusivities of SPD-processed nanocrystalline metals can be expected. The project will be performed in close collaboration with partners of the National Research Network (NFN S10400) on High-Performance Bulk Nanocrystalline Metals.

In this project lattice defects in bulk nanocrystalline metals could be identified, quantitatively analyzed, and characterized with respect to stability by means of a combination of macroscopic measurements high precision length measurements, so-called dilatometry and atomistic spectroscopic techniques (positron-electron annihilation). Part of the studies were performed at the high-intensity positron beam of the research neutron source FRM II which enabled in-situ studies of the defect kinetics on the same time scale as in the dilatometric measurements. In this project, among others, for the first time, the excess volume of grain boundaries a fundamental structural material parameter could be determined experimentally by the direct and absolute technique of dilatometry. Due to the ultrafine crystallite size, nanocrystalline metals may exhibit novel and improved properties in comparison to their coarse-grained counterparts. In this project, bulk nanocrystalline metals were prepared by severe plastic deformation, so-called high-pressure torsion (HPT). The macroscopic sample dimensions (on the length scale of cm) which can be achieved by HPT not only provide the base for technical applications but also allowed the novel access to material defects by the highly specific technique of dilatometry in the present project. Metals with body-centred cubic (Fe, Ta) and face-centred cubic structure (Cu, Ni) were prepared and studied. For all studied samples high concentrations of free volumes of the order of several 10-3 (thousandth part) could be detected. Correlating the results with positron annihilation spectroscopy and crystallite sizes as determined from scanning electron microscopy, the free volume could be attributed to the various types of lattice defects (lattice vacancies, dislocations, grain boundaries). In addition, the kinetics of defect annealing was analyzed quantitatively. The information and data derived from the present project may contribute to a more profound understanding of the processes of structural refinement during HPT as well as of the particular mechanical properties and the fast atomic diffusion in these ultrafine grained materials. The project was performed in close cooperation with groups of the Erich-Schmidt Institute (OeAD, Leoben), of the physics faculty of the university Vienna, and of the research neutron source Heinz Maier-Leibnitz (Munich-Garching). Results of the project were in part published in two papers in Physical Review Letters (see also FWF press release: http://www.fwf.ac.at/en/public_relations/press/pv201011-en.html).